Pilots Notes

I. GENERALITIES: 

For those whom are not particularly conversant in the field of engines, and more particularly aircraft engines, let us first go through a few very basic principles. To start with, there are no big differences between a car engine and an aircraft engine, at least not as long as piston engines (moteurs à pistons/zuiger-motoren) are concerned (fig.1). There are of course other sorts of power plants such as jet engines, but these are not presently used on the usual light civilian aircraft. In fact, the piston engines used on most basic trainers are rather unsophisticated units when compared to the equipment fitted in the average car.

As the word implies, the piston engine includes a number of pistons, usually four or six when light aircraft are concerned but possibly much more for heavier machines. Each piston is located in a cylinder and is connected to the crankshaft (vilebrequin/ krukas) by means of a connecting rod (bièlle/drijfstang). The piston reciprocates up and down in the cylinder and causes the
crankshaft to rotate by action of the connecting rod. This rotary motion can be clockwise or anti-clockwise, depending on the engines design. As far as aircraft are concerned, the crankshaft is connected to the propeller which provides the necessary thrust, and there is thus no complicated transmission system to the wheels, as is the case in the simplest of cars.

The crankshaft is suspended in a crankcase (carter/krukkast) by means of a number of bearings (roulements à billes/kogellagers), and which contains the necessary lubrication oil originating from an oil tank.

The lowest position of the piston is known as the Inner Dead Center, or IDC (Point Mort Bas - PMB/Onderste Dode Punt - ODP). When in this position, the space in the cylinder is at its maximum and corresponds to the total cylinder volume. This space is obviously reduced to a minimum when the piston is at its highest position, the Outer Dead Center, or ODC (Point Mort Haut PMH/Bovenste Dode Punt - BDP), and forms the combustion chamber (chambre de combustion/verbrandingskamer of compressieruimte). The piston's displacement during one single stroke determines the so-called cylinder capacity (cylindrée/cilinderinhoud) which is usually measured in cubic inches or cubic centimeters. Knowing the bore (alésage/boring) of the cylinder, i.e. its diameter, and the piston's stroke (course/slag), it is easy to calculate the displacement. If we consider for example the engine powering the Piper PA-28-161 "Warrior II'', the reported bore is 5,125 inches for a stroke of 3,875 inches. The resulting cylinder capacity (or displacement) is 319,8 cubic inches. Indeed:

• We know that the volume of a cylinder is determined by the relationship "pi" x R2  x H (where "pi" is   3,1416, R is the radius of the cylinder, and H (Height) corresponds to the stroke;
• R equals Bore/2, i.e. 5,125/2 = 2,5625 in.;
• As H is the stroke, it equals 3,875 inches
• Thus: 3,1416 x 2,56252 X 3,875 = 79,9374 cu.in. for one cylinder. As the engine includes 4 cylinders, the total cylinder capacity is 79,9374 x 4 = 319,7496 cu.in.

The piston/cylinder units can be connected to the crankshaft in various ways. They can for instance be located behind each other, the so-called in-line design, which is the case for the British "Gypsy" and "Cirrus" engines fitted on the Tiger-Moth and SV4 vintage aircraft. They are very often horizontally opposed, as is the case for the American Lycoming and Continental engines: one advantage of this design is that the forces which are exerted in the cylinders tend to cancel each other which give way to a somewhat smoother running power plant. Finally, the cylinders can also be mounted radially around the crankshaft: such radial engines (moteurs en étoile/stermotoren) are typical for the more powerful plants, but are sometimes to be found on light aircraft as well.

The piston's job, which consists of rotating the crankshaft, and consequently the propeller, requires the use of fuel. This fuel, gasoline, needs to be combined with air in order to form an inflammable mixture. Such a mixture is usually formed in the carburetor (carburateur/carburator of vergasser) from where it is ducted towards the cylinder whose top side is fitted with an
inlet valve (soupape d'admission/inlaatklep). As will be pointed out below, the carburetor is easily subject to ice formation and requires a heating system. This problem is eliminated on more advanced engines where the carburator is replaced by a so-called fuel injection system.

The events which take place in the cylinder include four motions, or strokes, of the piston. This four stroke cycle (cycle à quatre temps/viertaktcyclus) includes:

1°.- The induction stroke (aspiration/inlaatslag): the inlet valve is open, the piston leaves its highest point and moves downwards, literally aspirating the fuel/air mixture from the carburator towards the cylinder. When the piston reaches its lowest point, the crankshaft has rotated half a turn, i.e. 180°, and the cylinder is now completely filled with the inflammable mixture. The inlet valve closes, thus putting the fuel/air mixture in a hermetically closed space.

2°.- The compression stroke (compression/compressieslag): the piston leaves its lowest point and moves upwards. As the cylinder is hermetically closed, the fuel/air mixture is gradually compressed, up to a maximum value when the piston reaches its highest point again. By this time, the crankshaft has rotated a full 360°.

3°.- The power stroke (temps moteur/arbeidslag): an electrical spark, which results from the operation of the magneto(see below), occurs at the spark-plug (bougie/bougie) located in the top of the cylinder and gives way to the so-called ignition (allumage/ontsteking). The fuel/air mixture ignites and, as it was very strongly compressed during the previous stroke, it will now expand with great force because of the very high temperature, 1500 to 2000°c and exert the required pressure onto the piston, thus driving it back towards its lowest point and causing the crankshaft to rotate another 180°.

4°.- The exhaust stroke (échappement/uitlaatslag): the piston moves upward again. During this stroke a second valve opens, the exhaust valve (soupape d'échappement/uitlaatklep), which is also located on the top of the cylinder, and allows the consumed gases to be evacuated overboard through the exhaust pipe (tuyau d'échappement/uitlaatbuis). When the piston reaches its highest point again, the crankshaft has again rotated by 180°, the exhaust valve closes, the inlet valve opens once more, and the cycle starts all over again with a new induction stroke.

Note that, of these four strokes, only one directly drives the piston, namely the power stroke. The three others are solely due to the inertia of the power stroke in combination with the flywheel effect of the propeller. Note also that the so-called firing order (order d'allumage/ontstekingsvolgorde) of the various cylinders, and which is obtained through the distributor of the ignition system (see below), is arranged in such a way that the torsion stresses on the crankshaft are equally divided. Besides the flywheel effect of the propeller, the firing order is also instrumental in keeping the crankshaft rotating. Considering for instance the simple case of a four cylinder engine, when one cylinder is firing, a second one is on its compression stroke, a third one is on its induction stroke. while the fourth one is in the exhaust phase.

Let us also emphasize that the four stroke cycle, as explained here above, is extremely theoretical. Indeed, in reality it requires quite a number of adjustments regarding amongst others the correct moment for the opening and closing of the inlet and exhaust valves, not to speak about the rather complicated operating system of these valves, the correct timing for the ignition to occur, etc ...

II.THE CARBURETOR:

As the engine of your training aircraft is probably fitted with a carburetor, it is interesting to have some idea about its operating principle and the problems which may be involved, particularly the occurrence of icing.

Fuel is supplied to the carburetor from the fuel tank through a fuel shut-off valve which is controlled by the pilot, and through a fuel strainer, or filter. On high-wing aircraft, the fuel is supplied by simple gravity. On low-wing aircraft however, gravity feed is impossible and the fuel supply requires an engine driven fuel pump, an electric auxiliary pump or booster pump, used as an emergency standby in case the engine driven pump should fail during takeoff and landing, as well as a fuel pressure indicator. It should be noted that if the after takeoff checklist calls for the auxiliary pump to be switched off, THIS SHOULD NEVER BE DONE BELOW A MINIMUM OF AT LEAST 500FT AGL AND THE FUEL PRESSURE MUST BE VERIFIED: IF THE FUEL PRESSURE DROPS OR THAT THE ENGINE FALTERS, THIS INDICATES A FAILING ENGINE DRIVEN PUMP AND REQUIRES THE AUXILIARY PUMP TO BE RE-ACTIVATED AT ONCE !! Under such circumstances the flight should be discontinued and an immediate return for landing should be initiated.

As said earlier, the purpose of the carburetor is to form the fuel/air mixture which is then ducted towards the inlet valve of the cylinders through the inlet manifold. There are various types of carburetors. Broadly speaking, they can be divided into two families: the float-carburetors whereby the mixture is aspirated by the piston towards the cylinder and those, more sophisticated, which inject the mixture under high pressure in the cylinder. This latter type may not be confused with the so-called fuel injection system whereby there is no carburetor at all, and which is practically immune to icing. However, most light aircraft are fitted with the float type.

Figure 2 shows the operating principle of such a float-carburetor. Note that, once again, the system is much more complicated than the drawing may imply. For one thing, the carburetor must be able to produce an acceptable mixture at any power setting. Ideally, the fuel/air ratio is 1/15, i.e. one part of fuel for fifteen parts of air, but this ratio may vary between 1/11 and 1/18, depending on the power setting. Whenever the required ratio is disturbed, problems occur: if there is too much fuel, i.e. when the mixture is too rich, the engine runs roughly because of insufficient combustion; on the other hand, too little fuel, i.e. a too lean mixture, is equally detrimental to the normal operation. Furthermore, the altitude plays an important part and a mixture control system (see below), is needed as well.

The float-carburator mainly includes a float chamber (cuve à flotteur ou cuve a niveau constant/vlotterkamer), a jet system (système de gicleurs/ sproeiersysteem) and a mixing chamber (chambre de carburation/mengkamer) leading to the cylinder inlet manifold. A throttle (papillon/smoorklep), controlled by the pilot through the throttle lever, is located between the jet system and the mixing chamber.

The nozzles of the jet system discharge in the narrowest part of a venturi-like passage located in the air duct of the carburetor. The carburetor air inlet itself is often located underneath, the propeller spinner (moyeu d'hélice/schroefnaaf), as shown in figure 3, and includes a filter whose purpose it is to prevent dirt and dust to enter the fuel system. During the preflight inspection, it is important to verify the condition of this filter and to ensure that it is not blocked. The carburetor air inlet is sometimes located behind one of the engine cooling air inlets (prises d'air de refroidissement/inlaten voor koelingslucht). In this case it is hardly visible, if at all, and a baffle (plaques de guidage/leiplaten) system ensures that the air to the carburetor is effectively separated from the heated engine cooling air. At any rate, when the pilot selects the carburetor heating system to "HOT", the normal carburetor air inlet duct is closed by means of a valve and the engine cooling air, which passes through a baffle system around the exhaust pipe, and gets thus additional heating, is then directed towards the carburetor and prevents or eliminates any icing (fig. 2). The important matter to be aware of is that, in this case, the carburetor air filter is by-passed, thus allowing free passage to possible dust particles: this is why the use of the carburetor heating system should be restricted as much as possible when the aircraft is on the ground.

The downgoing motion of the piston creates an increase of the cylinder's volume, and consequently a pressure drop. This causes the air, whether normally filtered or heated, to be aspirated. Upon passing through the narrow part of the venturi it is further accelerated, thus causing the pressure to drop even more. In this concern, you know probably that a venturi is nothing more than an open duct, gradually narrowing to a small passage, then widening again. When air is forced through such a duct, its speed will increase, which goes along with a noticeable static pressure drop. This phenomenon was discovered by Daniel Bernouilli (1700-1782), a swiss whose research led the following famous statement: the sum of the dynamic pressure and the static pressure of air, or any other fluid for that matter, is a constant.
In other words, if the dynamic pressure increases (which it will if the speed increases: to make this clear, simply put your hand in a water jet under a tap and feel the difference in dynamic pressure when it is running fast or slow), the static pressure, i.e. the pressure on the duct's wall, decreases. You will encounter this so-called law of Bernouilli many times in other fields, amongst others in aerodynamics.

The float chamber is always fully filled with fuel (from there the french denomination "cuve à niveau constant") originating from the fuel tank. The fuel level in the float chamber remains unchanged because of the presence of a float - hence the name - fitted with a needle (pointeau/pen) which opens or closes the fuel inlet. Whenever the fuel level in the float chamber tends to decrease, the float and its associated needle goes down, thus allowing the fuel to enter the chamber: once the level is back
to normal, the needle will block the fuel passage again.

The float chamber is also in communication with the jet system and, as long as the engine in inoperative, both the float chamber and the jets are submitted to the normal atmospheric pressure.
Under these conditions, it is obvious that nothing happens and the fuel in the float chamber is at the same level as in the jets according to the principle of the communicating vessels (vases communicants/communicerence vaten).

Let us now consider three operating conditions, namely the cruise regime, idling and the acceleration:

1°) Cruise regime (régime de croisière/kruisregime): the throttle is open. Because of the down going motion of the cylinder in combination with the influence of the venturi, the pressure at the mouth of the main jet (gicleur principal/hoofdsproeier) is considerably lower than within the float chamber which remains under atmospheric conditions. As any fluid, fuel in this case, tends to move from an area of higher pressure to an area of lower pressure, the fuel is literally aspirated out of the jet and vaporized through the calibrated nozzle. The vaporized fuel mixes with the air flowing in the mixing chamber, thus
producing the necessary blending which is then sent through the manifold to the cylinder's inlet valve.

2°) Idling (ralenti/traagloop): when the engine runs at idle power, which is the situation represented on fig.2, the main jet is blocked by the throttle. None the less, even with the throttle in fully closed position, a small passage towards the mixing chamber still remains open and, in order to keep the engine running smoothly, a second jet, known as the idling jet (gicleur de ralenti/traagloopsproeier), is located at this spot. Note however that under these circumstances, the static pressure on the duct's wall is extremely low and particularly conducive to icing.

3°) Acceleration: the term "acceleration" is usually known as "reprise" in both french and flemish languages. If for some reason the pilot briskly advances the throttle lever (manette à gaz/gashendel) from idle to full power, the pressure drop at the main jet is so sudden that the aspiration through the normal channel lags and may cause the engine to falter. To prevent this, many carburetors are fitted with an acceleration pump. Broadly speaking this system works as follows: the throttle lever is connected to a piston located in a cylinder inside the float chamber. This cylinder is filled with fuel through a passage on its underside. Under normal operating conditions, the movements of the throttle lever cause the piston to move up or down without any side-effects. However, when the throttle lever is briskly opened, the fuel in the cylinder is unable to "make room" in time for the piston: the fuel is compressed and causes a calibrated valve to open a shortcut towards the main jet, thus allowing the fuel to reach the jet nozzle without undue delay. Note that assuming that such an acceleration pump is installed, the throttle lever can be used to prime the engine for starting: more about this below.

The vaporization of the fuel leaving the jet nozzle combined with the low pressure in the venturi gives way to a sharp temperature drop, some 20° to 30°C or even more. This implies that water vapor, which is always more or less present in the air, tends to turn to ice. In other words, carburetor icing is most likely to occur under high humidity conditions. For instance, this phenomenon will develop more easily during hot humid summer days, rather than in wintertime when the air is cold and dry. Keep in mind that it is the atmosphere's humidity which is the major causal factor for carburetor icing! and not the outside air temperature.

Carburetor icing causes the area around the venturi and the throttle to become gradually blocked, thus disrupting the fuel/air feed to the cylinders and causing a gradual power loss. On engines fitted with a fixed pitch propeller (which is the case of most elementary trainers), this power loss is easily noticed on the RPM indicator showing a decrease despite the unchanged position of the throttle lever. Moreover, the power loss is definitely audible. On the other hand, assuming a constant speed propeller, the associated governor tends initially to maintain the RPM, thus eliminating the audibility of the problem which can only be identified by a gradual (and inaudible) drop of manifold air pressure (MAP) rather than by an RPM decrease. It is obvious that carburetor icing must be taken care of as soon as possible by selecting the heating system to "HOT": any delay in doing so can only aggravate the situation and ultimately lead to total engine stoppage. Be warned:

ANY TIME YOU NOTICE AN UNEXPLAINED RPM OR MAP DROP, DO NOT INITIALLY RE-ADJUST THE THROTTLE LEVER, AS THIS WOULD ONLY EXPEDITE THE PROBABLE ICING PHENOMENON !!
APPLY CARBURETOR HEAT INSTEAD !!!

Be aware that carburetor icing may occur at any power setting, but particularly at idle power. This has to do with the very narrow passage in the venturi when the throttle is fully closed, and the ensuing drop of static pressure giving way to an extreme drop of temperature, not to speak about the fact that the ice formation can block this passage all together (as well as the throttle itself). Consequently, before any GLIDE DESCENT, the carburetor heating system MUST be selected to "HOT"! And there is more: during a prolonged glide descent, such as during a practice forced landing exercice, the cooling of the power plant is considerable. Besides the fact that this is extremely harmful to the engine, this additional cooling can be so intense that the heat of the exhaust becomes inadequate to ensure the proper operation of the carburetor heating system: it is thus strongly recommended to increase power somewhat for a few seconds, at least every one thousand feet, in order to maintain the engine's temperature within acceptable limits. Furthermore, keep in mind the following remarks and recommendations:

- Unless a carburetor heat indicator is available, never use partial carburetor heat: it must be either full "COLD" or full "HOT" ! It seems indeed that under some circumstances, the use of partial carburetor heat is likely to cause icing. This is amongst others the case when small ice particles are floating in the air: such particles are normally absorbed without any harm by the fuel/air mixture but, assuming partial carburetor heat, they are likely to melt and, being in contact with an extremely cold venturi, to transform instantaneously into sticking ice. However, if a carburetor heat indicator is available, partial heating may be used to maintain the temperature in the carburetor within indicated limits (usually outside a yellow caution range). The advantage of such an indicator is that excessive carburetor temperatures can be avoided.

- On account of the temperature inside the carburetor, some pilots argue that the prolonged use of carburetor heat, particularly the full "HOT" position, is harmful because it could give way to so-called detonation. Detonation is the phenomenon whereby the combustion of the fuel/air mixture in the cylinder, combustion which should be rapid but gradual, occurs in a rather explosive fashion. Detonation causes a typical noise, known as engine knock (le moteur "cogne"/het "kloppen" van de motor) and
occurs mainly at high power settings. Incidentally, this engine knock is not audible on board of an aircraft (as it is in a car, particularly when accelerating) and the most suitable instrument to provide a clue about possible detonation is the cylinder head temperature (CHT) indicator showing a rapid increase towards the maximum allowable value, and most probably exceed it if the power setting is not reduced in due time. Unfortunately no such indicator is available on most elementary trainers, in which case only an abnormal high oil temperature may result. Detonation is extremely harmful to the engine and, although it will probably not destroy it at once, it will soon lead to the failure of the power plant if not taken care of. It can be caused by a number of factors but, if it occurs, reducing power should be the initial corrective action in most cases.

