Fuel + Fuel Systems

Question 1

As mixture is progressively leaned from full rich condition, the engine will experience
[a] best power, then rough running, then peak exhaust gas temperature
[b] best economy, then best power, then rough running
[c] best power, then best economy, then rough running
[d] peak exhaust gas temperature, then best power, then best economy

Answer 1

C
The extra fuel supplied at the fully rich mixture setting assists in cooling the engine,
but it also reduces the combustion temperature. As mixture is leaned from fully rich
with a constant throttle setting, power output increases to produce best power.
Further leaning causes a drop in power, but also a drop in fuel consumption. Economy,
ie miles per gallon, increases until best economy is achieved. Finally, after the peak
combustion temperature is reached, the mixture becomes too lean to burn, producing
lean misfire.

Question 2

If mixture is correctly set to peak EGT, moving the mixture control to fully rich with throttle setting unchanged,
will cause power output to
[a] decreases continuously
[b] increase then decrease
[c] increase continuously
[d] decrease then increase

Answer 2

B
Best power always occurs between peak exhaust gas temperature and fully rich. Power
output will increase until best power is achieved, then it will decrease as the mixture
control continues to move toward fully rich.

Question 3

If mixture is correctly set for best power, an increase in altitude with no adjustment in mixture or throttle position
will produce
[a] a richer mixture and less power
[b] a leaner mixture and less power
[c] no change in mixture strength
[d] a richer mixture and more power

Answer 3

A
Efficient combustion requires the correct ratio between fuel and air. Increased altitude
reduces the mass of air induced because the outside air density is lower. If the amount
of fuel metered is not reduced, the mixture will become richer and power will reduce.

Question 4

Operating an engine with high power and the mixture very lean can cause
[a] plug fouling and overheating
[b] backfiring and overheating
[c] backfiring and low temperatures
[d] a decrease in indicated air speed and low temperatures

Answer 4

B
Lean mixtures take longer to burn. The slow burning charge is pushed back up into the
cylinder head during the exhaust stroke, increasing engine temperature. As the inlet
valve opens for induction, the still-burning charge ignites the fresh mixture in the inlet
manifold causing backfiring.

Question 5

On application of carburettor heat when ice is present, the manifold pressure will
[a] drop initially, then slowly increase
[b] rise initially, then slowly decrease
[c] drop continuously
[d] drop initially and remain at the lower value

Answer 5

A
Ice takes some time to melt. Initially, the engine will suffer from hot air and ice. The
manifold pressure will drop and then begin to rise again as the ice melts. The manifold
pressure will return to normal when the carburettor heat is returned to the cold position
after the ice has melted.

Question 6

For take off under very high altitude conditions
[a] the mixture must be left fully rich
[b] the mixture must be leaned to peak EGT
[c] the mixture should be leaned to best economy
[d] fully rich may be too rich and the mixture should be leaned to smooth running

Answer 6

D
A high strip and a hot day combined with fully rich mixture at take-off power, can
produce a mixture which is far too rich. Some leaning may be required to restore
smooth running. This should only be considered in extreme cases and then only after
checking the manufacturers recommendations.

Question 7

If throttle and mixture levers remain unchanged during a long climb from sea-level, the power available will
decrease because
[a] the decreased air density produces a leaner mixture
[b] volumetric efficiency reduces and mixture becomes too rich
[c] a richer mixture produces rising cylinder head temperatures
[d] decreased air density provides too little cooling of the engine

Answer 7

B
The reducing ambient air density reduces the mass of air induced during the induction
stroke, ie the volumetric efficiency is reduced. If no adjustment is made with the
mixture control, the resulting mixture will be too rich.

Question 8

Take off power is
[a] the same as maximum continuous power
[b] higher than maximum continuous power but may be used providing the mixture is fully rich
[c] used for short periods such as at take off with a time limit of about three minutes
[d] used only during an emergency take off

Answer 8

C
Take off power represents more than 100% power. Because of the increased engine
temperatures and decreased cooling airflow at take off, a time limit is normally
imposed.

