PP Fuel Metering Systems

What factor is not used in the operation of an aircraft gas turbine engine fuel control unit?

Compressor inlet air temperature.
Mixture control position.
Power level position.

In order to stabilize cams, springs, and linkages within the fuel control, manufacturers generally recommend that all final turbine engine trim adjustments be made in the

increase direction.
decrease direction.
decrease direction after over-adjustment.

When trimming a turbine engine, the fuel control is adjusted to

produce as much power as the engine is capable of producing.
set idle RPM and maximum speed or EPR.
allow the engine to produce maximum RPM without regard to power output.

An electronic engine control (EEC) is a system that receives engine operating information and

adjusts a standard hydromechanical fuel control unit to obtain the most effective engine operation.
develops the commands to various actuators to control engine parameters.
controls engine operation according to ambient temperature, pressure, and humidity.

A full-authority electronic engine control (EEC) is a system that receives all the necessary data for engine operation and

adjusts a standard hydromechanical fuel control unit to obtain the most effective engine operation.
develops the commands to various actuators to control engine parameters.
controls engine operation according to ambient temperature, pressure, and humidity.

In a supervisory EEC system, any fault in the EEC that adversely affects engine operation

causes redundant or backup units to take over and continue normal operation.
usually degrades performance to the extent that continued operation can cause damage to the engine.
causes an immediate reversion to control by the hydromechanical fuel control unit.

The active clearance control (ACC) portion of an EEC system aids turbine engine efficiency by

adjusting stator vane position according to operating conditions and power requirements.
ensuring turbine blade to engine case clearances are kept to a minimum by controlling case temperatures.
automatically adjusting engine speed to maintain a desired EPR.

What should be checked/changed to ensure the validity of a turbine engine performance check if an alternate fuel is to be used?

Fuel specific gravity setting.
Maximum RPM adjustment.
EPR gauge calibration.

Prior to performing engine trimming, you should

call the control tower to obtain current sea level barometric pressure and temperature.
observe the reading on the aircraft outside air temperature (CAT) gauge.
obtain a true temperature reading comparable to that of the air that enters the engine.

An aircraft should be facing into the wind when trimming an engine. However, if the velocity of the wind blowing into the intake is excessive, it is likely to cause a

false low exhaust gas temperature reading.
trim setting resulting in engine overspeed.
false high compression and turbine discharge pressure, and a subsequent low trim.

Generally, the practice when trimming an engine is to

turn all accessory bleed air off.
turn all accessory bleed air on.
make adjustments (as necessary) for all engines on the same aircraft with accessory bleed air settings the same – either on or off.

Which of the following best describes the function of an automatic mixture control (AMC)?

It compensates for change in the air density due to temperature and altitude.
It compensates for the air pressure above the fuel in the float chamber.
It compensates for the air pressure in the venturi of a float-type carburetor.

On a float-type carburetor, the purpose of the economizer valve is to

provide extra fuel for sudden acceleration of the engine.
maintain the leanest mixture possible during cruising best power.
provide a richer mixture and cooling at maximum power output.

The fuel metering force of a conventional float-type carburetor in its normal operating range is the difference between the pressure acting on the discharge nozzle located within the venturi and the pressure

acting on the fuel in the float chamber.
of the fuel as it enters the carburetor.
of the air as it enters the venturi (impact pressure).

If the main air bleed of a float-type carburetor becomes clogged, the engine will run

lean at rated power.
rich at rated power.
rich at idling.

Which method is commonly used to adjust the level of a float in a float-type carburetor?

Lengthening or shortening the float shaft.
Add or remove shims under the needle-valve seat.
Change the angle of the float arm pivot.

What is the possible cause of an engine running rich at full throttle if it is equipped with a float-type carburetor?

Float level too low.
Clogged main air bleed.
Clogged atmospheric vent.