- Coming back to the carburetor, it is often recommended to select the heating system several times for a short period instead of using the "HOT" position for a prolonged period of time to avoid detonation. It cannot be denied that excessive temperatures are never recommendable: hence the carburetor heat indicator on some aircraft. None the less, very serious research in this field has established that the chances for detonation due to prolonged use of the carburetor heat are nil, provided that the engine power is less than 75%. On the other hand, when using the carburetor heat, it is strongly recommended to lean the mixture (see below), but this has to do with the fact that hot air is less dense than cold air and that, as a consequence, carburetor heating gives way to a too rich condition, i.e. to a needless waste of fuel. The initial further power loss which occurs when the carburetor heat is selected to "HOT" is a direct result of this enrichment.

- The carburetor heating system is of vital importance. If it fails before departure, no takeoff may be initiated under any weather conditions ..... and if it fails during flight, which happily is most unlikely, better be ready to face an engine failure in the next few minutes. The system must be carefully checked during the engine run-up by selecting the carburetor heat to "HOT" at the specified power setting: this should cause a noticeable drop of about 75 to 100 RPM and possibly a variation in MAP when a constant speed propeller is involved. Assuming that the RPM drop is too little, have this checked out by maintenance before departure because this might indicate a partially open heating valve, which is not acceptable either as this would affect the takeoff performances.

- As said earlier, carburetor icing can easily occur at idle power. It is thus not unthinkable that it should happen during taxi and, let us stress it once more, particularly under high humidity conditions. The ice will obviously disappear during the engine run-up when checking the carburetor heat, but it is not unusual that after completion of the run-up, some time passes by before the takeoff clearance is obtained and during which icing can re-occur: with this in mind, it is not superfluous to re-activate the carburetor heating for a few moments just before line-up ..... but don' t forget to select in to "COLD" again before initiating the takeoff roll.

- Some pilots predicate the need to select the carburetor heating system to "COLD" during final approach "shortly before touchdown" in view of a touch-and-go, a go-around or of a possible balked landing. This method is usually not mentioned in the POH (pilot Operating Handbook): with later IFR operations in mind during which IFR approaches are carried out under actual IMC it seems preferable to leave the carburetor heating system to "HOT" until touchdown, and to select it back to "COLD" immediately after re-applying takeoff power.

III. THE PRIMING SYSTEM:

The priming system (système d'injection/voorinspuiting) is used to start the engine by injecting fuel in one or more cylinders (or in the inlet manifold). This can be done either through the so called primer, or downstreams from the carburetor by using the auxiliary fuel pump. On elementary trainers, the primer is usually a simple hand pump connected to the ducting between the fuel tank and the carburetor, beyond the so-called fuel strainer which is nothing else than a filter.

When sufficient fuel has been injected, the magneto's are switched on, and activating the starter causes air to be mixed with the fuel by aspiration: the spark-plug ignites the fuel/air mixture and the engine starts running. The problem with the primer is to know how much fuel must be injected: this information is to be found in the POH. When a common plunger hand pump is involved, a specified number of strokes is recommended: for instance, three strokes means to unlock the pump, to pull it completely out to aspirate the fuel, then to push it completely in to force the fuel towards the engine. This is the first stroke, which must then be repeated twice. After the third stroke, the pump must be kept pushed in and be rotated for about 1/4 turn so that the associated safety pin prevents the pump to move out, which could give way to an over rich mixture and a rough running engine.

The stipulated number of strokes is valid for engines which have been shut down for a significant period of time, say more than one hour, and for average outside air temperature conditions. In cold weather it is usually necessary to use a somewhat richer mixture, and thus to add one or two additional strokes. None the less, one must be very careful because too much fuel will prevent the engine to start, and especially because it may give way to leaks and a serious risk for engine fire. Therefore, and unless a specific procedure is laid down in the POH, the following procedure is recommended:

- Let us assume that you have added two strokes because of cold weather conditions, but that the engines does not start. It is possible that a third, perhaps even a fourth additional stroke is required ......  but it is equally possible that the engine is already "overprimed". In fact, you are not sure: is there too much fuel, or not enough ? It is evident that if there is too much fuel, and particularly if the smell of fuel is noticed, adding even more fuel is certainly not the right thing to do: as a matter of fact, this would be the best way to cause a major engine fire. Thus, play it safe and discharge the engine of possible excess fuel. The best way to do this is as follows and again, unless the POH specifies otherwise:

1°) Set the mixture control in idle cut-off (ICO) in order to prevent fuel feed to the carburetor.

2°) Open the throttle lever fully to allow as much as possible airflow in the carburetor.

3°) Verify that the magneto switches are still on, start the engine again and keep the starter engaged, thus allowing the propeller to turn around a few times: the combination of the mixture in ICO and full throttle will rapidly lead to the leaning of the mixture and subsequent engine start.

4°) As soon as the engine fires, set the mixture to full rich again and reduce the throttle lever to idle without delay to prevent the RPM of surging up to maximum.

- It is also possible that the previous procedure proves unsuccessful and, as soon as it becomes evident that the engine will not fire, there is no reason to keep the starter engaged any longer, firstly to avoid draining the battery, secondly to avoid overheating of the starter system. The most probable cause for this unsuccessful start attempt is that there was not enough fuel available: in this case you may repeat the priming procedure, and add one or two more strokes without undue risk.

- Although the possibility is rather remote, an engine fire during start is never to be ruled out. Consequently, it is strongly recommended to be fully cognizant with the associated emergency procedures published in the POH. Nevertheless, keep the following in mind: most POH's suggest that, in such a case, the starter should be kept engaged until the engine starts, to suck the fire from the carburetor into the cylinders, and thus keep the situation under control. In fact, assuming that the engine starts, it is quite possible that the pilot will not even be aware of the occurrence of a carburetor fire for the simple reason that he cannot see it. And in this case the flight can be carried out normally, at least if no other anomalies show up, particularly during the engine run-up. On the other hand, if flames become visible from the cockpit, chances are that things are seriously going out of hand and that evacuation of the aircraft should be considered, preferably after shutting down the fuel feed by selecting the mixture to ICO and closing the fuel shutoff valve, rather than fighting a lost battle. And this is probably the opportunity to remember the existence of the on-board portable fire extinguisher.

In order to start an engine that has been shut down recently, or under very warm outside air temperatures, priming is mostly not necessary. And if the engine fails to start, use only a fraction
of the normal priming.

When we discussed the carburetor, we noticed that it is often fitted with an acceleration pump which can be used in lieu of the normal primer. However, this method is not recommended because it does not allow to meter the fuel and that the carburetor can very easily be flooded. Furthermore, if an acceleration pump is available ( in order to avoid fire. the throttle lever should never be pumped during the engine start ! In fact, Even if no acceleration pump is fitted, pumping the throttle lever gives way to the opposite phenomenon, namely excessive leaning, and is therefore neither recommended.

IV. USE OF THE MIXTURE CONTROL:

In low countries such as Belgium, some pilots are only aware of the mixture control because they use this red coloured control lever to shut the engine down. Doing so, they make use of the so-called idle cut-off, or ICO, function. It must be realized that the mixture control implies much more than simply bringing the engine to a stop.

We know that the engine needs fuel which, for piston engines, is gasoline (benzine/essence). However, gasoline as such is absolutely non-inflammable unless it is mixed with air in adequate proportions, which is what happens inside the carburetor.

If we consider an aircraft at sea level under standard conditions, i.e. a temperature of 15°C and an atmospheric pressure of 1013,2 mb, the air has a specific density (densité/dichtheid) at which the mixture must possess a specific amount of air units for a specific amount of fuel units in order to be inflammable. If this same aircraft is operated at 5000 ft, the air density has already significantly decreased, i. e. the air has become thinner. If the fuel feed remains unchanged, the fuel units remain the same than at sea level, but with much less air units: the mixture becomes too rich in fuel, and consequently a high, thus an expensive fuel consumption results. At Even higher altitudes, the ideal fuel/air ratio is disturbed to such an extend that the inflammability of the mixture is seriously impaired, the engine' s power output decreases and there will be a moment at which the engine begins to "turn squarely" due to the excess fuel. The use of the mixture control allows to reduce this fuel feed, i.e. to
lean the mixture, and thus to prevent these detrimental effects.

How do we accomplish this ? Let us assume that the aircraft is cruising at 5000 ft at a given power setting with the mixture control still in "RICH". Although the engine is probably still running smoothly the fuel consumption is higher than normal. By pulling now the red mixture control gradually towards the "LEAN" position (and we stress the word "gradually" because if it is pulled too briskly it could be pulled too far and cause the engine's stoppage  this time due to lack of fuel; it would restart immediately when pushing the mixture control back in, but this does not exactly instils a lot of confidence in the mind of the passengers). By doing so the fuel supply is reduced the fuel/air ratio improves and the RPM increases. This process is continued until the highest possible RPM value is obtained and the mixture control is then left in this position  which corresponds to the so-called best power setting. The throttle lever may the be brought back to recover the initial RPM. In factl the RPM increase is not always clearly noticeable at lower altitudes but a lower fuel consumption will be obtained none the less.

Flying at best power setting is obviously better than keeping the mixture at full "RICH": the fuel consumption is reduced and, because of the better engine efficiency the airspeed is likely to be higher than it was before the leaning process. However the best power setting is not yet the most economical one as far as fuel consumption is concerned. The mixture can be leaned somewhat further. Now the shortage of fuel begins to make itself felt (remember that if you pull the mixture control to full "LEAN" or ICO position , the engine would simply stop running). This process should be continued until a specified RPM drop has occurred: this is the so-called best economy setting whereby the lower RPM is accepted because the fuel consumption is significantly less than at best power for only a slight airspeed decay. This setting allows the longest range (distance/afstand) at any power setting with the fuel available in the tanks. As a consequence considering these two different mixture settings  in order to operate the aircraft in the most efficient way climb should be carried out at best power to ensure the best rate of climb while best economy should be used for cruise.

With low powered engines fitted with a fixed pitch propeller such as those used on most elementary trainers the mixture control can be adequately set either at best power or at best economy by means of the RPM indicator or even simply by means of hearing. The only restriction is to keep from leaning as long as the power output exceeds 75%: this is due to cylinder cooling considerations which are discussed below. As an example the following recommendations are found in the POH of the C152 anno 1979:

"Lean the mixture for maximum RPM during climbs above 3000 ft."
"The mixture may be left leaned for practicing such maneuvers
as stalls"

"Lean the mixture for maximum RPM during all operations at any
altitude including those below 3000 ft, when using 75% power or less".

Moreover: "When cruising at 75% or less power the mixture may be further leaned until the RPM peaks and drops 25-50 RPM." This is especially applicable to cross-country training flights, but may also be practiced during transition flights to and from the "training area".

Finally: "Using the above recommended procedures can provide fuel savings of up to 13% when compared to typical operations at a full rich mixture".

Let us now consider more closely what happens inside the engine during the leaning process:

- The fuel in the fuel/air mixture happens to produce a significant cooling effect. Consequently, a lean mixture will give way to a higher temperature within the cylinder than a rich one. Because of this higher temperature the heat of the exhaust valve and of the so-called cylinder head tends to increase, which in turn produces hotter exhaust gasses.

- According to engine manufacturers, flying at best economy is not only the most suitable method to reduce fuel expenses, it is also strongly recommended because it lowers the maintenance costs. They claim that, at this mixture setting, the fuel in the cylinders is completely consumed because of the higher temperatures, that consequently no carbon deposits can occur and that, in the long term, less maintenance costs are involved. The only possible restriction for the use of best economy is that, on some power plants, it is not allowed above 65% of power. Such a restriction must obviously be mentioned in the POH.

- Care must be taken not to overdo the leaning as this can have very serious consequences in the long term, mainly because of the oxidation which affects the cylinder's inlet and exhaust valves. Leaning too much may also give way to detonation. Some engines, mostly the more powerful ones, from 180 BHP (CV/PK) upwards, are more sensitive to leaning and are often fitted with a cylinder head temperature, or CHT, indicator as well as with an exhaust gas temperature, or EGT, indicator. When the mixture is leaned, due to the decreasing cooling effect of the fuel, both the CHT and the EGT tend to increase. However, at one point, the fuel units are so reduced that the air units begin to prevail and, as also air is a coolant, the CHT and EGT begin to decrease again. In other words, there is a position of the mixture control whereby the EGT reaches the hottest value: this is the so-called peak EGT and corresponds to the best economy setting.

- Normally speaking, when peak EGT is reached, the value of the CHT is supposed to be still significantly lower than the maximum allowable value which must be published in the POH. The maximum CHT value may never be exceeded, neither is it recommended to operate for a prolonged period of time at near maximum value. None the less, it may happen that the CHT comes very close, or reaches the maximum value when leaning towards peak EGT, or that the engine begins to turn unevenly: in both these cases, the leaning process must obviously be stopped at less than peak EGT.

- Assuming the presence of an EGT indicator, in order to obtain best power, the procedure consists of leaning until peak EGT, then to enrich the mixture so that the EGT value decreases again by a specified value (see POH) , usually by 50° to 100° Fahrenheit (10° to 38° Celcius, but most EGT indicators are calibrated in degrees Fahrenheit). This is referred to as rich of peak.

Many "before takeoff" checklists call for mixture "RICH". This is in fact incorrect: it should be mixture "AS REQUIRED". Indeed, a full rich mixture may be acceptable up to 3000 or 4000 ft, but on airfields located at higher elevations it becomes necessary to lean in order to obtain the maximum takeoff power. In addition, be aware that you must take the density altitude into consideration, not simply the elevation above sea level ! If an adjustment of the mixture control is required for takeoff, it is
necessary to perform a full power check during the engine run-up and to adjust the mixture until full power is obtained.

When flying at high altitudes with the mixture leaned, remember to enrich the mixture gradually during descent. Failing to do this might cause the engine to falter, particularly if high power is required at low altitude (for instance during a go-around).

Finally, a few words regarding the ICO function. Selecting the mixture control to full "LEAW' cuts off the fuel feed to the carburetor, and causes the engine to stop immediately ..... which is the reason why this lever is painted red. The idle cutoff system is much more efficient to stop the engine than switching off the magnetos. Indeed, by this latter method fuel still flows to the cylinders, albeit at a much lower rate due to the near idle power throttle setting, until the propeller stops completely. This may give way to self-ignition, or pre-ignition, due to hot carbon deposits in the cylinder, and cause the engine to continue turning unevenly for a while, a phenomenon known as "dieseling", and possibly even to produce harmful backfiring (terugslag/retour de flamme).

V. THE MAGNETO:

The magneto ensures the ignition of the fuel/air mixture in the cylinder. The operation of the magneto can broadly be described as follows: 

1°) Let us consider a rotating magnet, known as the rotor, located between the ends of an armature (stator/anker) made of soft iron. The magnet is connected to the engine's crankshaft. It remains thus stationary as long as the engine is inoperative, in which case nothing particular occurs (fig.4a). Note however that, as the armature is made of soft iron, it is strongly influenced by the magnetic field of the magnet.

2°) As soon as the crankshaft starts turning, the magnet rotates as well and the armature is submitted to an alternating polarization, i.e. its positive pole becomes negative and the negative pole becomes positive (fig.4b).

3°) The armature is surrounded by a number of so-called primary windings of thick copper wire, of which one end is grounded (mis à la masse/aan de massa gelegd-aarding), i.e. connected to the aircraft's structure. The other end is connected to the contact-breaker (rupteur/onderbreker) which is discussed here-after. The alternating polarization; often referred to as flux reversal, induces a low voltage (±50 V at normal engine RPM) alternating current into the primary winding (fig.4c).

4°) This low voltage alternating current is too weak to produce a spark which is strong enough to ignite the mixture into the cylinder. Therefore, further making use of the electrical induction property, a great number of so-called secondary windings, this time made of very thin copper wire, is wound around the primary windings (fig.4d). One end of the secondary winding is also grounded while the other end is connected to the distributor (distributeur/verdeler).

5°) As said previously, the primary winding is connected to the contact-breaker. Broadly speaking, this is an eccentric wheel which is driven by the crankshaft as well, and designed in such a way that it interrupts the primary alternating current each time that it reaches a maximum value: this results in a considerable variation of voltage in the primary winding which, still by induction, causes an enormous surge of voltage in the secondary winding, going up to 25.000 V or even more. The
purpose of the distributor consists of directing this high voltage current in due time to the spark-plugs (bougies) of the various cylinders where a real strong spark now occurs between the so-called electrodes, thus igniting the mixture.

6°) Induction phenomenons may produce detrimental side effects. One could believe that, when the contact breaker interrupts the current in the primary winding, no more current flows in it. Nothing is less true however: the sudden drop in voltage itself has the property to induce a current in the primary winding. And this current, known as follow-up current, can be of such intensity that when the so-called contact points (vis platinées/onderbrekerspunten) of the contact-breaker separate, a strong spark occurs between them. Such a spark is likely to burn out the contact points very rapidly, besides that its occurrence may impair the current rupture as well. As this spark cannot be completely avoided, the contact points are made of heat resistant platinum to protect them as much as possible. In addition, the spark is very significantly weakened by the presence of a condenser (condensateur/condensator) in the primary system, whose purpose it is to absorb the greatest part of the follow-up current's energy and to deliver it again at a more suitable moment (fig.4e).

7°) Figure 4e. represents a nearly complete diagram of the magneto. Still, a very important sub-part is missing: the magneto switch used by the pilot. This switch is located on the primary system and is also grounded. Its purpose is to by-pass the contact-breaker, effectively putting it out of service. Electrical current is of somewhat lazy nature and tends to follow the shortest way. Figure 4f. shows that when the switch closes contact C, the shortest way goes directly to the ground. However, when contact C is open, the current is compelled to travel toward the contact-breaker and ignition can occur. In other words, if the magneto switch is in "OFF" position, contact C is closed, the current travels directly to the ground, the contact-breaker is out of the game, the ignition is prevented and the engine stops running. When the magneto switch is in "ON" position, contact C is open, thus enabling the magneto to operate normally. And here lies one of the major dangers related to the magneto !!! Assuming that the propeller is rotated by hand for some reason, the magnet rotates as well and, if the magneto switch happens to be in "ON" position, ignition is likely to occur. Indeed, rotating the propeller by hand causes the pistons to move up and down in the various cylinders, allows them to get filled with the inflammable fuel/air mixture (particularly if the mixture control is not in ICO) , and if the spark occurs ... !!! It is thus an elementary precaution to verify that the magneto switch is in "OFF" position before handling a propeller by hand. BUT EVEN IF IT IS, THIS IS NO GUARANTEE THAT THE ENGINE WILL NOT FIRE!!! Indeed, the magneto switch itself might fail or, and this is far from being unusual, the primary system may break at the level of the switch so that, even with the magneto switch properly in "OFF" position, the primary winding leads to the the contact breaker anyway, with all the possible consequences. Thus:

NEVER ROTATE A PROPELLER BY HAND ASSUMING THAT
EVERYTHING IS SAFE ! ALWAYS KEEP lN MIND THAT,
EVEN WITH THE MAGNETO SWITCH lN "OFF" POSITION,
IGNITION MAY OCCUR. !!!