Question 9

The purpose of a booster pump is
[a] to provide extra power at take off
[b] to transfer fuel from one tank to another
[c] to prevent the mixture from becoming too lean at high altitude
[d] to guard against fuel vapour accumulation [ vapour locking]

Answer 9

D
Booster pumps assist the fuel to flow through the lines without breaking up into
bubbles of vapour. This is especially likely when demand is high and the fuel is warm.

Question 10

One disadvantage of a carburettor as compared to a fuel injection system is
[a] uneven mixture distribution to the cylinders
[b] it is prone to fuel vaporisation
[c] it requires a high pressure prime pump for starting
[d] its relatively high cost

Answer 10

A
Low atmospheric pressure also encourages vapour formation.
The carburettor mixes fuel and air some distance away from the cylinders. Initially,
much of the fuel is still in the form of liquid droplets. If some of these droplets stick to
the walls of the pipes, the mixture strength can change on route to the cylinders.
Because the induction pipes running to the cylinders from the carburettor are not all
the same length, mixture strength can vary from cylinder to cylinder [not a serious
problem in smaller engines].

Question 11

If an engine is flooded and difficult to start, an appropriate technique would be
[a] crank continuously and swear like mad
[b] mixture fully rich, throttle closed and crank
[c] mixture to idle cut off, throttle wide open, crank and re-set after start
[d] crank initially with magnetos off, then turn magnetos on

Answer 11

C
If mixture is too rich to burn, the situation can be remedied by cutting off the fuel
supply and providing maximum airflow. Again the manufacturer’s recommendations
should be considered and use of the starter motor should be limited to short periods to
prevent overheating.

Question 12

Application of carburettor heat when no ice is present on an engine fitted with a CSU will be accompanied by
[a] an initial drop, then an increase in manifold pressure at constant RPM
[b] a drop in manifold pressure and RPM
[c] a drop in manifold pressure with constant RPM
[d] an initial drop, then an increase in manifold pressure and RPM

Answer 12

C
The initial drop is caused by the introduction of hot air into the induction system. If no
ice is present, the drop will be observed while ever the carburettor heat is applied.
There will be no subsequent change in manifold pressure due to melting ice. The
propeller governor will maintain constant RPM throughout.

Question 13

The colour of pure gasoline is
[a] green
[b] red
[c] blue
[d] clear

Answer 13

D
Pure gasoline is actually a colourless liquid.

Question 14

The colour of 100/130 AVGAS is
[a] green
[b] red
[c] blue
[d] clear

Answer 14

A
The colour is a dye introduced to assist in visual identification.

Question 15

The colour of 100 octane low lead aviation fuel is
[a] green
[b] red
[c] blue
[d] clear

Answer 15

C
The colour is a dye introduced to assist in visual identification.

Question 16

One difference between an exhaust gas temperature gauge and a cylinder head temperature gauge is that
[a] the exhaust gas temperature gauge responds more slowly to combustion temperature change
[b] the cylinder head temperature gauge responds more slowly to combustion temperature change
[c] the exhaust gas temperature gauge is always colour coded
[d] the cylinder head temperature gauge is a better indicator of mixture condition

Answer 16

B
The cylinder head temperature gauge is connected to the cylinder body, usually at one
of the spark plugs. A large metal object such as a cylinder, cannot suddenly change its
temperature, so changes in combustion temperature take time to be reflected as
changes in cylinder head temperature. The exhaust gas temperature gauge [EGT], on
the other hand, senses temperature via a thin metal probe in an exhaust pipe. It is able
to change its temperature almost immediately the temperature of the exhaust gas
changes. This gives the pilot a much more direct indication of mixture condition.

Question 17

The theoretical maximum range [nil wind], for a piston engine aeroplane is obtained by operating at the IAS that produces the best lift/drag ratio and flying:
[a] as low as possible
[b] as high as possible
[c] at full throttle height
[d] with fully rich mixture.

Answer 17

C
Full throttle height is the height at which the use of full throttle produces the required
power. Since most light aircraft enjoy the best lift/drag ratio at a speed of about 65 or
70 knots [best gliding speed], full throttle height would be the height at which the use
of full throttle produced that IAS. This is purely a theoretical consideration as the fuel
used to climb to such a height would probably be more than you would save by being
there!

Question 18

The fuel flow gauge on many general aviation aeroplanes actually measure fuel pressure with a transducer and
indicate an equivalent fuel flow. One consequence of this is
[a] the gauge is absolutely reliable as an indicator of fuel flow since fuel flow and fuel pressure must
always be proportional to each other
[b] the gauge does not measure fuel flow directly and may give incorrect readings if the filters or
discharge nozzles become blocked.
[c] the gauge is absolutely reliable at normal power setting but may be incorrect at high power
[d] the gauge is likely to over read at high power settings

Answer 18

B
Many small engines use a fuel flow gauge which actually measure fuel pressure and
displays it on a gauge calibrated in flow. Such a system assumes that the fuel flow is
proportional to fuel pressure. This is a pretty safe bet provided the size of the orifice in
the injector nozzle or fuel lines remains the same. Any partial blockage due to dirt will
cause pressure to rise while flow actually falls. The pilot will usually notice a fluctuation
in fuel pressure.

Question 19

When using the carburettor heat control on an engine not fitted with a CAT gauge you should -
[a] use only enough heat to melt the ice.
[b] use partial heat throughout the flight to prevent ice from forming.
[c] use full heat only if the engine is at low power or at idle.
[d] use full heat when gliding or when symptoms are noticed and then return to the ‘off’ position.

Answer 19

D
The correct technique without a CAT gauge is to wait until the symptoms appear and
then apply full carburettor heat to melt the ice. Return the control to ‘off’ and repeat if
symptoms reappear. It is good practice to apply full heat whenever the aircraft is
gliding as a preventitive measure.

Question 20

The flight manual specifies that the fuel required for the aircraft is 100LL. During a daily inspection you notice
that the fuel obtained in the fuel drain is red. Engine operation with this fuel would be:
[a] normal since 100LL should be that colour.
[b] likely to result in reduced engine performance and detonation.
[c] likely to result in oil fouling of the spark plugs.
[d] likely to result in overboosting and engine damage.

Answer 20

B
A red colour indicates that the fuel is Mogas. This is normal motor car fuel and is not
permitted for use in aircraft engines unless it is specifically indicated in the aircraft’s
flight manual. If Mogas is used in an engine that normally requires 100LL, reduced
performance and detonation is likely.

Question 20

Fluctuating fuel pressure indications when all other instrument indications are normal is a likely indication of;
[a] fuel vaporization
[b] detonation
[c] carburettor ice
[d] overboosting

Answer 20

A
As the vapour pockets pass through the system the fuel pressure falls to zero. It then
returns to normal when fuel passes through. This causes fluctuating pressure readings.

Question 20

The colour of 100 octane low lead aviation fuel is
[a] green
[b] red
[c] blue
[d] clear

Answer 20

C
The colour is a dye introduced to assist in visual identification.

Question 21

Throttle ice is the name given to ice that forms as a result of:
[a] adiabatic cooling due to the sudden pressure drop across the throttle butterfly
[b] supercooled water droplets striking the external components of the induction
system
[c] the cooling effect of evaporating fuel
[d] flying when the outside air temperature is below freezing.

Answer 21

A
Adiabatic cooling occurs when a gas suffers a sudden pressure drop. The pressure
behind the throttle is much lower than the pressure before it - especially when the
throttle is closed as in a glide descent.

Question 22

Vaporising of fuel in the fuel lines (vapour locking), can be caused by
[a] high engine temperatures and high power
[b] excessively lean mixtures
[c] using the boost pump at low engine power
[d] using rich mixtures at high altitudes

Answer 22

A
The hotter the fuel and the faster it is pulled through the fuel lines, the more likely it is
break down into vapour.

Question 23

If an engine begins to run roughly because of vaporisation of fuel during flight the pilot should
[a] operate the boost pump
[b] richen the mixture
[c] check that the boost pump is turned off and reduce power
[d] place the mixture in idle cut-off and turn the boost pump on

Answer 23

A
The boost pump reduces the stress on the fuel as it flows through the lines, preventing
it from breaking down into bubbles of vapour.