Although figure 4f. gives a rough idea of how the magneto works, it is obvious that the combination of the windings, the contact breaker and the distributor requires very precise tuning and a lot of technical adjustments. Furthermore, the engine's RPM is an important aspect, as the ignition, thus the occurrence of the spark in the cylinder must be obtained at different moments depending on whether the engine runs at full power or at idle: this gives way to the magneto's so-called timing problems. Without going into complicated details, it is perhaps interesting to note the following:

- As the operation of the magneto depends on the rotation of a magnet, one may wonder about the efficiency of the system at very low RPM, particularly during the engine start, and more so during "handstarting". In theory, as long as the engine rotates at less than ±200 RPM, a magneto such as shown in the diagram cannot produce the required voltage necessary to ignite the mixture. Of course, electrical starters are sufficiently powerful to rotate the crankshaft fast enough. None the less, a variety of arrangements can be provided to improve the starting efficiency, amongst those the so-called impulse starter. Broadly speaking, this is a spring (ressort/veer) which is connected to the magnet, and which winds up when the crankshaft begins to rotate. The spring snaps loose at the adequate moment, producing a typical metallic "clang" noise, and causes the magnet to rotate at high speed for a few moments, thus enabling it to produce the required voltage. The system is designed in such a way that, once the engine is running normally, the impulse system disconnects automatically.

- We mentioned earlier the importance of the timing of the magneto, i.e. the exact moment at which the spark is supposed to occur. In the part "Generalities", we suggested that the spark occured at the beginning of the power stroke, i. e. when the crankshaft is at 000°. This is purely theoretical: in reality, the combustion of the mixture is gradual and requires a certain time lapse. Consequently, as the engine can be operated between ±700 RPM up to something like 2800 RPM, it is necessary to obtain the spark at various moments to ensure as complete a combustion as possible. Therefore, the mixture is normally not ignited at the beginning of the power stroke, but during the last phase of the compression stroke. This is known as spark advance (avance à l'allumage/voorontsteking). The value of the spark advance, which is expressed in degrees of the crankshaft before it reaches the Upper Dead Center, is very small when the engine runs slowly, but increases as the RPM increase up to a maximum of about 20° to 30° before the UDC. The value of the spark advance is automatically obtained by means of a linkage between the throttle lever in the cockpit and the contact-breaker. Poor magneto timing, which is likely to be noticed during the magneto test, may lead at short notice to the total destruction of the power plant if not taken care of. Note that during starting, unlike during the normal engine operation, there is no spark advance at all and the spark indeed occurs at the beginning of the power stroke. However, a mismatch can always take place and, assuming that spark advance should occur during the engine start, backfiring may occur, causing the propeller to kick back viciously. Such backfiring is extremely harmful to the engine and, needless to say, utterly dangerous if the engine is started by rotating the propeller manually.

Testing the magneto

It would be better to title "Testing the magnetos", in plural: indeed, two of these are required because of safety considerations. This implies that each cylinder is fitted, not with one, but with two spark-plugs, each one activated by its own magneto. Both magnetos must operate properly, notwithstanding that theengine runs perfectly well on a single one, albeit at the cost of a slight decay of the power output due to the fact that when only one magneto is operative, the combustion of the mixture is less even. This lessened power output can be noticed during the magneto test: when one of the magnetos is switched off, it causes a drop of 100 to 200 RPM.

The magneto switch includes at least four positions labelled "OFF", "LEFT", "RIGHT", and "BOTH", and mostly a fifth "START" position unless an independent starting knob is available:

- The "BOTH" selection allows both magnetos to operate at the same time: this is the normal position when the engine is running.

- The "LEFT" and "RIGHT" positions are normally used for test purposes, although any of these positions may be used in flight if one of the magnetos should fail or cause any other problem.

- The "OFF" position is self-explanatory: the engine stops running because the primary windings of both magnetos are grounded. None the less, as said before, the engine is usually shut down by means of the mixture control instead of the magneto switch, and it is recommended to select the magnetos to "OFF" only after the propeller is fully stopped.

The POH of each aircraft type stipulates the RPM value at which the magnetos are to be tested, the maximum allowable drop when one magneto is selected to "OFF", and usually the maximum difference in drop between both magnetos. The magneto test itself should be carried out as follows: beginning from the "BOTH" position at the recommended RPM setting, select "LEFT" and verify the RPM drop with only the left magneto in operation; go back to "BOTH" and allow the RPM to stabilize at the original value, then select "RIGHT" and verify the RPM drop with only the right magneto in operation; go back to "BOTH" and ensure that the original RPM value is again obtained.

It is important to re-select "BOTH" after having tested the first magneto: indeed, when only one magneto is operative, there is a possibility that sorne of the fuel/air mixture is not completely consumed and that hot carbon deposits occur on the inoperative spark-plug. When the inoperative spark-plug is back in operation, such deposits are likely to seriously impede the spark, which may result in a sputtering engine with considerable RPM drop when selecting the switch directly from one magneto to the other.

Hot carbon deposits are always to be expected anyway at low power, say below 1000 RPM, such as when idle power is used during a prolonged taxi, and the subsequent magneto test may very well give way to such sputtering. This situation can easily be normalized by selecting the "BOTH" position, increasing the power for a few moments, possibly to full power, then to recommence the test. If it is still not satisfactory, with the magneto switch back in "BOTH" position, lean the mixture until a significant RPM drop occurs, let the engine run for a few moments at this reduced RPM, then go back to "RICH", and try again: the leaning process causes the temperature in the cylinders to increase which will most likely burn out any remaining deposits. Obviously, if this method still fails, nothing else is left but returning to the apron for further inspection by the maintenance.

The effect of hot carbon deposits was already mentioned previously in relation to the engine shutdown. It should be noted that the resulting self-ignition, or pre-ignition (auto-allumage/ voorontsteking) is caused by the fuel/air mixture igniting before the spark-plug's discharge. As said earlier, this phenomenon can be extremely harmful. It should be noted that carbon deposits may occur anywhere and anytime in the cylinder, and for a nurnber of reasons. In other words, pre-ignition is not only the result of a low power setting, it could also happen during flight at higher power settings, in which case it can be extremely destructive.

Incidentally, because of the possibility of carbon deposits, it is not recommended to leave the engine running at idle power for a prolonged period of time. This is sometimes forgotten, particularly during practice forced landings from 2000, 3000 feet or more, whereby the engine is often kept at idle for the whole duration of the exercice. And besides possible deposits, the engine suffers a harmful cooling process as well: if a touch-and-go or a go-around is to be carried out, it is not unlikely that the engine will falter at the moment that full power is selected. Thus ..... caution !!!

It may happen that the magneto test shows no drop at all when one magneto is switched off. This is indicative of a broken primary winding at the level of the switch, or a failure of the switch itself. In other words, the magneto cannot be switched off. This can be confirmed by the so-called dead cut check, i.e. rapidly switching to "OFF" then back to "BOTH" to verify whether the engine tends to stop or not. Assuming that is doesn't, are you allowed to takeoff in such a case? Fact is that, as long as the flight is normal, there is strictly nothing wrong with the fact that you cannot switch off one of the magnetos. Fact is also that most POH's consider a maximum allowable drop, but dont' say a word about the "zero drop" case. On the other hand, assuming that an actual forced landing should become necessary, those same POH's mention the "ignition switches off" requirement. The answer to this question is thus: NO ! ..... and certainly not when departing from the home base.

Another possibility is that the drop is so small as to be hardly noticeable. This may be due to a wrongly set spark advance value causing the mixture to ignite too soon, and maybe harmful to the engine as well. In addition, it may affect the power output. Therefore, it is recommended to open the throttle lever fully to verify whether normal takeoff power can be obtained or not. If it can, so much the better, but the drop value should be reported to maintenance after the flight none the less. If it can't, this may confirm a problem with the spark advance and there is no other option than to return to the apron for investigation.

An excessive drop, and particularly when accompanied by vibrations, may be indicative of carbon deposits on a spark-plug, problem which we discussed already earlier. It may however be the result of a failure in the ignition system as well (faulty sparkplug, faulty electrical harness, etc ... ). At any rate, most pilots will be sufficiently wise to realize that there is a serious problem and cancel the flight.

Most POH's mention a so-called maximum differential between the drop of the two magnetos. This again is a means to verify whether the setting of the spark advance is properly adjusted and also in this case, assuming that the maximum differential turns out to be unsatisfactory, a full power check should be carried out.

Finally, be aware that the magneto test must not necessarily be carried out on the ground, it may as well be in flight. Assuming that engine vibrations should occur while airborne, the magneto test may determine whether these originate from the ignition system or not. If they do, the faulty system may be switched off and cause the vibrations to disappear. None the less, flying with one single magneto is not very safe, and a landing should be carried out at the first suitable airfield for repair.

VI. COWL FLAPS:

Cowl flaps are movable panels mounted mostly underneath the engine nacelle. The pilot can open or close them, either partially or fully, by means of an associated control lever.

The purpose of the cowl flaps is to regulate the cooling air in the engine, and thus to control the cylinder head temperature which is shown on the associated CHT indicator, as well as the oil temperature.

According to most checklists, the cowl flaps must be open for taxi, takeoff and climb, whereas they should be closed for cruise and descent. This makes sense at first glance, considering the fact that the engine is supposed to run hot during taxi, as well as in flight at high power and reduced airspeed. But this is not necessarily the case, particularly not when very cold OAT's prevail. Keep in mind that too low CHT can be equally harmful to the engine as a too high value. In other words, whenever you select the cowl flaps one way or another, always do it with an eye on the CHT indicator: if it is too low, keep them closed, if it is too high, open them as required to bring the temperature back to an acceptable value. Don't open or close them simply because the checklist says so !

Regarding the case of high CHT, note that besides opening the cowl flaps, other ways to reduce it are to enrich the mixture if it was previously leaned, to reduce power somewhat, or to descend at increased airspeed.

Note also that opened cowl flaps produce a significant drag and, in cruise, give way to a somewhat reduced airspeed. In other words, if you notice that your cruising speed is lower than the expected value, have a look to the cowl flaps lever: maybe you simply forgot to close them.

VII. FUEL INJECTION SYSTEM:

We have seen that, due to its design, the float type carburetor is easily affected by icing. This problem is eliminated by the use of a fuel injection system which replaces the carburetor on a number of more modern aircraft. Additional advantages of the fuel injection system are the fact that the fuel feed to the engine is never affected by the aircraft's attitude, even not by sustained inverted flight (which is impossible with the usual float type carburetor), and that fuel metering is more precise, resulting in a better fuel/air ratio and an improved fuel economy. Finally, as no inflammable mixture is formed outside the cylinders, the fire risk is considerably reduced.

With this system, metered fuel is continuously injected directly into each individual cylinder intake ports located in the combustion chamber. Although the design may slightly vary from one engine type to another, and except for the absence of a carburetor heat control, operating a fuel injected engine does not require very different procedures as compared to its carbureted counterpart. However, besides a fuel pressure gauge you may often find a fuel flowmeter whose precise use must be checked in the POH. Furthermore, note that on most fuel injected engines, because of the fuel system design, no priming is possible by pumping the throttle (a procedure which is not to be recommended anyway), and no primer handpump is available, priming being carried out by use of an auxiliary fuel pump. At any rate, study carefully the applicable engine starting procedure in the POH.

Although the fuel injection system eliminates carburetor icing, impact ice, i.e. ice on the airframe, might still be a factor. This possibility is practically non-existant as long as the aircraft is not operated in IMC and, even so, this phenomenon is rather remote, particularly on turbocharged engines (see below). Impact ice may occur when ice crystals or supercooled water droplets hit the filter located at the inlet of the air induction and can be detected in the same fashion as carburetor icing: by a gradual decrease in RPM or MAP. This is why, instead of a carburetor heat control, you will often find an "ALTERNATE AIR" lever. This lever controls a two-way baffle in the induction system allowing the entry of unfiltered air, heated in the engine compartment.

Many alternate air systems include a so-called suck-in door which opens automatically whenever the primary air source becomes blocked for any reason. On some aircraft, for instance on the Cessna 172R, anno 1997, no alternate air lever at all is available and the pilot has to rely entirely on the automatic system.

One last piece of advice: assuming that your training aircraft is fitted with a fuel injected engine, you are unconcerned by carburetor icing. BUT DON'T REMAIN UNCONCERNED IF YOU REVERT TO AN AIRCRAFT FITTED WITH A NORMAL CARBURETOR !!!

VIII. RAM AIR CONTROL:

Ram air is the dynamic pressure which flowing air exerts on an object. For instance, the air entering the Pitot tube during flight is ram air.

Some aircraft, notably the Mooney, are equipped with a control lever whose purpose is to allow ram air to enter directly in the intake manifold, by-passing the main air filter. The purpose of this installation is to increase the MAP.

The problem with this so-called ram air control is that its efficiency is rather questionable. Indeed, for it to be most effective, it is necessary that the throttle lever is fully opened, which is normally the case only at higher altitudes, say from about 7000 feet, where the atmospheric pressure has dropped sufficiently. However, at such altitudes, and precisely because of the lesser air density, the increase in MAP is considerably less than it would be at lower altitudes ........ where the throttle lever is normally never fully opened.

Another matter to remember is that using the ram air control provides unfiltered air, a fact which is of no importance during flight, but which might cause problems if the ram air control is left opened on the ground, exactly in the same way as a carburetor heating system (or an alternate air door for that matter).

IX. THE TURBOCHARGER:

The power of a normal aspirated engine falls off as altitude increases. Assuming a climb at full power from sea level, for instance 28 In.Hg. and 2700 RPM, you would notice a gradual decrease in MAP requiring repetitive adjustments until the throttle reaches the full open position at about 7000 feet density altitude. The turbocharger, which fits a number of more advanced light aircraft, prevents such a rapid power decay, up to the so-called critical altitude, which is much higher, and at which moment the
MAP cannot be maintained any longer.

The turbocharger (fig.5) uses the energy of the engine exhaus't gases to rotate a turbine wheel and, through it, a compressor, both capable of spinning up to ±100.000 RPM. Outside air is ducted through a filter towards the compressor where it is pressurized, and further ducted towards the intake manifold.

A so-called wastegate is included in the exhaust gas duct. This wastegate can be either of the fixed or of the controllable type, the latter being either manually controlled or, as is the case on some more expensive designs; electronically controlled and fully automatic. Assuming a manually controllable type, the wastegate must be selected fully open at sea level, thus allowing the exhaust gases to by-pass the turbine: under these conditions the turbocharger is inoperative, the engine operates as a normal aspirated unit, and the throttle lever can be opened fully without risk of over-excessive MAP. During climb, the wastegate can be gradually closed, the exhaust gases are now allowed to rotate the turbine and partially compressed air is sent to the intake manifold, thus preventing the MAP to drop.

Once a specific (density) altitude is reached, ±12000 to 15000 feet, depending on the design, the wastegate must be fully closed and all exhaust gases are used to activate the turbine at maximum RPM. At this stage, if the altitude continues to increase, the MAP will now begin to fall off because of the decreased energy of the exhaust gases and consequent slower rotation of the turbine.

A number of turbocharger designs are fitted with a fixed waste-gate which is preset to obtain a specified MAP, at a specified density altitude, and at full throttle, for instance for the Piper PA-34-200T, SENECA II: 40 In.Hg. at 12.000 feet. Note that when a fixed wastegate is used, the turbocharger operates as soon as the engine is started. This means that, during takeoff at sea level, the MAP exceeds the ambient air pressure (±29 In.Hg.) by a considerable amount. This means also that, during takeoff, you must take care not to exceed the maximum allowable MAP published in the POH, despite the fact that a safety valve is preset to open if this value is inadvertantly exceeded.

One of the advantages of the turbocharger is that it allows to reach higher altitudes than with a normal aspirated engine of similar power. However, this has no real significance, unless the aircraft is equipped with oxygen or a cabin pressurization system. Of course, another advantage is that the engine's power output is much less affected by airport elevation, as is the case with normal aspiration. In fact, and particularly when a fixed wastegate is involved, most takeoffs, including those at high elevation airports, can be carried out with an MAP in excess of the value at sea level, which greatly improves the takeoff and climb performances (but this does not mean that the required runway length remains unaffected at higher altitudes).

Turbocharged engines must be handled with great care, particularly when operating at altitudes above 10.000 feet. Remember that compressed air can be extremely hot and, because of this, CHT and oil temperature must be closely watched to avoid exceeding the red line: this is where the proper use of cowl flaps and mixture control becomes of paramount importance. Note that, because of the high internal temperatures involved, despite the fact that the turbocharger allows to maintain the MAP at a specified value during climb (at least until the critical altitude), the engine's power output will decrease anyhow as climb progresses, but this power decrease is much less marked than with normal aspirated engines. One advantage of these high temperatures, is that the chances for impact ice (see "Fuel injection") are close to nil.

X. AIRCRAFT GASOLINE:

When discussing the carburetor, we mentioned the detonation phenomenon. Detonation may be due to a number of causes which all come down to the same basic reason: an excessive temperature within the cylinder. This may be due amongst others to inadequate engine cooling, to a poorly tuned magneto or to a too lean mixture. Another possibility for detonation is the use of the wrong gasoline.

The choice of the gasoline depends on the so-called compression ratio (taux de compression/compressieverhouding) of the engine, i.e. the ratio between the volume of the cylinder when the piston is at the IDC and when it is at the ODC. This ratio is expressed as being 6/1, 7/1, 8,5/1, etc ... The power output of the engine increases with the compression ratio, but this results in higher temperatures during the power stroke and consequently gives way to a higher risk for detonation.

The degree of resistance of gasoline against detonation is obtained by the blending of two chemical additives: octane whose property it is to be very knock-resistant, and the much less knock-resistant heptane. Depending on the ratio between these two chemicals, the gasoline is said to contain so much or so much octane. Broadly speaking, gasoline graded at 80 octane means that it contains 80% of octane and 20% of heptane. The figure 80 is thus indicative of the "anti-knock" quality of the fuel.

The octane grading is sometimes indicated by two numbers, for instance 100/130: the first number refers to the knock-resistance for a leaned mixture, the second one for a rich mixture. However, upon recommendation of the ASTM, or "American Society for Testing and Materials" , this dual rating is disappearing and only the first number remains.

Note also that the earlier 80 octane graded gasoline which was commonly used on most light aircraft is usually replaced nowadays by the so-called 100LL, i.e. a higher octane rating but a lower lead content, LL standing for "Low Lead" (faible teneur en plomb/laag loodgehalte). Gasoline with a higher than required octane grading may be used. This is why some aircraft approved for 80 octane fuel currently use 100LL because of non-availability of 80. At any rate, read in the POH which types of gasoline are adequate for your aircraft as well as their associated colors. For instance, 80 octane gasoline is red colored and 100LL is blue colored.

Besides using the proper type of gasoline, it is equally important to ensure that the quality of the fuel is satisfactory: despite the presence of the strainer and a number of other filters located inside the engine, it may not contain visible dirt particles. Another important matter concerns the always possible presence of water in the fuel which may lead to the sudden stoppage of the engine, and has been the causal factor of a number of fatal accidents. It is extremely important to carefully drain the fuel tanks and, wherever possible, the fuel ducts BEFORE EACH FIRST FLIGHT OF THE DAY. Indeed, condensation water is likely to occur when the aircraft has been parked for an extended period of time, for instance overnight. Draining is also recommended after each refueling. Note however that the chance for presence of water in the fuel is nil at major fueling stations which are continuously controlled in this concern, but it may be a problem at small or remote airfields, particularly if the fuel is delivered from jerrycans or other containers. Obviously, refueling in rainy conditions should be avoided at any time.

XI. ENGINE OIL:

Also engine oil is a very important item. Oil lubricates the various moving parts of the power plant and decreases the friction effects, thus keeping the operating temperatures within acceptable limits.

Engine oils are usually of mineral origin, although synthetic counterparts are available on the market as well, and are graded in so-called viscosity. High viscosity oil, or "thick" oil, can raise problems when attempting to start a cold engine during freezing conditions, and may even prevent starting altogether. Consequently, wintertime calls for lower viscosity gradings.

You will find the recommended oil viscosity gradings according to the ambient temperatures in the POH of your training aircraft. These are reported as SAE 20, 30, 40 or 50, SAE standing for "Society of Automotive Engineering", which is the international organisation for the classification of oils. Some oils, known as multigrade or multiviscosity have the property of being equally adequate at low and high ambient temperatures. Another type is AD oil, or Ashless Dispersant oil, containing a detergent which enables to keep metal parts in clean condition by bringing any carbon deposits in suspension and carrying them away so that little or no residue remains into the filters.

As many different types of oil are available, it is important to know which one is used in your aircraft. In this concern, a word of advice: during navigation flights implying one or more stops before returning to your home base, see to it that you have a few reserve oil cans on board. Indeed, the indications on the oil cans are not always very explicit or very clear, or do not always exactly correspond to the recommendations of the POH. Assuming that you are on an airfield where no expert is available, you might easily top up the oil tank with the wrong kind of product, which might be extremely detrimental to the engine.

As far as engine oil is concerned, remember the following:

1°) When starting a cold engine under low outside air temperature (OAT) conditions, the oil pressure is likely to go up to the red mark. This is perfectly normal because of the increased viscosity in such conditions. Keep the engine running at about 1000-1200 RPM until the oil has warmed up sufficiently and that the oil pressure has dropped to a normal value. Under normal OAT conditions, the oil pressure should stabilize at its normal value within 30 seconds.

2°) The oil pressure gauge is particularly important because a low reading is the first indication of impending oil starvation and subsequent engine failure. However, gradual oil starvation goes along with a gradually increasing oil temperature. In other words, if the oil pressure decreases, the oil temperature increases and will rapidly reach the red mark if the pressure drops to zero. The only way to counter a decreasing oil pressure is to reduce power significantly and, if it definitely approaches the zero mark, to shut the engine down completely before it grinds to an extremely damaging stop by itself. Of course, this means a forced landing on a single engined aircraft. Both oil pressure and oil temperature must always be considered together. Albeit it a rather rare occurrence, it may happen that the oil pressure gauge drops to zero despite the fact that the engine is perfectly sound and runs absolutely smoothly, and that the oil temperature remains normal: this indicates a failure of
some kind of the oil pressure indicator and does not require the untimely engine shutdown which, on a single engined aircraft, would obviously result in a questionable forced landing.

I Generalities

Your instructor certainly emphasized the need to verify the condition of the propeller during the preflight inspection. Most often quite some rugosity can be felt on the leading edges of the blades, particularly when metal propellers are involved. Indeed, the propeller tips are rather close to the ground surface on most nose wheel equipped light aircraft and, when the engine is running, it is quite possible that loose gravel or other rubbish is aspirated and causes such unevenness: therefore, the engine run-up before takeoff should preferably be executed on a clean spot. Unfortunately, that kind of damage is practically unavoidable but is no cause for real concern: at any rate, propeller blades are regularly verified and, when necessary, are "re-conditioned" during the regular obligatory maintenance inspection periods.

Propeller blade damage becomes a very serious problem if sharp nicks are discovered: in this case, and because of the stresses imposed during high power operations, it is not unlikely that the blade might break at this spot. This would completely destroy the balance of the propeller and, if it should happen during flight, the resulting vibrations are likely to tear the engine away from its mounts even before the pilot has got the time to reduce power to full idle. Needless to say that such an event would most probably have a fatal outcome. It is thus vital to carefully inspect the propeller before starting the engine and, if you have any doubt about its condition. have it verified by an approved technician. In extreme cases the blade will be beyond repair, but most usually the nick will smoothened out as shown in figure 1. Considering the involved balance problems, such an operation requires the utmost care and must obviously be carried out by an approved maintenance shop.

The propeller is NOT the means to push or to pull the aircraft on the ground!!! This is what tow bars are for! If no tow bar is available, which unfortunately is all too often the case, using the propeller roots to move the aircraft may be considered as acceptable but is certainly not to be recommended, particularly not if a variable pitch propeller is involved (neither is it recommended to exert push or pull forces anywhere on fuselage, wings or tailplanes: light aircraft are very sensitive to this kind of treatment which may easily give way to premature fatigue of the material).

Also remember that a propeller is an extremely lethal object (see PILOT NOTE 1): never turn it around by hand without being absolutely sure that the magnetos are off, that the mixture is in ICO, that the throttle lever is in full idle. that the parking brakes are set and. preferably, that chocks are in place!!! You have perhaps witnessed engines being started by swinging the propeller manually: never try this without having been properly briefed on the applicable procedure!

When we look at a propeller blade section, we notice that its shape is very similar to that of a wing. And, in fact, a propel1er blade is nothing else than a wing, but a rotating one. Just as is the case for the common wing, a propeller blade produces lift and drag which are the components of its total aerodynamic reaction R. This total force R, in turn, decomposes into the thrust of the propeller, as well as its so-called torque effect (fig.2) which we will discuss hereafter.

As is the case for the usual wing, the propeller blade section has a chord line (koorde/corde), i.e. an imaginary line connecting the leading edge with the trailing edge, which forms an angle of attack with the relative wind. However, for a propeller blade section, the relative wind is produced by two different motions: the forward motion of the aircraft itself which we identify as Vt, and the rotary motion of the propeller Vr. Figure 2 shows the resultant relative wind which forms the angle of attack "a" with the chord line, as well as the angle P between the chord line and the propeller's rotation plane: this angle P is known as the pitch angle, or blade angle (schroefspoed/pas de l'hélice) of the considered blade section.

Once again just as for the wing, the angle of attack "a" must be such that the maximum lift is obtained for the least drag and, still as for the wing, the angle of attack "a" may not exceed a specific value (±16°) in order to avoid stalling symptoms.

The rotation speed of the propeller blade tip is obviously much higher than the rotation speed of the propeller blade root(schroefwortel/pied de pale). In other words, the value of Vr varies all along the blade's length. Now, assuming that angle P, thus the pitch or blade angle, would be the same along the whole blade length, such a blade would be utterly inefficient: as shown in figure 3a, towards the tip the angle of attack "a" would by far exceed the maximum allowable value of about 16° and result in stall phenomena which would completely destroy the propeller's thrust in this area. On the other hand, towards the propeller root (fig.3b) we see that the angle of attack "a" is not only exceedingly high, but that it is completely negative as well, a situation which is equally destructive for the propeller's efficiency. Consequently, as the angle of attack "a" must remain as close as possible of the ideal value along the full length of the blade, it is necessary to gradually vary the pitch angle P from a very high value at the blade root to a low value at the propeller tip. This explains the twisted aspect of propeller blades(fig.4).

Let us now consider figure 2 again: we notice that, just as for a normal wing, the total force R results from the lift Fz which acts at right angles to the relative wind, and from the drag Fx acting in the prolongation of the relative wind. As far as a propeller blade is concerned, this total force R can be decomposed in a "horizontal" component CA which determines the value of the propeller's thrust (trekkracht/traction), and a "vertical" component CB which determines the value of the torgue. The thrust is self-explanatory the torque is nothing else than the tendency whereby the aircraft tends to rotate around its roll axis, opposite to the propeller's rotation, and which will be discussed below.

II The fixed pitch propeller

 

In the early days of aviation, all propellers were of the fixed pitch type (vaste schroef/hélice à pas fixe). Nowadays, this system is only used on elementary training aircraft. The blades of a fixed pitch propeller cannot rotate around their axis, as is the case for their counterparts on the so-called variable pitch propellers (verstelbare schroef/hélice à pas variable) and constant speed propellers (regulateurschroef/hélice à vitesse constante).

The only advantage of the fixed pitch propeller is its simplicity of construction, which obviously makes it a comparatively cheap design. Fixed pitch propellers used to be made of wood, particularly for earlier light aircraft, but at the present time metal construction is generally preferred.

You will also notice that most light aircraft are fitted with two-bladed propellers: this is the case when fixed pitch propellers are concerned. When a variable pitch or constant speed system is involved, a three-bladed propeller is sometimes used. As a matter of fact, some aircraft manufacturers offer the very same aircraft type with the possibility for the owner to choose either a two- or a three-bladed propeller. The use of more blades becomes particularly important on engines developing 1000 HP (PK/CV) or more, but as far as light aircraft are concerned it makes not much difference whether two or three blades are fitted: the only advantage of a three-bladed propeller is to reduce the noise and vibration levels somewhat and to provide a more sophisticated look. However, these advantages may be seriously questioned in view of the additional expense and maintenance costs.

Coming back to the fixed pitch propeller, its aerodynamic efficiency is considerably less than that of its variable or constant speed counterpart. It can be designed for maximum efficiency during climb, which will result in poor efficiency in cruise, or the other way around, if it is designed for maximum efficiency in cruise, the climb performance will suffer. The design of a fixed pitch propeller is usually a compromise for both cases, but inevitably allowing only average performances in climb as well as in cruise.

Another disadvantage of the fixed pitch propeller is the fact that the RPM is influenced by the aircraft's airspeed. Let us assume cruise flight at a given power setting, say 2000 RPM: if the airspeed increases by lowering the aircraft's nose, the angle of attack "a" decreases as well (fig.5a). As a result, both Fz and Fx decrease and, because of the reduced drag, the propeller has a tendency to turn faster and causes the RPM to increase. Incidentally, during a dive at high speed, as happens during aerobatics, the angle of attack becomes quickly negative, particularly if the engine is reduced to idle power, which gives way to braking forces and vibrations that are extremely detrimental to the engine. The opposite effect happens when the aircraft's airspeed decreases by pulling on the stick: the angle of attack increases (fig 5b), Fz and Fx increase and, because of the increased drag, the RPM decrease.

Because of the effects of airspeed on a fixed pitch propeller, assuming that a full power check is required for some reason during the engine run-up (see PILOT NOTE I), the maximum rated RPM cannot, and may not, be reached as long as the aircraft is at a standstill. If we consider for instance the POH of the Cessna 152, anno 1979, 2550 RPM is the rated maximum. None the less, an engine run-up at full power is reported to produce only 2280 to 2380 RPM which is known as the static RPM, i.e. the maximum RPM when the aircraft is stationary. And indeed, assuming that 2550 RPM should be obtained under these circumstances, the maximum rated value would be significantly trespassed during the subsequent takeoff roll as a result of the increasing speed.

III The variable pitch propeller

In order to improve the efficiency of the propeller, the idea of rotating the blades around their axis in order to vary their pitch angle made its way. In the early days, the propeller blades were set manually before starting the engine to obtain either maximum climb or maximum cruise performance. Of course, once the blades were positioned on the ground, the pilot could not change anything to their setting during flight.

This elementary system was soon improved in such a way that it became possible to mechanically select the blades in one position or the other during flight by means of an additional propeller control lever, the usual throttle lever being used to select the required manifold pressure, or MAP (inlaatdruk/pression d'admission). The propeller control lever had (and still has on the present systems) two extreme positions: one labeled "LOW PITCH" or "HIGH RPM", corresponding to the smallest blade angle, thus resulting in the smallest angle of attack, and which provided the highest possible RPM; the other, labeled "HIGH PITCH" or "LOW RPM" corresponding to the highest blade angle and consequently the lowest RPM. Note that the term "HIGH PITCH" is sometimes referred to as "COARSE PITCH", whereas "LOW PITCH" may be replaced by the term "FINE PITCH".

As time passed by, the mechanical system was replaced by more sophisticated devices, mostly electrical or hydraulic systems, of which the latter became the most common one. The hydraulic system made use of the normal engine oil in very much the same fashion as for the present so-called constant speed systems.

Although the original variable pitch propeller, and particularly with the hydraulic system, was in itself a considerable improvement as compared to the basic fixed pitch design because it allowed to select the most suitable pitch angle for both climb and cruise, it still remained subject to RPM variations with the aircraft's airspeed. This setback was ultimately eliminated as well with the introduction of the constant speed propeller.

IV The constant speed propeller

Nowadays the constant speed system is the standard design for engines fitted with variable pitch propeller. Although the operation is very similar, the major improvement offered by the constant speed version is the presence of a so-called governor (regulateur/regulateur).

Constant speed propellers are designed in two different ways, depending on whether they are intended to be fitted on single-or on multi-engined aircraft.

1) Singled-engined aircraft case

1.1) Single-engined aircraft

As explained earlier, an inherent tendency for any propeller is to rotate faster under influence of an increasing airspeed (Vt). If the propeller is of the constant speed type, this tendency is prevented by means of the governor as shown in figure 6:

- This drawing shows that the engine oil is pressurized by a pump, then directed towards a pilot valve which, in a simple variable pitch propeller, is directly operated by the propeller control lever. The pilot valve is composed of an inlet and an outlet as well as of a dual piston located in a cylinder and which is designed either to close both the inlet and the outlet of the pilot valve, or to open its inlet while closing its outlet or conversely, to close the inlet and open the outlet.

- The pressurized oil entering the system through the pilot valve inlet (the outlet being blocked) exerts pressure on the forward face of a piston located in the hub. This piston is linked to a pin which moves in a curved gap. This curved gap is located in a second cylinder which is able to rotate as a result of the aft and fore motion of the piston, and which is coupled to the propeller blades roots by a geared wheel system.

- Moving the propeller control to "HIGH PITCH/LOW RPM" causes the dual piston to open the inlet of the pilot valve while its outlet remains closed. This allows the pressurized oil to enter the hub, to exert pressure on the piston, and to displace it rearwards: the associated movement of the pin in the curved gap causes the second cylinder to revolve and to rotate the propeller blades to the highest blade angle, to increase their angle of attack, thus their drag, and to lower the RPM.

- Moving the propeller control to "LOW PITCH/HIGH RPM" causes the dual piston to close the inlet of the pilot valve and to open its outlet. The incoming oil is returned to the pump through a by-pass while the pressurized oil within the hub is allowed to return to the oil tank. The pressure on the piston's face drops, the propeller blades are free to move through natural tendency towards their smallest angle causing the piston to move forward and, due to the reduced drag, the RPM increase.

The system is designed in such a way that the pilot can select any intermediate position between "HIGH PITCH" and "LOW PITCH", and obtain any required RPM (for a given MAP), i.e. he can set the propeller blades at the ideal pitch angle for all operations, whether in climb or in cruise.

This far the description matches that of a basic variable pitch propeller which, once a specified RPM value is selected, still remains influenced by airspeed variations. Let us now examine the governor which, on the constant speed system, prevents the propeller to turn faster or slower under influence of Vt:

- The governor (of which all components are located within the propeller hub, unlike the drawing might suggest) is composed of a spring system, known as speeder spring, a so-called speeder rack linked to the propeller control, and two flyweights connected to a plate rotating at the same speed as the engine's crankshaft and linked to the pilot valve. The flyweights have a tendency to slant outwards under the influence of the centrifugal force. This "slanting" is however prevented by the tension of the speeder spring, tension which is set by the pilot through the propeller control. The system is designed in such a way that whenever a specific RPM has been selected AND that the RPM matches this setting, both flyweights are in upright position as shown in figure 6. Under this condition, bath the inlet and the outlet of the pilot valve are closed. Let us now see what happens if the aircraft's airspeed (Vt) varies:

a) The airspeed increases:

The propeller has a tendency to turn faster. The centrifugal force increases and the flyweights slant to the outside. Doing so, they push the plate upwards which compresses the speeder spring, thus causing the inlet of the pilot valve to open while the outlet remains closed: pressurized oil is admitted to the hub, causes the piston to move rearwards and the propeller blades to rotate towards high pitch. As a result, the RPM decrease, so does the centrifugal force, the speeder spring resumes its preset tension and the pilot valve's inlet closes again.

b) The airspeed decreases:

The propeller has now a tendency to turn slower. The centrifugal force decreases and the flyweights slant to the inside. This allows the speeder spring to extend and to push the plate downwards, thus causing the outlet of the pilot valve to open while the inlet remains closed: pressurized oil flows away from the hub and returns to the oil tank, thus allowing the propeller blades to move to low pitch and the RPM to increase. The centrifugal force increases, the flyweights return to the upright position causing the plate to move up, compressing again the speeder spring and closing the pilot valve's outlet.

2) Multi-engined aircraft case

On multi-engined aircraft, the constant speed system is somewhat different than on single-engined models. Indeed, assuming a governor failure is a single, the normal result will be that the propeller will move to the extreme low pitch position and allow the pilot to operate the engine at high RPM, possibly even to go to full power by opening the throttle lever. Note however that, on some single-engined aircraft, counterweights are attached to the blade roots to the purpose of making the propel1er move to high pitch in case of a governor failure.

In a multi-engined aircraft, if an engine failure of some sort occurs, the pilot must be able to feather the propeller (schroef in vaanstand/hélice en drapeau) in order to bring it to a complete stop. When feathered, the propeller blades assume a 90° angle thus positioning them parallel to the airflow (fig.7) The feathering capability provides two important advantages:

2.1) In case of trouble, the propeller can be brought to a complete standstill and further damage due to windmilling without oil pressure is prevented;

2.2) A feathered propeller offers very little drag as compared to a windmilling one and does not excessively affect the aircraft's performances.

Figure 8 shows the mechanism of a constant speed propeller with feathering capability. The major differences are as follows:

- The natural tendency of the propeller blades to move towards low pitch/high RPM is prevented by pressurized nitrogen (or a calibrated spring) in the hub, in front of the piston. Because of this, pressurized oil is now used to move the piston against the nitrogen pressure towards low pitch.

- If the propeller turns too fast, the governor causes the pilot valve's outlet to open while the inlet valve remains closed, thus allowing the pressurized oil to flow away: the piston will move rearwards under influence of the nitrogen pressure (or the spring), thus causing the blades to move towards high pitch.

- Note also that in case of oil pressure loss, for instance during a normal engine shutdown, the nitrogen (spring) pressure would cause the propeller to move beyond high pitch towards the feathered position. This is prevented by a centrifugal pin which locks into position when the RPM drops to a certain value (often ±800 RPM). This means that, in order to be able to feather the propeller in flight following an engine failure, it is imperative to prevent this pin to engage before this RPM value is reached: this is done by moving the propeller control lever to the "FEATHER" position.