Question 24

Carburettor refrigeration ice is most likely to form
[a] only with outside air temperatures below freezing
[b] only with outside air temperatures above freezing
[c] behind the throttle
[d] around the fuel nozzle

Answer 24

D
Refrigeration ice is caused by the evaporation of fuel. It forms around the carburettor’s
fuel discharge nozzle where most of the evaporation is occurring.

Question 25

A
On application of carburettor heat in an aircraft with suspected carburettor icing, the engine immediately starts
misfiring and running roughly. The correct action would be
[a] leave the carburettor heat on allowing the ice to melt and the engine to return to normal once
the ice has dissipated
[b] lean the mixture because the application of carburettor heat has richened the mixture
[c] richen the mixture to overcome the loss in power caused by the rough running
[d] return the carburettor heat to the off position and descend to a lower altitude and warmer air

Answer 25

A
The ice doesn’t melt immediately so for a few seconds you have ice and hot air. This
causes the engine to lose even more power and run more roughly until the ice has
melted - usually about 5 to 10 seconds.

Question 26

Yellow band on carby air temperature gauge indicates ice is
[a] about to start
[b] possible
[c] well advanced
[d] unlikely

Answer 26

B
The yellow band on the CAT gauge indicates the temperature range where ice is likely.
No instrument can indicate ice is certain to occur just as no instrument can indicate
that it is impossible. The green band on the CAT indicates that ice is not likely.

Question 27

The onset of fuel vapour locking in an engine would be indicated by
[a] failing manifold pressure
[b] fluctuating RPM only
[c] fluctuating fuel pressure
[d] high fuel consumption

Answer 27

C
As the vapour bubbles pass through the lines the fuel pressure drops to zero. When fuel flows it returns to normal. The gauge will flick between zero and normal indication producing a fluctuating reading.

Question 28

Vaporisation of fuel would be most likely to occur, at high power settings
[a] with hot fuel and low atmospheric pressure
[b] with cold fuel and high atmospheric pressure
[c] with hot fuel and high atmospheric pressure
[d] with cold fuel and low atmospheric pressure

Answer 28

A
All liquids will evaporate more readily when the ambient atmospheric pressure is low.
Heat of course will also promote evaporation.

Question 29

If fuel vaporisation occurs during flight, the appropriate pilot action would be
[a] place the mixture control to full rich
[b] increase engine RPM
[c] operate the fuel boost pump
[d] change fuel tanks

Answer 29

C
The boost pump is always the best remedy for fuel vaporisation.

Question 30

If a forward facing fuel tank vent is bent through 180 degrees so that it is facing backwards
[a] the flow of fuel will be hindered by reduced pressure in the tank
[b] the mixture will become too lean
[c] the engine will not develop any power
[d] fuel could leak from the tank through the vent

Answer 30

A
A forward facing vent is designed to catch the on-comming air flow and direct it into
the space above the fuel. This produces a slight positive pressure which helps the fuel
to flow freely. If it was bent so that it faced backwards it would create a suction which
would reduce the fuel’s ability to flow freely.

Question 31

A blocked fuel injector in an aircraft fitted with a pressure transducer fuel metering system can be indicated by
[a] a higher than normal fuel pressure reading
[b] a higher than normal Exhaust Gas Temperature
[c] a lower than normal fuel flow reading
[d] a lower than normal fuel pressure reading

Answer 31

A
The blockage makes it more difficult for the fuel to flow through the lines. If the pump
tries to push it and it can’t get out pressure is built up in the lines.

Question 32

Carburettor icing in an engine fitted with a CSU can be detected by which of the following cockpit indications
[a] loss of RPM, but unchanged MP
[b] loss of RPM, but slightly increased MP
[c] loss of MP, but unchanged RPM
[d] loss of MP and RPM

Answer 32

C
Ice restricts the flow of air through the carburettor so less air gets through into the inlet
manifold. Manifold pressure drops. This means the pistons are being pushed with
less force so the power drops. If nothing is done with the propeller control RPM will
remain constant.