To conclude the theory of the constant speed propeller (as well as of the basic variable pitch propeller), a few considerations regarding its practical use:

a) The engine control levers are threefold: the normal throttle lever, the propeller lever and the mixture control lever. Which, incidentally, makes six levers on a twin-engined

aircraft ....... thus ample possibility to actuate the wrong one. Be warned!

 

b) The normal position of the propeller control lever during ground operations, as well as during takeoff and landing, is "LOW PITCH" or "HIGH RPM". Although the propeller control lever is directly related to the RPM indicator, engine power (up to maximum is mainly obtained as for a fixed pitch propeller: by means of the usual throttle lever. Note that the reason for selecting "LOW PITCH" during landing is to be ready for a possible go-around whereby maximum propeller efficiency is required.

 

c) Besides the RPM indicator, the pilot has at his disposal a manifold pressure, or MAP, indicator: this is the pressure measured between the carburetor (or injection system) and the cylinders. The MAP is usually calibrated in inches of mercury (In.Hg.). Note that when the engine is stopped, the MAP indicator shows the ambient air pressure which, at sea level, is about 29,92 In.Hg. When the engine is idling, the MAP drops to about la In.Hg. This is due to the fact that the "air consumption" in the cylinders is much higher than the quantity of air passing through the manifold. This air quantity will increase as the throttle lever is opened, up to a value which, at full power, will come close to the ambient air pressure but without reaching it (unless the engine is of the turbocharged type - see PILOT NOTE l - in which case the ambient air pressure can even be exceeded by a considerable amount). Note that for British designed engines, the manifold pressure, referred to as boost, is usually expressed in pounds per square inch (lb/sq.in.), the standard sea level atmospheric pressure then being 14,7 lb/sq. in.

d) During flight, the MAP (or boost) is set by means of the throttle lever and the desired RPM by means of the propel1er control. Various settings are published in the POH to obtain specified power settings (expressed in percent) and considering various density altitudes. Note also that different combinations in MAP and RPM may produce the same power.

e) The standard procedure to set the power is as follows:

+ to increase power , first increase the RPM to the desired value, then increase the MAP;

+ to decrease power , first reduce the MAP, then reduce the RPM.

Be aware however that on some engines this procedure must be reversed due to specific structural consideration.

A

Always verify the POH in this consideration.

 

f) Many pilots claim that, in order to avoid "overboosting the engine" the MAP should never have a higher value than the RPM divided by 100. For instance, assuming 2300 RPM, the maximum allowable MAP should be 23 inches, anything more being considered as overboosting (no similar "rule of the thumb" has ever been expressed in relation to lb/sq.in). At first glance, the power charts of most American constant speed propeller equipped aircraft seem to confirm this statement. Indeed, if we consider for instance the Cessna 182Q "Skylane", anno 1979, the reported cruise performances at 2000 feet under standard temperature vary from 2400 RPM and 22 inches producing 74% power, to 2100 RPM and 18 inches producing 46% power. A closer look reveals that for instance 66% power can be obtained either with 2300 RPM and 21 inches, or with 2200 RPM and 22 inches, but as well with 2100 RPM and .... 23 inches, this latter setting being in contradiction with the principle mentioned hereabove. If we consider turbocharged engines, such as on the PA-34-200T "Seneca Ii", we read even allowable power settings of 2300 RPM and 31,6 inches. In other words, the danger of damaging the engine because of overboosting is more of a legend than actual fact, unless perhaps one should open the throttle fully with the propeller lever at, or close to, the high pitch limit.

 

g) The question is often asked whether the propeller setting must be adjusted to a higher RPM when a power increase is required to make a minor correction in altitude (or vertical rate): the answer to this is: No ! Leave the propeller lever alone and simply increase the MAP as required, unless full power becomes necessary. Neither is it necessary to decrease the RPM when the power must be reduced: leave the propeller lever still alone and simply decrease the MAP as required, unless you are in final descent for landing in which case you should move the propeller lever to "LOW PITCH/HIGH RPM" in preparation of a touch-and-go or go-around.

 

h) As far as the various possible combinations of MAP and RPM for one and the same percentage of power are concerned, the choice remains all yours. Just keep in mind that the advantage of using the lowest possible RPM is to significantly reduce the noise level (as well in the aircraft as for third parties on the ground) . However, many engines show a tendency for roughness at low RPM which should be avoided as well. But generally speaking, using the lowest possible RPM seems the most recommendable.

 

i) Another item to remember is that the ambient pressure decreases with altitude. Taking again the Cessna 182Q as an example, the recommended climb power setting is 2400 RPM and 23 In.Hg or full throttle. What exactly does this mean? At sea level, the MAP of 23 inches can indeed be obtained with a certain opening of the throttle lever. Incidentally, note that this recommended climb power setting corresponds to 75%, which means that the mixture may be leaned as necessary (see again PILOT NOTE I). But if this throttle opening remains unchanged, as the altitude increases, the ambient pressure decreases and the MAP will gradually decrease as well to 22, 21, etc... thus causing a gradual power drop. In order to maintain 75% it is necessary to readjust the MAP to 23 inches at regular intervals, say every 1000 feet, until the throttle lever is fully open, at which moment it becomes impossible to maintain 75% any longer and the available power irremediably decreases with further climb. Note that the full throttle position is normally reached at about 7000 ft density altitude. The opposite is of course equally true, particularly during high speed descents whereby a specific rate of descent is initiated without reducing the throttle lever position: if the throttle lever position remains unchanged the MAP, and the power as a consequence, will gradually increase due to the increasing air density.

 

j) An important consideration regarding particularly engines with constant speed propeller is the problem of carburetor icing. Indeed, with a fixed pitch propeller, carburetor icing can be detected by an audible decrease in RPM. This may or may not be the case if a constant speed propeller 1S involved because the governor tends to maintain the RPM unchanged. With this type of propeller the best way to timely detect carburetor icing is by means of the MAP indicator decreasing gradually without any other obvious reason.

 

k) As far as the engine run-up is concerned, the MAP must be increased by means of the throttle lever, the propeller control lever being set at the "LOW PITCH/HIGH RPM" position, until a specified RPM is reached. The magneto check and other required verifications are then carried out in the usual way. However, these verifications include an additional item, namely the cycling of the propeller control from "LOW PITCH/HIGH RPMII to "HIGH PITCH/LOW RPM", which must cause the RPM to drop to a specified value, then back to "LOW PITCH/ HIGH RPM" which must cause the RPM to recover its initial value. This cycling verifies the proper operation of the pilot valve. It is recommended to perform this cycling two or three times, particularly after a prolonged parking period under cold weather conditions, in order to allow the cold oil in the hub to mix with the hot oil from the engine lubrication system. It should be noted that the propeller cycling does in no way verify the proper operation of the governor system: to this purpose, with the engine still operating at normal run-up power, it is necessary to reduce the RPM by ±200 RPM by means of the propeller control, then to increase the previously set MAP and observe that the RPM value remains unchanged (note that the testing the governor is usually not required by the checklist).

 

l) There is an interaction between MAP and RPM. This interaction is ever present but is particularly noticeable when cycling the propeller during the engine run-up: reducing the RPM causes the MAP to increase, and conversely. As said, this interaction is ever present. Let us now assume that, having reached the cruising altitude, we wish to reduce to cruise power, say for instance from 25 inches and 2500 RPM to 22 inches and 2200 RPM: as we first reduce to 22 inches, by the time that 2200 RPM is obtained the MAP is no longer 22 but something like 23 inches; and needs to be re-adjusted. With some practice this re-adjustment can be avoided by reducing the MAP initially to slightly less that the intended value so that, once the RPM is decreased, it settles by itself at the desired setting.

 

V PROPELLER EFFECTS

 

Any aircraft which is fitted with a propeller is subject to a number of so-called propeller effects which, under certain conditions, cause the aircraft to rotate around its yaw axis (topas/axe de laçet). This yawing tendency is indicated by the ball (kogel/bille) moving more or less sidewards.

 

It must be noted that if the propeller is rotating clockwise (as seen from the cockpit), the propeller effects mainly cause the aircraft to yaw to the left, thus a deflection of the ball to the right. This is the case on most American manufactured engines. If the propeller is rotating anti-clockwise, as on most British engines, the propeller effects are reversed: they mainly cause the aircraft to yaw to the right, thus a deflection of the ball to the left.

 

The propeller effects are due to four causes, namely the torque, the slipstream, the so-called P-factor and the gyroscopic effect.

 

1) The torque

 

As mentioned earlier in this note, the torque effect is the phenomenon whereby the aircraft tends to turn around its roll axis (rolas/axe de roulis), opposite to the propeller rotation. In other words, if the propeller rotates to the right, the aircraft tends to roll to the left, and conversely. The torque is in fact an illustration of the famous Newton's law stating that an action in one direction causes a reaction in the opposite direction. Torque can easily be demonstrated by means of one of those little toy aircraft fitted with an elestic driven propeller: when the elastic band expands and that we hold the aircraft model, the propeller rotates in one direction, say to the right; on the other hand, if we hold the propeller, the aircraft model rotates to the left.

 

When airborne, the torque effect, as such, causes little problem to the pilot. As a matter of fact, the manufacturer has a number of technicalities at his disposal to counter this rotational tendency. However, these often come down to increase the lift on the downgoing wing (in the early days of aviation, the rigging of the wings was slightly different to one side as compared to the other) but, as increasing lift always causes an increase in drag, the roll tendency is replaced by a yaw tendency which, in turn, can also be countered, at least to some extend.

 

It is mainly during the takeoff roll that the torque makes itself felt, particularly when the trottle lever is briskly opened to full power: assuming a clockwise rotating propeller, the aircraft tends to roll to the left in spite of all correctives, compressing the left landing gear wheel on the ground surface, thus increasing its friction as compered to the right wheel, and the aircraft tends to yaw to the leftas a result. 

 

On some aircraft, particularly high powered models such as WWII single-engined fighters, the torque effect can be particularly dangerous when applying full power from near idle, such as during a go-around or a balked landing: the roll tendency can be so violent that the aircraft instantly flicks onto its back if the throttle is opened too briskly.

 

2) The slipstream

 

This is a rather well known phenomenon: the propeller's slipstream winds in spiral around the fuselage and, if a clockwise rotating propeller is involved, it exerts a pressure on the left side of the vertical tailplane, thus pushing it away to the right and causing a yaw motion to the left. Incidentally, this push on the vertical tailplane also helps somewhat to counter the torque effect.

The spiralling slipstream is much more compact at high RPM and low speed. Because of this, its intensity is very significantly reduced at normal cruising speeds and can even be eliminated altogether under these circumstances, again by means of constructional technicalities such as slightly offsetting the vertical tailplane or even the engine's thrust axis. On the other hand, at low speed and high RPM, such as during climb or horizontal flight at very low speed, the slipstream effect becomes particularly strong, and guite some rudder input is required to keep the ball centered.

We mentioned the use of constructional technicalities to totally eliminate the slipstream effect at normal cruising speeds. This is fine, but raises another problem: during glide flight with the engine at idle, there is practically no more slipstream to be dealt with; and during descent at high speed with power on, the spirals of the slipstream become so stretched that they hardly y have any effect on the vertical tailplane. Still the aforementioned technicalities remain. In other words, during such situations the aircraft might have the tendency to yaw to the right with the ball deflecting to the left.

The existence of the slipstream effect and the corrective constructional details, whether or not in combination with the P-factor which we discuss next, are the cause that the flight control inputs, and particularly the rudder input to keep the ball centered, may be significantly different during turns to the left as compared to turns to the right.

3) P-factor

The P-factor (P standing probably for "propeller") is the phenomenon whereby the downgoing propeller blade develops more thrust than the upgoing one. To understand this statement, let us to make a comparison with the helicopter theory, and consider the main rotor which turns in the horizontal plane. Let us assume that, as seen from above, it turns as shown on figure 9.

Figure 9 assumes that the helicopter is in stationary flight. In this case, the rotor is solely influenced by the relative wind due to the rotation speed VR and the resulting lift is equally distributed over both blades. However, when the helicopter moves forward (fig.1Q), blade A is submitted to the relative wind VR plus the relative wind V due to the forward motion: in helicopter terminology, blade A is called the advancing blade. On the other hand blade B, the so-called retreating blade, is submitted to VR minus V. In other words, because of the difference in relative wind, blade A develops more lift than blade B, a situation which causes quite a few problems and requires a number of expensive technical adjustments.

But the helicopter is not our concern at present. Let us now assume that we tilt the propeller of figure 9 forward, as seen from point C, until its axis becomes horizontal as is the case for an aircraft flying at normal cruising speed (fig.11): blade A becomes now the downgoing blade, and blade B the upgoing blade, i.e. the pilot sees the propeller turning clockwise. The propeller plane is now at right angles to the relative wind caused by the forward motion of the whole. Under these circumstances, there is no longer a difference in lift as was the case before, and the lift developed by the propeller (and which decomposes in thrust and torque) is equally distributed over both blades. This means that the effect due to the advancing blade and the retreating blade is maximum when the propeller rotates in the horizontal plane, and is non-existent when it rotates in the vertical plane. In other words, as far as aircraft are concerned, this effect, the so-called P-factor, increases gradually as the angle of attack of the aircraft increases.

Considering now an aircraft flying straight and level at a normal cruising speed, nothing unusual happens. However, when is flies at low speed, we know that this requires a high angle of attack. The propeller's rotating plane is thus tilted as shown in figure 12 and the phenomenon of the "advancing" and "retreating" blades makes itself felt: as the downgoing blade (thus the right blade) develops more lift, thus more thrust, than the upgoing blade (the left one), the aircraft tends to yaw to the left.

Note also that at low speed, high angle of attack, and consequently high power, the P-factor and the slipstream effects combine, which requires quite some rudder input to maintain the ball centered

4) The gyroscopic effect

The word "gyroscope" is usually related to aircraft instruments such as the attitude, heading and turn indicators in which rather heavy small wheels are spinning at very high speed around their axis. These are gyroscopes in the full sense of the word. But any object rotating at high speed around its axis reacts like a gyroscope, IS is fact a gyroscope: a kid's spinning top, the wheels of your car, the wheels of your bicycle, etc ... And so is an aircraft propeller.

Gyroscopes possess three fundamental properties amongst which rigidity in space (standvastigheid in de ruimte/rigidité dans l'espace), precession (precessie/précession) which in fact is a direct result of rigidity, and the third one, nutation, which is of no interest to the propeller theory.

Rigidity in space is, as the word "rigidity" implies, the property whereby the gyroscope's axis remains aligned in the same direction as long as its rotational speed remains adequate. A spinning top provides a good example of gyroscopic rigidity (and of nutation for that matter, but that's another story): when it is at rest and that we try to put its axis in equilibrium on a table, it falls sidewards as soon as we let it go; but if it is spinning at high speed, then the axis will remain vertical (until its rotation speed begins to decrease), which is rigidity. Incidentally, note that the principle of rigidity in space is used in both the attitude indicator and in the directional gyro.

If we try to disturb the alignment of a gyroscope's axis, precession occurs. What is precession? Let us clarify this with an example: figure 13 shows a gyroscope rotating clockwise, thus to the right as seen from "a", and whose axis is tilted. According to the principle of rigidity, the gyroscope's axis tends to maintain this specific position.

Let us now assume that we try to bring the axis in horizontal position by means of couple AB or, which comes down to the same thing, by exerting a force C on the upper edge of the rotating wheel (fig.14). We may expect that the gyroscope will reach the horizontal position and maintain it as soon as force C is removed, as shown in figure 15. But what happens? We can indeed succeed in bringing the axis in level position, but at the same time it deflects to the left. In fact, things happen as if the original force C moves by 90° in the direction of rotation of the gyroscope before actually exerting itself on its edge. (fig.16). This sideward motion to the left is the precession effect which, incidentally, increases at higher spinning speeds, as well as with the strength of force C. Note that precession is the basic principle on which the turn indicator operates.

In which way doesall this affect the aircraft? AS said earlier, the propeller reacts in a similar way and, although the gyroscopic effect may be disregarded for all practical purposes on aircraft fitted with a nose wheel (unless aerobatics are involved), it may become quite another story during takeoff with a tailwheel equipped aircraft. Indeed, it is necessary to raise the tailwheel in order to pick up speed and to avoid that the aircreft becomes airborne before reaching the required lift-off value. It is thus necessary to push the stick forward to this purpose and, because of the propeller's gyroscopic effect, the aircraft tends to yaw to the left, exactly in the same fashion as described hereabove. Furthermore, note that in this case the gyroscopic effect combines with the P-factor and the slipstream effect and, on some aircraft types, particularly the more powerful ones, this combination may give way to rather unpleasant surprises. One piece of advice in this concern: on tailwheel equipped aircraft, never push the stick too brisky forward during the takeoff roll, and be ready to apply rudder without delay in order to maintain the aircraft in aligment with the runway axis!

Propeller effects in general may take themselves felt more or less strongly depending on a number of factors such as for instance the engine power, the diameter of the propeller, the lenght of the fuselage, etc... As said earlier, they are particularly noticeable during climb as well as during turns, especially climbing turns and descending turns at idle power during which the difference in rudder input to maintain the ball centered is really startling. Not to speak about the gyroscopic effect during takeoff on tailwheel aircraft.

At any rate, propeller effects are often disregarded and all too many pilots indulge in flying with the ball out of center because of these. Don't be one of them!!

VI QUESTIONARY

1. Sharp nicks in a propeller blade can be dangerous. Why?

2. You wish to rotate the propeller by hand. State the various precautions to be taken.

3. The pitch angle, or blade angle, of the propeller is the angle between ....... and the ......

4. The angle of attack of every section of a propeller blade must remain as close as possible to the ideal value. How is this accomplished?

5. The lift developed by the propeller blades acts at right angles to the rotation plane and provides the thrust. True or false?

6. What do you understand by the propeller's torque?

7. Some light aircraft are fitted with three-bladed propellers. The advantages of this design are: a) less noise and less vibrations, b) considerably improved performances, c) a and b.

8. Assuming a fixed pitch propeller, increasing the airspeed by diving causes the RPM to ......

9. What do you understand by "static RPM"?

10. A variable pitch propeller is not necessarily a constant speed propeller. True or false?

11. A constant speed propeller is a variable pitch propeller. True or false?

12. The extreme positions of the propeller lever on a single engined aircraft are "PITCH" or "..... RPM" and "...... PITCH" or "......RPM".

13. Hydraulically controlled variable and constant speed propellers make use of: a) an independent oil system, b) normal engine oil, c) a or b.

14. Assuming a basic variable pitch propeller, the selected RPM remains constant with varying aerodynamic loads. True or false?

15. The major difference between a variable pitch propeller and a constant speed propeller is the presence of a ...... in the latter system.

16. Constant speed propeller designs are different for multi-engined aircraft as compared to (most) singles. True or false?

17. State the two advantages of the feathering system.

18. Assuming a loss of oil pressure on a feathering propeller, it will feather automatically. True or false?

19. The normal position of the propeller control during ground operations, as well as for takeoff and landing is: a) "LOW PITCH", b) "HIGH PITCH".

20. When the engine is stopped the MAP should read about ...... In.Hg. Assuming a British boost pressure indicator, it should read about ...... lb/sq.in.

21. The power settings and resulting performances considered in the POH are based on: a) the true altitude, b) the pressure altitude, c) the density altitude.

22. Normally, to increase power, first increase ...... then ....... To decrease power, first decrease ....... then ......

23. Assuming a selected value of 2300 RPM, the MAP should never be more than 23 In/Hg. True or false?

24. You wish to adjust your cruising altitude from 4800 ft to 5000 feet. You need more power and you must increase both RPM and the MAP. True or false?

25. You wish to reduce your cruising speed from 140 to 100 kts. You need less power and you must decrease both the RPM and the MAP. True or false?

26. What is the advantage of using the lowest possible RPM with a variable or constant speed propeller?

27. During climb, what happens to the MAP and to the engine power if the throttle lever position remains unchanged? Consequently, what should you do?

28. The climb power setting expressed in RPM and MAP in the POH corresponds to ..... % power, which can be maintained until about ..... feet.

29. Assuming a constant speed propeller, you descend at high speed from 8000 to 1500 ft with the throttle lever position slightly reduced. What should you check as far as MAP is concerned?

30. During cruise, you notice that the MAP decreases without reason. What is the most propable cause?

31. The engine run-up with a constant speed propeller implies the cycling of the propeller. To which purpose?

32. During the engine run-up with a constant speed propeller, you wish to verify the proper operation of the governor. How do you accomplish this?

33. When the RPM decrease, the MAP ......., and conversely.

34. Considering a clockwise rotating propeller, the aircraft tends to yaw to the ..... under certain conditions, causing the ball to deflect to the ....

35. Considering an anti-clockwise rotating propeller, the aircraft tends to yaw to the ...... under certain conditions, causing the ball to deflect to the ......

36. State the four sorts of propeller effects.

37. The propeller torque contributes to counteract the torque effect. True or false?

38. The slipstream effect contributes to counteract the torque effect. True or false?

39. The slipstream effect is the most marked at: a) high power, b) low speed, c) a and b.

40. The aircraft designer uses a number of technicalities to eliminate, or reduce, the torque and slipstream effects. Which consequence can this entail during descent?

41. The downgoing blade of the propeller produces more thrust under specific circumstances than the upgoing one. Can you explain this statement.

42. What do you understand by a gyroscopic rigidity?

43. What do you understand by gyroscopic precession?

44. Assuming a clockwise rotating propeller, when you PULL on the stick, the aircraft tends to yaw to the .....

45. The propeller's gyroscopic effect can be particularly critical on tail wheel equipped aircraft during the ...... phase of the flight.

 

 

 

 

I DISTRIBUTION OF FORCES DURING CLIMBING AND GLIDING FLIGHT

1) Climbing flight

We know that when an aircraft is flying straight and level at a constant airspeed, the lift Fz equals the weight G, and that the thrust T equals the drag Fx. Such a state of things is shown in in figure 1.

We also know that, if the thrust becomes greater than the drag, the airspeed will increase: this causes the lift to exceed the weight, and the aircraft tends to gain height. Conversely, if the thrust becomes smaller than the drag, the airspeed decreases, so does the lift, and the aircraft tends to loose height. These phenomena easily lead to the belief that, during climb, the lift must be greater than the weight. It may thus come as a surprise that, when climbing, at least as far as a normal climb at steady speed in concerned, the lift is in fact less than the weight.

Let us look again at figure 1: the forces shown can be represented in another way, namely by connecting them to each other by putting the "nose" of one into the "tail" of the other. To this purpose, we draw the weight G on top, instead of below, and showing downwards : we connect the thrust T to the "nose" of the weight G, the lift Fz to the "nose" of the thrust T, and finally the drag Fx to the "nose" of the lift Fz. Doing so, and because the forces balance each other, we obtain a so-called closed figure, in this case a perfect rectangle, as shown in figure 2.

Note that whenever the forces exerted on an aircraft are such that the airspeed remains unchanged, these forces must balance each other and, when connected to each other, they must form a closed figure. Keeping this statement in mind, let us now consider figure 3, representing the forces acting on an aircraft during climb:

- The strength of force G, the weight, is known and can be expressed in so-called Newtons (see note below) . The direction in which the weight acts is also known, namely towards the center or the Earth. In other words, force G can be represented graphically in both direction and strength;

- The strength of the thrust T is also known, and can as well be expressed in Newtons. As its direction is also known, namely the aircraft's longitudinal axis, force T can as well be represented graphically in both direction and strength;

- The strengths of the lift Fz and the drag Fx are so far unknown, which is why they are represented in dashed lines as only the direction in which they act can be established: the lift Fz acting upwards, at right angles to the thrust, and the drag Fx acting opposite to the thrust;

- And incidentally, anyone who should believe that, during climb at steady speed, the thrust is equal to the drag (as is the case in horizontal flight) would be as mistaken as the one who believes that the lift is greater than the weight. Indeed, let us now connect these forces as we did in figure 2, by placing the weight on top and connecting the thrust to the "nose" of the weight. As we know only the direction in which the lift acts, we can connect it to the "nose" of the thrust, but without any graphical strength. However, as the drag acts opposite to the thrust and that we want its "nose" connected to the "tail" of the weight vector, we can easily construct a closed figure (fig.4). The fact that this figure is no perfect rectangle, as in level flight, has no importance: because it is a perfectly closed figure, it shows that all forces acting during a steady climb balance each other and that, consequently, the speed remains unchanged.

Figure 4 allows us to graphically measure the strength of both lift and drag. But even without doing so, it clearly shows that, during a steady climb, the lift is smaller that the weight, and that the thrust is significantly greater than the drag. In fact, it is the excess thrust (which depends on the power available of the engine) which allows the aircraft to climb. THE LIFT PLAYING NO PART AT ALL.

Note: The mass of an aircraft, i.e. the amount of matter of which it is composed, is expressed in kilograms, and is usually wrongly referred to as weight. The mass as such, whether the mass of an aircraft or anything else, is NOT a force. But any mass is submitted to gravity, which attracts it to the Earth's center.

Gravity causes the mass to increase its speed (in fact causes it to fall) each second by 9,81 meters per second, i.e. it causes an acceleration of 9,81 m/sec²•

Any mass which is accelerated undergoes a force, in this case the force of gravity. The strength of the force equals the masse in kilograms, multiplied by the acceleration. expressed in m/sec²,and results in Newtons. For instance, an aircraft with a mass of 1000kgs is accelerated to the Earth's center at a rate of 9,81 m/sec², which represents a weight of 9810 Newtons.

As the excess thrust is the fundamental means to climb, think about this: if the drag Fx is increased for some reason, and that the climb speed is to be kept unchanged, it is evident that the thrust must increase. As long a adequate power is available, there is no problem. But what if the maximum thrust is already used, for instance during takeoff? If the aircraft maintains the same climb angle, due to the increased drag, the airspeed will drop and give way to a loss of lift which, in turn, causes the aircraft to sink. Because of this, the resulting relative wind causes the angle of attack to increase, which might very quickly lead to the stall. So, as the thrust is already maximum, the only way to regain the recommended airspeed is to decrease the climb angle "a", which obviously deteriorates the climb performances.

And it is equally obvious that, if the drag becomes sufficiently strong, no climb performance at all will remain.

The question is: why would the drag increase? There are a number of reasons for that (a dirty aircraft for example, but it would hopefully not be that dirty). The most striking reason for the drag to increase is the use of flaps (as well as a lowered landing gear if the system is retractable). We know that a small deflection of the flaps produces a considerable increase of lift, but inevitably also some increase of drag and, although this drag augmentation is very small, the pilot must be aware that it takes its toll on the aircraft's climb capabilities.

In fact, there is only one reason to use partial flaps for takeoff: to reduce the ground roll (mostly because of an extremely short runway). As said, partial flaps causes the lift to increase and allows the aircraft to leave the ground at a lower speed but, as soon as the aircraft is airborne, the detrimental effect of the drag makes itself felt and gives way to a reduced climb performance as explained above.

Another example of increased drag: the final approach with flaps full down. In this configuration, the flaps produce a considerable increase of drag, for only a very small increment of lift.  This ensures a steeper glide path without increasing the approach speed, and a shorter ground run. But, in case of a balked landing requiring an immediate go-around, chances are that, even with full power, as long as the flaps remain in full down position, the climb capability is dangerously reduced, hence the need to retract them to an intermediate position to get rid of the major part of the drag in order to en sure a safer situation.

Incidentally, remember that the retraction of the flaps always implies a loss of lift. Assuming that you would retract them at once to full up, and considering the very low airspeed at this very moment, the loss of lift would be significant enough to cause the aircraft to sink and cause it to stall.

Thus far the basics regarding the forces acting during climb. But if you are interested, besides finding their respective strengths graphically, Fz and Fx can be calculated by a few simple mathematics. To this purpose, let us examine the drawing of figure 5: it is similar to figure 4, except that a straight line has been drawn from the "nose" of Fx, parallel to Fz, and produces a right-angled triangle. In this triangle, the hypotenuse equals the weight G while the adjacent side equals the lift Fz. Note also that angle "a'" equals climb angle "a". According to the basic principles of trigonometry:

cos. a' = adjacent side/hypotenuse

thus, cos a' = Fz/G

or, Fz = G x cos a'

As the value of the cosine of an angle of less than 90° smaller than 1(it varies from 1 at zero degrees to 0 at 90°), it is evident that the lift Fz must be smaller than the weight G. As the strength of G is a known value, as well as the climb angle, the strength of Fz can thus easily be calculated.

Let us go one step further: assuming that the aircraft climbs vertically, this means that the climb angle is 90°. Now, as the cosine of 90° equals zero, we have Fz = G X 0, i.e. the strength of lift Fz is zero. Let this latter consideration deal once and for all with the not uncommon belief that, during climb, the lift should be greater than the weight.

Regarding the drag Fx: the strength of the thrust is known and, as can be seen on figure 5, during climb the thrust equals the drag Fx, plus a "certain value" which we call X. If we know the value of X, all we need to do in order to know the strength of Fx, is to subtract X from the value of thrust T. Still on fig.5, it appears that X is in fact the side of the right-angled triangle which is opposite to angle "a'''. Another basic principle of trigonometry is that:

sin. a' = opposite side/hypotenuse (note that the sine of an angle of less than 90° is less than 1 but, unlike the cosine, it varies from zero at 0° to 1 at 90°), thus:

sin. a' = X/G

or, X = G x sin a'

Consequently:

Thrust T = Fx + G (sin a')

or, Fx = T - G (sin a')

 

Assuming that the aircraft would climb vertically (at a steady speed), the climb angle a would then be 90°. As the sine of 90° equals l, the thrust would be equal to the drag plus the weight. On the other hand, when the aircraft travels in level flight, the climb angle "a" is zero. As the sine of 0° is zero, G(sin a')becomes zero, and the thrust T equals the drag Fx as originally shown in figure 1.

2) Gliding flight

During gliding flight, considering that thrust is no longer present, only three forces act upon the aircraft: the weight G, whose strength and direction are known, the lift Fz and the drag Fx which are only known in direction. Another known value is the glide angle "a". All these parameters can be represented as on figure 6, with Fz and Fx shown in dashed lines.

Here again, the previously stated principle applies: whenever the forces acting on an aircraft in flight are such that the airspeed remains unchanged, these forces must be in balance and, when they are connected to each other, they must form a closed figure. This leads to figure 7 where the weight G has been put on top, with the lift Fz connected to its "nose". As said before, only the direction of the lift and of the drag is known, not their respective strengths. However, we know that the Fx acts parallel and opposite to the descent path, and that it is to be connected to the "tail" of the weight; we also know that Fz acts at right angle to the descent path: consequently, we can obtain figure 7 which graphically shows the strength of all three forces, and in which angle "a'" equals the glide angle "a".

Referring to figure 7, and again calling upon some elementary mathematics, the strengths of Fz and Fx can be calculated as follows:

cos a' = Fz/G

or, Fz = G x cos a'

In other words, during glide flying, the lift equals the weight multiplied by the cosine of the glide angle.

On the other hand, we also notice that:

sin a' = Fx/G or, Fx = G x sin a'

In still other words, during glide flying, the drag equals the weight multiplied by the sine of the glide angle.

So far the numerical calculation of Fz and Fx during the glide descent. But there is more to it: both lift and drag are depending on the aircraft's airspeed and, as no thrust is available during the glide, the airspeed depends solely on the glide angle "a". This interrelation between Fz, Fx and the glide angle can be expressed by using the trigonometry's basic tangent rule which states that:

tan a' = opposite side/adjacent side (note that the tangent of an angle varies from zero at 0° to infinity at 90°), thus:

tan a' = Fx/Fz

Consequently, tan a' is at its minimum, i.e. the minimum glide angle will be obtained, when:

Fx/Fz is minimum, i.e. when Fz/Fx is maximum.

The ratio Fz/Fx, known as lift/drag ratio, represents the strength of the lift compared to the strength of the drag and, when the maximum lift is obtained for the minimum drag, the ratio Fz/Fx is said to be maximum and the best lift/drag ratio is thus obtained.

As you probably know, the lift/drag ratio varies with the angle of attack (which, in the specific case of gliding, must not be confused with the glide angle), and there is only one value of this angle of attack which results in the best lift/drag ratio: any deviation of this angle of attack causes the lift/drag ratio to deteriorate and the glide angle to increase.

 

Flying at best lift/drag ratio is particularly important during a glide descent: it results in the best gliding speed, i.e. the gliding speed which gives way to the longest glide distance.

 

Let us now revert to figure 7, and consider that the represented glide angle "a" results from the best lift/ drag ratio. Let us now assume a heavier weight: in order to counteract the effect of the increased weight, we can:

a) increase the angle of attack to get more lift but, as said earlier, doing so would inevitably cause the glide angle to increase despite the fact that the three forces, weight, Fz and Fx would ultimately come back in balance between each other (see note below) ;

b) if we wish to maintain the glide angle unchanged, the only alternative is to increase the speed, thus increasing both lift and drag, until the three forces are in balance again. In other words, the higher the weight, the higher the gliding speed must be in order to obtain the minimum glide angle and the longest gliding distance. And indeed, if we refer for instance to the POH of the Cessna 172RG, we see that the reported best glide speeds are:

- for 2650 lbs: 73 kts

- for 2250 lbs: 67 kts

- for 1850 lbs: 61 kts

In fact, for each weight there is a specific best gliding speed ensuring the best lift/drag ratio. None the less, the POH of many aircraft reports only one single best gliding speed. In fact this value is valid for the maximum landing weight (which often equals the maximum takeoff weight) and should be decreased by ±5 knots for each 500 lbs (227 kgs). On light two-seat trainers, the weight variations are not sufficient to consider more than one best gliding speed.

As, when flying at best lift/drag ratio, the glide angle remains unchanged at any weight, only one factor is likely to affect the maximum possible distance travelled: the wind. It is obvious that gliding with headwind will result in a lesser gliding distance. Another interesting point is that, when the weight is increased, the ensuing higher gliding speed required to maintain the best lift/drag ratio produces an increased groundspeed and, for a given period of time, the distance travelled will be greater. This is a principle which is often applied in glider flying: ballast is taken on board when the aim is flying for maximum distance.

Finally, let us consider an increase in drag during a glide descent, such as when the flaps or the landing gear are extended: this will cause the airspeed to decrease but, referring once more to figure 7, a steeper glide angle allows to maintain the speed unchanged.

 

Note: We know that increasing the angle of attack would give way to a shorter glide distance, but on the other hand it would allow to decrease the rate of descent. It is thus possible to stay longer in the air, albeit at the expense of the travelled distance. Obviously, assuming that you would increase the angle of attack until the rate of descent would show zero, this ideal situation would not last for very long, as you would rapidly reach the stall. However, there is an angle of attack, slightly higher than the one for best lift/drag ratio, which gives way to a slightly reduced but safe airspeed, and allows you to reach the ground after a longer period of time: this angle of attack provides the so-called maximum endurance instead of the maximum range, another operating principle which is very well known by glider pilots.

II CLIMBING AND DESCENDING TURNS

You have perhaps noticed that during climbing turns the bank angle shows a stronger tendency to increase by itself than is the case for level turns and that, consequently, the resulting tendency for overbanking must be counteracted in a more marked way. Nothing really unusual with that: after all, when studying the level turns (even already when studying the secondary effects of the flight controls) you have been made aware of the fact that, because of its higher angular speed, the outer wing develops more lift than the inner one, hence this behavior.

However, you may have noticed as well that, during descending turns, the opposite happens, i.e. the bank angle tends now to decrease and it is necessary to maintain the aileron input to keep the bank angle unchanged. This latter phenomenon may seem rather strange at first as, indeed, the angular speed of the outer wing is still higher than that of the inner wing, no matter whether the turn is carried out in level flight, during climb or during descent. So, how comes that this tendency for the bank angle to increase is so strongly present during a climbing turn? And what happens in a descending turn? The explanation of these phenomena is that during climbing turns, besides the higher angular speed of the outer wing, its angle of attack is considerably higher than that of the inner wing, hence the tendency for the bank angle to increase, whereas during descending turns, the angle of attack of the outer wing is considerably smaller than that of the inner wing, hence the tendency for the bank angle to decrease. But what causes these differences of the angle of attack?

The core of the problem is that, the wings are submitted to a relative wind A which in both cases is the resultant of two motions: the forward motion B of the aircraft, and its upgoing or downgoing motion C. During straight climb or descent, both wings have obviously the same angle of attack, i.e. the angle between the chord lines of both wings and the resulting relative wind is identical, as shown in figures 8 and 9.

Assuming now that a climbing turn is carried out, say to the left, the outer wing has a faster angular speed than the inner one although the upward motion of both wings remains the same: figure 10 shows clearly that this combination results in a lower angle of attack for the inner wing, as compared to the outer wing. Consequently, during a climbing turn, on top of the fact that the outer wing travels faster and develops more lift, the smaller angle of attack of the inner wing causes it to suffer an additional loss of lift, hence the marked tendency for the bank angle to increase.

A similar reasoning applies for descending turns. Figure 9 shows the angle of attack of both wings during a straight descent. In this case, the relative wind C due to the descent acts upwards instead of downwards and, in combination with the forward motion B, produces the resultant relative wind A. Assuming now that the aircraft performs a descending turn to the left, here again the outer wing has a faster angular speed than the inner one but, as the downward motion of both wings remains the same, this results in a higher angle of attack for the inner wing (fig.11), thus producing an increased lift which counteracts the effect of the higher angular speed of the outer wing, hence the tendency to roll out of a descending turn.