Question 33

Which of the following is the most likely consequence of using too low a grade of fuel in an aero-engine
[a] increased lead fouling problems
[b] slightly increased power output with fewer lead fouling problems
[c] detonation and power loss
[d] increased susceptibility to fuel vaporisation or icing

Answer 33

C
Octane rating is a measure of the fuel’s ability to withstand heat without detonating. If
the octane rating is lower than that recommended by the manufacturer, the possibility
of detonation increases.

Question 34

The most likely cause of icing in a float type carburettor is
[a] low to medium power operations on warm humid days
[b] high power operations on days when air temperature is around 15°C. with high relative
humidities
[c] very cold sub zero air temperatures
[d] partial or fully closed throttle operations on a very hot day

Answer 34

A
When the engine is operating at low power the throttle butterfly valve is partly closed.
This produces a rapid adiabatic expansion of gas as it flows past the throttle. This
expansion causes a temperature drop increasing the risk of icing. High humidity pro
vides the other ingredient required - water vapour.

Question 35

Which of the following would be the most likely cause of a fluctuating fuel pressure indicator
[a] a fuel pump seizure
[b] a sticking carby inlet valve
[c] a blocked fuel tank vent
[d] an fuel vapour lock in the fuel line

Answer 35

D
As the vapour bubbles pass through the lines the fuel pressure drops to zero. When
fuel flows it returns to normal. The gauge will flick between zero and normal indication producing a fluctuating reading.

Question 36

The presence of carburettor icing may be verified by maintaining constant throttle and propeller pitch setting,
then applying carby heat and noting an immediate
[a] increase in MP, with no further change
[b] increase in MP, then a gradual decrease
[c] decrease in MP, with no further change
[d] decrease in MP, then a gradual increase

Answer 36

D
Manifold pressure drops further at first because the engine is coping with both hot air
and ice. When the ice melts, the manifold pressure increases.

Question 37

In which of the following operational conditions is the formation of carburettor ice most likely
[a] maximum rate climb
[b] low power descent
[c] taxi
[d] high speed level cruising

Answer 37

B
When the engine is operating at low power the throttle butterfly valve is partly closed.
This produces a rapid adiabatic expansion of gas as it flows past the throttle. This
expansion causes a temperature drop increasing the risk of icing. High humidity pro
vides the other ingredient required - water vapour.

Question 38

A fuel tank vent becomes blocked in an aircraft with a pump-fed fuel system. This
[a] may cause excessive fuel flow by increased fuel pressure
[b] cannot cause any fuel system abnormality as long as the pump is
working properly
[c] may cause tank to collapse by reduced tank pressure
[d] may cause fuel tank leaks due to increase tank pressure

Answer 38

C
Some vent designs require a fairing to be placed over the opening to prevent suction
occurring as the air passes over it. Fuel tanks which contain a rubber cell can collapse
if the pressure within the tank drops low enough.

Question 39

Select the conditions most likely to cause impact ice to form in the induction system
[a] operation in high humidity with the throttle partly closed and carb
urettor heat off
[b] gliding in the vicinity of cloud with the throttle fully closed
[c] cruising in clear humid air
[d] flying in rain with an outside air temperature of -3°C

Answer 39

D
Impact ice forms when supercooled water droplets impact with the induction system.
It requires liquid water [rain or cloud] to be present in temperatures below zero. This
type of induction system icing is likely to form on the air intake and air filter. It is not
possible to remove it with carburettor heat.

Question 40

If an aircraft fitted with a rubber fuel cell suffers a blockage of the fuel tank vent, continued operation may cause
[a] the pressure in the cell to increase causing the cell to collapse
[b] the pressure in the cell to decrease causing the cell to collapse
[c] the pressure in the cell will increase causing the cell to inflate
[d] the cell may collapse and send too much fuel to the engine

Answer 40

B
The rubber cell is not a rigid structure so as fuel is removed it will tend to collapse as
the pressure within drops.

Question 41

Cavitation occurs when fuel vapour forms
[a] between the fuel pump and the engine
[b] within the fuel pump
[c] between the fuel tank and the fuel pump
[d] within the fuel tank

Answer 41

B
Cavitation occurs when the fuel vaporises on the inlet side of the fuel pump when the
fuel pressure is low and the fuel temperature is high.