III QUESTIONARY

1. During climb, the lift is: a) greater then the weight, b) smaller than the weight, c) equal to the weight.

2. Using flaps for takeoff: a) increases the climb performances due to the additional lift, b) lengthens the takeoff roll due to the additional drag, c) neither a nor b, d) a and b.

3. Using the flaps for takeoff: a) decreases the climb performances due to the additional drag, b) shortens the takeoff roll due to the additional lift, c) neither a nor b, d) a and b.

4. When the aircraft climbs vertically, the lift is zero. True or false?

5. During a go-around following a balked landing, it is necessary to initially retract the flaps slowly to an intermediate position before retracting them fully. Explain the reason for this process.

6. Climbing at a steady airspeed is possible because of: a) excess lift, b) excess thrust, c) a and b.

7. During a glide descent, the lift is: a) greater than the weight, b) equal to the weight, c) smaller than the weight.

8. During a glide descent, the best lift/drag ratio is obtained at: a) a single gliding speed, b) a single angle of attack, c) a and b.

9. During a glide descent at best lift/drag ratio, and under no-wind conditions, the weight of the aircraft does not influence the gliding distance. True or false?

10. The angle of attack providing the best lift/drag ratio results in: a) the longest gliding distance, b) the best endurance, c) a and b.

11. During a level turn, the bank angle tends to: a) increase, b) decrease, c) remain unchanged. Explain

12. During a climbing turn, the bank angle tends to: a) increase, b) decrease, c) remain unchanged. Explain.

13. During a descending turn, the bank angle tends to: a) increase, b) decrease, c) remain unchanged. Explain.

 

 

 

 

 

 

 

 

 

 

 

The study of the aircraft's electric system is usually rather superficial during the elementary training of most pilots and, especially for those whom are not particularly acquainted with these matters, it often remains a little mysterious, despite the fact that it is an important part of the aircraft's equipment. We might even add that, when IFR operations are involved, a fair knowledge of the electric system becomes essential. With this note, we will try to clarify a few things without digging too deeply in the theory of electricity.

 

BASIC PRINCIPLES OF ELECTRICAL CURRENT

Any substance is made of a countless number of small particles known as atoms, each of which is composed of even smaller particles orbiting around it in very much the same way as the planets orbit around the sun, amongst which electrons and protons.

Broadly speaking, and by convention, electrons are said to be negatively charged, whereas protons are positively charged. The ratio between the number of electrons and protons determines the so-called electric potential of the atom: if there are more electrons than protons, the atom is said to be negatively charged, if there are more protons than electrons it is said to be positively charged.

In some substances, the atoms include a considerable number of so-called free electrons which can easily be attracted and migrate from one atom to another. Such substances, such as copper, silver, gold, or acid liquids, are known as electrical conductors (geleiders/conducteurs). If no, or very few electrons are involved in this migration process, the substance is known as an insulator (niet geleider/isolant ou diélectrique).

The main characteristic of a conductor is that free electrons continuously jump from one atom to the other, thereby releasing free electrons from the second to a third one, etc ... This migration of free electrons happens in a random mat ter between the various atoms, without any definite direction. However, if a conductor, say a copper wire, is connected between two so-called electrodes, i.e. two masses whose atomic structure has a different electric potential, the positive electrode being referred to as the anode, the negative (or rather, the less positive) one as the cathode, the free electrons will move in an orderly way from the cathode to the anode, i. e. from the "-" to the "+". This flow of free electrons is known as electric current.

The intensity of the electric current is measured in ampères (A), or amps, and depends on the potential difference between the anode and the cathode. The potential difference, usually referred to as voltage, is measured in volts (V).

The intensity of the current also depends on the resistance which the conductor opposes to the passage of the current. The resistance itself depends on the nature of the conductor, its length, and its cross section, and is measured in ohms (represented by the greek letter omega).

The relationship between the current's intensity (I), the voltage (V) and the resistance (R) is expressed by the following basic formula, known as Ohm's law:

 

1 = V/R

In other words, the current's intensity is directly proportional to the potential difference between the electrodes, and inversely proportional to the resistance. This can of course be expressed V = IR, or R = V/I, and allows a number of basic calculations with regard to electric current.

However, don't believe that Ohm's law is universal, which would be a little too easy: it applies only to certain type of conductors, such as metals, known as linear conductors; it does not apply for instance to neon gas or to a carbon-arc lamp. Neither does it apply to electrolyte (see "Battery")

Keep in mind that electric current generates a lot of heat. This can be extremely harmful to a number of appliances. Adequate cooling is therefore absolutely indispensable to avoid malfunctions or premature failure, particularly with regard to radios and navaids, and more so if these are stacked upon each other.

The electrical appliances in light aircraft, such as the engine starter, a number of instruments, the radio equipment, etc..., operate on the type of current described hereabove, namely direct current, or DC (gelijkstroom/ courant continu). Direct current owes its name to the fact that it always flows in the same direction, as opposed to the alternating current, or AC: (wisselstroom/courant alternatif), which alternatively flows in one direction then in the reverse direction. Although alternating current is not really better or worse than direct current, the use of DC is merely dictated by practical purposes where light aircraft are concerned. For instance, direct current allows to start the engine on the aircraft's own electrical power without necessarily the need for external and costly ground power devices, or for an additional and weighty on-board auxiliary power unit (APU). None the less, alternating current offers a number of technical advantages (for instance a lighter wiring system), which is why AC is usually the basic current used in the heavier transport aircraft.

GENERAL LAYOUT OF THE ELECTRIC SYSTEM

DC power is produced by the aircraft's battery, which is connected by means of a battery switch to a distribution system known as the bus bar. The current is then supplied from the bus bar to the various electrical users through a number of circuit-breakers and individual switches.

On aircraft fitted with an extensive radio equipment, the bus bar is split into two parts: the primary bus and the avionics bus. The primary bus (which is sometimes split in additional busses), provides the current to the common users, such as the landing lights, the navigation lights, the Pitot heating system, the engine starter, etc ... , whereas the avionics bus supplies the radio equipment such as VHF, ADF, VOR, etc... The avionics bus is connected to the primary bus bar by means of a radio master switch.

Anyone knows that the battery's capability to deliver electric current is of rather short duration: assuming that all electrical users are operating together, the battery would be completely drained after about 30 minutes. Its charge must thus be maintained: this is achieved by a generator or alternator which is driven by the engine.

The proper operation of the electric system must be monitored by appropriate instruments, at least an ampèremeter, or ammeter, sometimes a voltmeter, and one or more warning lights.

THE BATTERY

Let us imagine a single cell, i.e. two metal plates of different atomic structure which are immersed in a solution of water and sulphuric acid. The solution, known as the electrolyte, is also a conductor and produces a chemical reaction, or electrolysis, whereby the plates very slowly decompose in a number of ions, i.e. positively or negatively charged atoms. Depending on their electric potential, these ions are attracted to one plate or to the other, thus creating a potential difference, of about 2 V between both plates. This potential difference is referred to as the electromotive force, or EMF, which is likely to produce a current when both plates are connected by a wire.

The battery contains of a number of such cells of which each plate is connected to either the negative or the positive terminal, thus giving way to a total EMF of mostly 12 V or 24 V.

The most common light aircraft battery is similar to the one fitted in your car, and is thus likely to suffer the same problems. It is of the so-called lead (lood/plomb) type whereby the plates of each cell are made respectively of lead and a derivative known as lead dioxide. Unlike simple cells, a car or aircraft battery can be recharged (either by action of a generator/ alternator or, if necessary, by an external source of power).

The recharging process of the lead type battery goes along with the gradual evaporation of the electrolyte and requires the battery to be refilled once in a while with pure water. A resulting disadvantage of this evaporation is that _the battery must be vented, which brings along the possibility for the extremely corrosive electrolyte to leak. Although such leaks are improbable under normal operating conditions, they would certainly occur if the battery is put upside down. Therefore, aircraft certificated for aerobatics where negative G's are involved are fitted with a so-called nickel-cadmium (or other suitable combination) battery whose main advantage is to be completely free of any leakage risk, even if the aircraft is operated in sustained inverted flight.

As the battery is regularly checked for condition and servicing during the obligatory periodic maintenance sessions of the aircraft, this item is not a required part of the pilot is preflight inspection. However, some basic precautions are required to ensure its proper operation: for instance, failing to select the master switch to "OFF" after flight may lead to the premature draining of the battery. Indeed, a number of appliances is not fitted with an individual switch and, whenever the master switch is on, these appliances are automatically powered. This is usually the case for the fuel gauges, the turn indicator, possible warning lights, etc ... Note also that some appliances, such as the clock, are continuously powered by the battery, even with the master switch in off: this means that, when the aircraft is parked with ail individual switches AND the master switch in "OFF" position, a very weak current keeps flowing and causes the battery to discharge, albeit at a very slow rate. In other words, leaving the master switch on will increase the discharge rate, particularly if a number of individual switches are inadvertently left in operating position.

A few words of advice with regard to battery care:

1)  Most aircraft are fitted with a so-called ground service receptacle allowing to connect an external DC source. Although this system is mainly used when maintenance work is to be carried out, the use of an external source may be considered for normal operations as well, particularly for the preflight inspection and subsequent engine start during darkness, as well as for cold engine starts under near freezing conditions. Read the procedure related to external power use in your POH.

2)  During the preflight inspection, the functional check of appliances such as navigation lights, taxi and landing lights, Pitot heater, etc ... should be kept brief, and ail associated switches should be returned to the "OFF" position as soon as possible. If the preflight inspection is carried out during darkness and that no external power source is available, use your flashlight as much as possible instead of the on-board lighting system.

3)  When the engine is cold and that starting is to be carried out in near freezing temperatures, it is recommended to rotate the propeller by hand a few times (be sure that all necessary precautions have been taken in this concern - see PN I: "LIGHT AIRCRAFT ENGINES") to cause the engine oil to move about in the cylinders and allow it to become somewhat less stiff. Using an external source to start the engine is also recommended under such circumstances.

4)  After each flight, prior to turn the master switch off, make sure that absolutely ail individual switches, including the panel and other lighting systems (which are very often overlooked and are hardly noticeable in daylight), are turned off.

5)  Conversely, before each flight and prior to select the master switch to "ON", verify that ail individual switches, including panel and other lighting systems, have indeed been turned off. Remember that a lot of DC power is required to activate the engine starter: once the master switch is "ON", only the absolutely essential electrical circuits should be selected, all the others should be left off until after the engine is running and that the generator/alternator has been put on line (see below).

6)   After the flight, the radios and navaids are often all switched off together by means of the radio master switch, the respective individual switches being left on. Of course, once the radio master switch is subsequently switched on, all the associated equipment is put on line at once. This procedure, which is favoured by a number of pilots, is harmless when the radio master switch is turned on AFTER the engine start and that the generator/alternator is operating. However, it might become a source of problems, and can even be damaging to the radio equipment if the radio master switch is turned on before the engine start. Indeed:

a) Assuming that an ATIS is to be picked up and/or, as may be the case for IFR flights, that a start-up clearance happens to be awaited for a significant period of time, all the radio equipment is needlessly operating meanwhile, perhaps even together with the navigation lights and internal lighting when night operations are involved. This causes rapid battery draining, possibly to such an extend that subsequent engine starting may be impossible;

b) Starting the engine causes significant voltage variations in the electric system. These are extremely harmful to the radio equipment if it is not switched off.

7) Assuming that the aircraft is fitted with an alternator instead of a generator, and that the master switch is of the split type, only the battery switch should be selected to "ON", the alternator being put on line only after the engine is running (see below).

THE GENERATOR/ALTERNATOR

As said earlier, the lifetime of the battery is limited and would hardly reach 30 minutes if the entire network is powered without recharging. In addition, this theoretic duration depends on the initial battery's condition and can be significantly reduced. Therefore, whenever the battery is the sole source of electrical power, it is important to switch off all unnecessary appliances. As long as the engine is running, it is the purpose of the generator/alternator to maintain the battery's charge unchanged.

You are probably aware of the fact that a generator/alternator system produces alternating current, unlike the battery which produces direct current. It is thus necessary to rectify, i.e. to change, the AC into DC: this is usually accomplished by a very simple mechanical system located at the outlet of the generator/ alternator. The subsequent direct current is then supplied bymeans of the generator/alternator switch to the battery which, in turn, supplies the bus bar.

Most present light aircraft electrical systems are fitted with an alternator instead of a generator. The alternator is in fact a more sophisticated generator and is more efficient in a number of areas than the common generator. One of the most notable differences (at least as far as light aircraft are concerned) is that the generator uses a permanent magnet driven by the engine to produce an alternating current (in very much the same fashion as the engine's magneto), whereas the alternator uses a so-called electromagnet, i.e. a wire wound around an iron core which becomes a magnet when a current passes through the wire. In other words, unlike the common generator, the alternator needs to be electrically powered to produce AC. This electric power, known as alternator field excitation, can be provided by only one source: the battery.

The presence of an electromagnet in the light aircraft's alternator is not free of any ill consequences. Indeed, as the ignition system is usually totally independent of the electric system, assuming a "dead battery", most engines can be started by turning the propeller manually with the magneto switch in "BOTH" position. In the early days, when electric starters were not available, this used to be the standard procedure, either using a cranking handle or, as is still the case nowadays ort some very light vintage aircraft, by swinging the propeller by hand, (incidentally, never try to start an engine by swinging the propeller without being properly trained to do it). Now, assuming a common generator, because of the permanent magnet system, it will produce electric power as soon as the engine is running at an adequate RPM and, once the generator switch is turned on, it will re-charge the battery. However, assuming an alternator instead of a generator, once the engine is running and that the alternator switch is on, the electric system will still remain unavailable because the battery is unable to supply power to the electromagnet, and that consequently the alternator remains useless. In other words, despite the fact that the engine is running and that the alternator switch is on, no electrical appliances will be operative.

THE VOLTAGE REGULATOR

The purpose of the voltage regulator (spanningsregelaar/regulateur de tension) is to control the output of the generator/alternator according to the number of appliances which are in operation and the resulting total demand of current. It also ensures that the battery's charge does not exceed its limits.

The voltage regulator is mostly hidden from view, usually behind the instrument panel, operates automatically and cannot be controlled by the pilot.

Besides regulating purposes, the voltage regulator also ensures, in combination with an overvoltage relay, that the generator/alternator automatically disconnects in case a current surge

shouldoccur. The combination voltage regulator/overvoltage relay is sometimes referred to as the alternator control unit.

THE MASTER SWITCH

In many light aircraft, the master switch is not a single selector, but a red colored dual one: one half is the battery switch, the other half being the alternator switch. Both these switches are usually of the interlock type, i.e. they are connected between each other in such a way that the battery can be switched on while the alternator switch remains off, but that the alternator cannot be switched on without automatically engaging the battery switch as well. None the less, some installations allow both switches to be operated independently of each other.

Prior to start the engine, the checklist usually calls for "MASTER SWITCH ON": consequently, when a dual switch is involved, most pilots select both switches together: nothing basically wrong with this, and the starting procedure usually takes place without any problem. Still, it should be kept in mind that, as soon as the alternator is switched on, current is taken from the battery to supply the electromagnet which, as long as the engine is not running, is also a "needless current consumer". It is true that the electromagnet is only a very minor consumer, but it could make the difference between a subsequent successful or unsuccessful engine start if a significant period of time elapses between the initial selection of the alternator switch and the actual starting procedure. This is particularly true for IFR flights whereby the start-up clearance is to be awaited. In fact, as long as the engine is shut down, only the battery should be switched on and, as said before, all other unnecessary equipment, including the alternator switch, should be kept off until after the engine is running.

THE CIRCUIT-BREAKERS

Any electrical appliance must be individually protected against possible current surges. Such surges are usually the result of a short circuit and could be extremely dangerous because, were it not for the presence of the circuit-breakers, they could easily lead to a fire.

Most circuit-breakers are located on the instrument panel, within easy reach of the pilot. Each one is accompanied by the specification of the circuit which it protects, and carries the value of the current intensity, expressed in ampères, for which it is designed. Some circuit breakers may be located elsewhere. This is for instance the case for the Cessna C152, anno 1979: in this aircraft, two circuit-breakers (fuses) are located in the engine compartment, adjacent to the battery, and protect the external power, clock and flight hour recorder circuits.

Circuit-breakers can be of the push-pull type, i.e. these can be pulled out or pushed in at will by the pilot. ln other words, push-pull circuit-breakers allow any circuit to be connected to, or disconnected from, the network even when no individual switch

is available. This possibility may prove very useful under certain abnormal situations (such as an electrical smoke from unknown origin) . The most common type is the push-to-reset circuit-breaker: these cannot be pulled out, but can easily be pushed in if they have popped out and disconnected their associated circuit from the network for some reason.

Normal switches are in fact circuit-breakers as well, but most of them will not automatically go to the off position if a short circuit occurs (this is why a switch is ..... a switch, not a circuit-breaker as such). However, some switches do go to the off position automatically in case of problem, and are therefore known as circuit-breaker/switches. For instance, in the Cessna C182Q, anno 1979, the radio master switch is such a one: it will automatically go to the off position if a major malfunction occurs in the radio or navaid equipment.

Some circuit-breakers are in fact so-called fuses (smeltbeveiligingen/fusibles). These are wrapped in a tiny transparent glass cover fitting in a little protective cap which is screwed into the instrument panel. The replacement of such a fuse, of which a number of reserves must be available on board, is a rather unpractical affair, particularly if such a replacement is to be carried out while airborne. Therefore, it is preferable to do this while safely at a standstill on the ground, unless in the unlikely case that such a replacement during flight would be an absolute necessity for safety reasons.

Burned out fuses left aside, and which cannot be instantly replaced anyway, whenever a circuit-breaker pops out or that a circuit-breaker/switch moves by itself to the off position, it is allowed to reset it once, after a cooling period of about 2 minutes. If it does not hold after the resetting, it should be left alone for the remainder of the flight, and the associated circuit is to be considered as unserviceable. A circuit-breaker should in no case be forced in the operating position: this would almost certainly give way to a fire.

Prior to start the engine, the pilot must ensure that all circuit-breakers located on the instrument panel are in. As far as
fuses are concerned, he should simply verify that no one is missing (there is no need to unscrew them to verify their condition, although doing so is certainly not a bad idea) and check that replacements are available on board.

To conclude, note that any time that an electrical appliance remains inoperative, first action is to verify whether it is not due to a popped circuit-breaker or burned out fuse. If this is the case, the problem can often be corrected by resetting or replacing the involved item.

THE AMMETER

The most usual indicator related to the electric system is the ampèremeter, or ammeter, which shows the intensity of the current in ampères, or amps.

Two types of ammeters are available: one whose graduation goes from zero to a maximum value, the other whose zero value is in the center of the indicator with a "-" indication on the left side, and a "+" indication on the right side. For convenience, we will call the latter the "+/-" ammeter.

The first type shows the total intensity of the required current and is connected downstreams of the generator/alternator. The "+/-" ammeter is connected between the battery and the bus bar: it shows the intensity of the current from the generator/alternator to the battery with "+" or, assuming that the generator/alternator is inoperative, from the battery to the bus bar with "-", i.e. it shows a charge (+) or a discharge (-) of the battery. Usually, the first type of ammeter is usually found on Piper aircraft, the second one on Cessna aircraft.

When the engine is shut down, and that the battery switch is in on position, the first type of ammeter shows exactly the same as when the battery switch is turned off: zero. None the less, when the battery switch is turned on, current flows from the battery to a number of appliances and, as the engine is not running, the battery gradually discharges. Under similar circumstances, the "+ / -" ammeter will deflect to "-", and more so as more appliances are put in operation. In fact, the "+/_11 ammeter provides more information regarding battery discharge than the graduated ammeter, as the latter always shows zero, whether a low or high rate of battery discharge is in progress.

After engine starting, and assuming that the generator/alternator is put on line, the graduated ammeter will usually show a rather high value. This indicates that the generator/alternator is operating properly and that the battery, which can be considerably drained by the engine start, is re-charging. As the battery charge reaches its normal value, the ammeter's indication decreases and stabilizes at a value corresponding to the number and current requirement of the various operating appliances. Indeed, as far as the graduated ammeter is concerned, the more appliances are in operation, the more current is to be supplied to the bus bar, and the more current must be supplied by the generator/alternator to maintain the battery's charge.

If a "+/-" ammeter is in use, it will also initially show a considerable deflection towards "+" after engine starting and that the generator/alternator is put on line, thus indicating the charging of the battery: this deflection will gradually decrease, and the needle will soon stabilize close to the central zero value, slightly deflected towards "+". This is the normal indication of a "+/_11 ammeter during all operations.

THE VOLTMETER

Some installations include a voltmeter, although this indicator is not really necessary on single engined aircraft.

 

We know that, whereas the ammeter shows the current intensity, the voltmeter indicates a potential difference, or voltage (spanning/tension). In a single engined aircraft, and if present, it usually refers to the battery voltage and provides an additional verification of the voltage regulator's operation and of the ammeter's.

The voltmeter becomes more useful on multi-engined aircraft (although it is not necessarily installed), as it can indicate a voltage difference between both generators/alternators, thus a difference in load sharing, a situation which is detrimental for a number of reasons.

THE WARNING LIGHT SYSTEM

The warning light system is usually composed of a single red light (two on twin engined aircraft), labeled "ALT" or "LOW VOLTAGE", which illuminates any time that the battery is switched on and that the generator/alternator is not operating. In other words, it indicates that only the battery is feeding the network, either because:

a) the generator/ alternator switch is off;

b) the engine RPM is too low and that the output of the generator/alternator is insufficient, a normal occurrence during taxi at low power, particularly at night when panel lighting, navigation lights and taxi/or landing lights are in use;

c) of a malfunction in the system.

Sometimes an additional warning light labeled "HIGH VOLTAGE" is available. lt illuminates only if a surge in the generator/ alternator output causes the voltage to exceed a predetermined value, in which case the generator/alternator is automatically disconnected from the network to prevent damage to the various electrical circuits. Note that, assuming that such an occurrence has taken place, the "ALT" or "LOW VOLTAGE" light will illuminate as well.

SYSTEM MALFUNCTIONS

Electrical system malfunctions are, just as any other malfunction, a rather rare occurrence. In addition, and as far as the private pilot operating only in VMC is concerned, such problems including the complete loss of the electrical system, do not imply very dramatic consequences. This is probably the reason why the electric system is often somewhat neglected during the elementary flight training. However, when it comes to IFR operations, such malfunctions are likely to become an entirely different story, as indeed flying IFR heavily relies on a properly operating electrical system: a major failure in this concern occurring in IMC may lead to tragedy for the unprepared pilot.

Electrical malfunctions may vary from a popping circuit-breaker or burned out fuse to a major problem involving the cockpit being filled during flight with electrical smoke from unknown origin. It is essential that the IFR rated pilot is cognizant with the recommended procedures regarding electrical problems as laid down in the POH of his particular aircraft ... and that he knows a little more about the subject in order to be able to take proper measures if ever he is confronted with a major problem.

We discussed already the case of the popped circuit-breaker or burned fuse. We discussed also the case of the "dead battery" when on the ground. What else could happen, particularly during flight?

1) Generator/alternator failure

This may be due a loose or broken driving belt (rather unlikely), or to an internal failure of some kind, such as for instance a transient overvoltage condition. At any rate, this will cause the battery to be the sole supplier of electric power and is indicated by the illumination of the "ALT" or "LOW VOLTAGE" warning light, and by the ammeter showing either zero, or a discharge (-).

First thing to do is to check the generator/alternator circuit-breaker and to reset it if it has popped out. Assuming that the circuit-breaker is still in, try to put the generator/ alternator back on line. To this purpose it is recommended to reduce the electrical loads as much as possible, then to reset the generator/alternator switch by recycling it from ON to OFF and back to ON again. If the procedure proves successful, as indicated by the ammeter showing anew a charge and the warning light extinguishing, the flight may be theoretically be continued to the intended destination. None the less be suspicious and, particularly at night or in IMC, it is strongly recommended to avoid too much electrical load and to land at the first suitable airport where the system can be checked out by an approved maintenance shop.

If the procedure mentioned hereabove turns out to be unsuccessful, the generator/alternator switch should be switched off definitively, thus leaving the battery as sole power source: all non-essential electric equipment should then be switched off in order to spare the battery. This may simply be a mere annoyance for a VFR/VMC flight. For IFR/IMC operations, and particularly in solid IMC conditions, such a situation becomes a major emergency. Indeed, remember that the battery will hold only for a restricted period of time: this means that you must imperatively land as soon as possible, using the absolute minimum of radios, navaids and other electrical appliances. An emergency must immediately be declared to the ATC: "MAYDAY, MAYDAY, MAYDAY, THIS IS OO-XXX, ELECTRICAL FAILURE", after which the following way of action is suggested:

a)  Inform the ATC of your intentions (proceeding to your destination, or diverting to another airport), and of the fact that your transponder might stop operating;

b)  Assuming that the forecasted weather is more or less satisfactory, you might consider to simply switch off the battery and proceed to your initial destination, using only time and heading (this is why a good keeping of the flight-log is a must). On the way, you still have the possibility to put the battery back on line once in a while for a brief position check and/or ATC report. Acting this way might ensure sufficient battery power available at arrival for an instrument approach, if necessary;

c) Assuming that you are not too far away from a radar equipped airport, request vectoring to bring you in. This means that you have to keep the battery switch on: switch every- thing else off (including external lights at night, and use your flashlight to illuminate the instrument panel) except for one VHF transceiver and the transponder. You might even consider to notify the ATC that you will switch off the whole electric network between each subsequent call for a radar heading, until close to the airport.

2) Abnormal high ammeter reading - Overload

An overload is indicated by an abnormal high ammeter reading. We discussed already the case of the ammeter reading following the engine start: in this case, the temporary high ammeter reading is perfectly normal and should stabilize to a normal value within the next 5 minutes at most. After engine start, it is important to verify that this stabilization has indeed occurred: if not, it indicates a problem and the flight should be cancelled for maintenance check. Let us take this opportunity to stress that any engine start consumes a lot of electrical power and, assuming that the engine does not fire fast enough for some reason, keeping the starter engaged, or using it repetitively, for a prolonged period of time may quickly lead to the complete draining of the battery, not to speak about the harmful overheating of the starter itself.

The starting procedure left aside, we have seen that an excessive output of the generator/alternator automatically causes its disconnection from the network, the illumination of the "LOW VOLTAGE" warning light and a zero, or discharge reading on the ammeter. Assuming that a "HIGH VOLTAGE" light is also available, it will illuminate as well, thus notifying the pilot that the generator/alternator has been lost due a current surge rather than a mechanical failure. If automatic disconnection has occurred, the cause may be of transient nature and there is a fair possibility that the procedure mentioned in (a) above will be successful.

The automatic disconnection is supposed to occur at a predetermined value. However, it may happen that this relay fails to operate and that the ammeter shows a continuous excessively high rate of charge: this would rapidly give way to a dangerous overheating condition of the battery. Another possibility is that, due to a failing voltage regulator, a continuous abnormal high rate of charge occurs, albeit without exceeding the value at which the disconnection is activated: this situation would be equally harmful for the battery in the long term. In fact, a number of reasons may cause an abnormal high ammeter reading, thus an overload. If such an ammeter indication is noticed, the procedure to be followed depends on the design of the electric system, particularly on whether or not the battery and generator/ alternator switches are interlocked. In both cases, first action is to reduce the number of electrical consumers: if this contributes to bring the ammeter reading within a more normal value, the flight may be continued, keeping both the generator/alternator and the battery available, but with reduced availability of the various electric appliances. None the less, and particularly for night flights or IFR operations, diverting to the nearest suitable airport is strongly recommended.

 

If no decreased ammeter reading results, proceed as follows:

 

a) Interlocked system

In the interlocked system, the only option is to switch off the generator/alternator, thus leaving only the battery as sole power source: you are thus in the situation described in (a) above. Although most checklists do not mention the possibility, it must be remembered that the generator/alternator has simply be switched off but that it is still operative, albeit producing an overload: in the worst case, according to the principle that desperate diseases need desperate remedies, one might consider to re-activate the generator/alternator, at least intermittently, if absolutely necessary.

b) Non-interlocked system

The battery and generator/alternator switches are not always interlocked: in some installations they are fully separated, and even some split type master switches allow independent operation of both sides, i.e. the generator/alternator is able to supply the bus bar with the battery switch in off position. If such is the case, the battery should be switched off: doing so should cause the ammeter reading to decrease, in which case it is still recommended to keep the amount of electrical consumers to a minimum. The flight may be continued, but diverting to the nearest suitable airport is again strongly recommended.

It should be noted that, in installations allowing the generator/alternator to supply the bus bar, as the generator/alternator's voltage is slightly higher than the battery's, this increased voltage is not very sound, neither for the radio equipment, nor for the electric system as a whole. This is one more reason to avoid operating too long with the battery switch off.

If switching the battery off does not reduce the ammeter reading, the generator/alternator should be switched off and the battery switched back to on: once again, the battery becomes the sole power source.

3) Electrical smoke or fire

Although some checklists may be a little more explicit, most so-called ELECTRICAL SMOKE OR FIRE procedures imply only the most basic actions: master switch to be turned off in order to stop the smoke, cabin vents to be turned open, heating to be turned off in order to avoid possible electric smoke in the engine compartment to enter the cabin, and to land as soon as possible. Applying such a simplified procedure during an IFR flight in solid IMC does not solve a lot: it simply leaves you "in the soup" without radio, without navaids, without transponder, without navigation lights, without stall warning, usually without turn indicator, and also without Pitot heater, which might lead to an additional loss of the airspeed indicator. In other words, an electrical smoke (or fire - see "additional notes" below) can be an extremely serious emergency, so let us discuss it some more.

Firstly, note that smoke of electrical nature produces a typical and easily recognizable smell and, if corrective action is unduly delayed, may give way to a subsequent fire. Obviously, assuming that the culprit can be detected immediately, simply turn it off by means of the associated switch and/or circuit- breaker (this is why push-pull circuit breakers are much more interesting than push-to-reset types).

One or more circuit-breakers may pop out in association with an electrical smoke occurrence. This will probably stop any further smoke development and these circuit-breakers should in no case be pushed in again: the definitive loss of the associated equipment must be taken into consideration.

Electrical smoke can also occur without any circuit-breaker popping out. If the origin of the smoke cannot be readily determined, act as follows:

a)  Prepare yourself to continue to navigate temporarily by time and heading only. Again, time and conditions permitting, transmit the emergency call "OO-XXX, MAYDAY, MAYDAY, MAYDAY, ELECTRICAL FIRE", then turn off both the battery and generator/alternator switches without further delay;  

b)  With no electrical power at all, the smoke will usually stop and dissipate. Turn off all the individual switches. Assuming that the circuit-breakers are of the push-pull type, pull them out as well. However, be aware that some equipment remains supplied by the battery even with the master switch off and might be the cause of the smoke, particularly if the associated circuit is located in the engine compartment and that the cabin heat is operating: in this case the smoke might continue to enter the cabin (possibly even with the cabin heating valve in closed position if the system is not absolutely tight) , and you have no other option but let it happen: the chances for a serious fire to develop are rather remote (although not entirely excluded) but ensure that the cabin heat is in closed position and that the ventilation is open.

c) The next step is to re-activate all circuits one by one, allowing a pause of a few minutes between each other to ensure that the smoke does not re-occur. The first item to be selected is obviously the battery switch. If smoke re-occurs, the situation depends again on whether or not the master switch is of the interlock type:

- Interlocked system

In this case, the situation is likely to become critical, particularly if no push-pull circuit breakers are available (which is unfortunately often the case). Indeed, the smoke emission might originate from the battery or from an appliance which has no individual switch (such as the fuel gauges), and if the circuit-breakers are of the push-to-reset type they cannot be pulled. In both these cases only two options, both equally questionable are available:

+ either to switch off the battery and continue the flight according to the flight-plan, using time and heading, and hoping to find improving weather conditions later on;

+ or to leave the battery on, to notify the ATC of your decision to divert to a nearby radar equipped airport, then to proceed to your new destination, switching the battery off between each subsequent call for radar vectors, hoping that the battery will hold (you can always re-engage the generator/ alternator if absolutely necessary), that the smoke will not become suffocating, and that no fire will result.

Assuming that push-pull circuit-breakers are available, and that you pulled them out during the initial step, the smoke re-occurrence is definitely coming from the battery or its wiring. You are again to choose between the two options mentioned hereabove. However, re-check that the circuit-breakers are indeed pulled out: maybe you overlooked the one which causes the smoke.

- Non-interlocked system

As this system allows to use the generator/alternator to supply the necessary DC, select the generator/alternator to ON then, and only then, switch off the battery (see further above).

d) Assuming that, with the battery switch on, no· smoke re-occurs, engage the generator/alternator: if no smoke, things are brightening. If smoke re-occurs, select the generator/alternator switch off again: you are back is the situation where- by the battery is the sole power source.

e) Assuming that both the battery and the generator/alternator become available, select the appliance which you judge the most urgent. Smoke? Switch if off immediately! And so on, until you have retrieved at least the most essential equipment.

Additional notes

- An electrical fire in the cabin is usually preceded by smoke: normally speaking an actual fire will only occur if no action is rapidly taken to stop the smoke. Cabin fire is obviously a life threat but on the other hand keep in mind that using the fire extinguisher (as may be called for by the check- list) can be equally threatening if the chemical known as Halon is used. Indeed, the property of Halon is to eliminate the oxygen to stop the fire ... but eliminating the oxygen in the confined cabin area may also dangerously affect the occupants. In other words, if the situation degrades to the extend that a fire must be extinguished, try to smother it using a piece of clothing or anything similar, rather than using the fire extinguisher.

- Hair raising situations such as the total loss of electrical power, or electrical smoke or fire in IFR/IMC, are extremely remote. But, if ever they happen, your only salvation is to REMAIN CALM: this is the only way to take you out of your predicament ..., and remember that, if you have passengers on board, a good part of your energy might be required to avoid panic on board.

OUESTIONARY

1. The positively charged particles orbiting around an atom are known as ........; negatively charged particles are known as .....

2. Electrical condutors include a significant number of electrons known as ......

3. In an electrical circuit, the current flows: a) from the positive electrode to the negative electrode, b) from the negative electrode to the positive electrode.

4. The positive electrode in a circuit is known as ......; its negative counterpart is known as .....

5. The potential difference between the two electrodes in an electric circuit is measured in .....

6. The intensity of an electric current is measured in ......

7. Any conductor offers a certain resistance to the passage of the electric current: this resistance is measured in .....

8. State the basic formula of Ohm's law.

9. Ohm's law is applicable to all type of conductors. True or false?

10. Navaids are often stacked upon each other. Which precaution does this require?

11. The aircraft's battery produces: a) direct current, b) alternating current.

12. What is the difference between alternating current and direct current?

13. What is the main advantage of using DC on light aircraft?

14. Lead batteries are composed of cells immersed in a solution of water and sulphuric acid known as ......

15. The acid solution in a lead battery causes a phenomenon known as .....

16. The chemical reaction in a lead battery produces negatively or positively charged atoms known as ....

17. The potential difference which is chemically obtained between the battery's cathode and anode is known as EMF, i.e. .....

18. State the EMF of the battery fitted in your training aircraft (POH).

19. What causes the gradual evaporation of the electrolyte in a lead battery?

20. Lead batteries are unfit for aerobatic aircraft. Why?

21. When the battery switch is in 'off' position, all electrical appliances are inoperative. True or false?

22. When the battery switch is in 'on' position, a number of appliances are automatically supplied. True or false?

23. Assuming that the whole electric system is powered by the battery only, how much time may reasonably be expected before the battery is fully discharged?

24. The ratio master switch (if present) connects the ..... bus to the ...... bus.

25. What do you understand by a 'bus bas'?

26. When is it particularly recommended to use an external power source?

27. Although the network is operating on DC, the battery is recharged by an alternator or generator producing AC. Explain this 'discrepancy'.

28. As far as light aircraft are concerned, what is the difference between a generator and an alternator?

29. Assuming a 'dead battery', and assuming that the engine can be started by swinging the propeller, the electric system can always be restored once the engine is running. True or false?

30. State the purpose of the voltage regulator.

31. What do you understand by an interlocked master switch.

32. State the purpose of the circuit-breakers.

33. Any individual switch is a circuit-breaker. True or false?

34. State four types of circuit-breakers.

35. If a circuit-breaker pops out, you should reset it after a cooling period of .....

36. Following the first reset of a circuit-breaker, it pops out again. What should you do?

37. Assuming that the battery is the only source of power, the ammeter always indicates a discharge. True or false?

38. The ammeter might show an unusual high rate of charge. When is such a situation to be considered as normal?

39. An electrically operated appliance turns out to be inoperative. What is the first thing to verify?

40. The low voltage light illuminates. This always indicates an inoperative generator/alternator. True or false?

41. The generator/alternator is inoperative. Which electrical appliances are lost?

42. Assuming the presence of both a low and high voltage warning light, both are illuminated, this happens when: a) the engine is not running, b) a mechanical failure of the generator/alternator, c) a short-circuit in one of the electrical appliances, d) none of these.

43. Some electrical systems can be supplied by the generator/alternator only, with the battery off. True or false?

44. The low voltage light illuminates during flight. No high voltage lights is available. State your way of action.

45. Both low voltage and high voltage lights illuminates during flight. State your way of action.

46. You notice electrical smoke of unknown origin during flight in solid IMC. You switch the master switch off but the smoke continues. What is happening? What can you do about it?