One of the things a metering orifice in a main air bleed helps to accomplish (at a given altitude) in a carburetor is

pressure in the float chamber to increase as airflow through the carburetor increases.
a progressively richer mixture as airflow through the carburetor increases.
better fuel vaporization and control of the discharge, especially at lower engine speeds.

A punctured float in a float-type carburetor will cause the fuel level to

lower and enrich the mixture.
rise, and enrich the mixture.
rise, and lean the mixture.

The back-suction mixture control system operates by

varying the pressure within the venturi section.
varying the pressure acting on the fuel in the float chamber.
changing the effective cross-sectional area of the main metering orifice (jet).

If an aircraft engine is equipped with a carburetor that is not compensated for altitude and temperature variations, the fuel/air mixture will become

leaner as either the altitude or temperature increases.
richer as the altitude increases and leaner as the temperature increases.
richer as either the altitude or temperature iincreases.

Float-type carburetors which are equipped with economizers are normally set for

their richest mixture delivery and leaned by means of the economizer system.
the economizer system to supplement the main system supply at all engine speeds above idling.
their leanest practical mixture delivery at cruising speeds and enriched by means of the economizer system at higher power settings.

If a float-type carburetor becomes flooded, the condition is most likely caused by

a leaking needle valve and seat assembly.
the accelerating pump shaft being stuck.
a clogged back-suction line.

If an engine is equipped with a float-type carburetor and the engine runs excessively rich at full throttle, a possible cause of the trouble is a clogged

main air bleed.
back-suction line.
atmospheric vent line.

What occurs when a back-suction type mixture control is placed in IDLE CUTOFF?

The fuel passage to the main and idle jets will be closed by a valve.
The float chamber will be vented to a negative pressure area.
The fuel passage to the idle jet will be closed by a valve.

Which of the following best describes the function of an altitude mixture control?

Regulates the richness of the fuel/air charge entering the engine.
Regulates the air pressure above the fuel in the float chamber.
Regulates the air pressure in the venturi.

Select the correct statement concerning the idle system of a conventional float-type carburetor.

The low-pressure area created in the throat of the venturi pulls the fuel from the idle passage.
Climatic condiitions have very little effect on idle mixture requirements.
The low pressure between the edges of the throttle valve and the throttle body pulls the fuel from the idle passage.

On an engine equipped with a pressure-type-carburetor, fuel supply in the idling range is ensured by the inclusion in the carburetor of

a spring in the unmetered fuel chamber to supplement the action of normal metering forces.
an idle metering jet that bypasses the carburetor in the idle range.
a separate boost venturi that is sensitive to the reduced airflow at start and idle speeds.

The economizer system of a float-type carburetor performs which of the following functions?

It supplies and regulates the fuel required for all engine speeds.
It supplies and regulate the additional fuel required for all engine speeds above cruising.
It regulates the fuel required for all engine speeds and all altitudes.

How will the mixture of an engine be affected if the bellows of the automatic mixture control (AMC) in a pressure carburetor ruptures while the engine is operating at altitude?

It will become leaner.
No change will occur until the altitude changes.
It will become richer.

The fuel level within the float chamber of a properly adjusted float-type carburetor will be

slightly higher than the discharge nozzle outlet.
slightly lower than the discharge nozzle outlet.
at the same level as the discharge nozzle outlet.

The metered fuel pressure (chamber C) in an injection-type carburetor.

is held constant throughout the entire engine operating range.
varies according to the position of the poppet valve located between chamber D (unmetered fuel) and chamber E (engine-driven fuel pump pressure).
will be approximately equal to the pressure in chamber A (impact pressure).

Select the statement which is correct relating to a fuel level check of a float-type carburetor.

Use 5 pounds fuel pressure for the test if the carburetor is to be used in a gravity fuel feed system.
Block off the main and idle jets to prevent a continuous flow of fuel through the jets.
Do not measure the level at the edge of the float chamber.

What carburetor component measures the amount of air delivered to the engine?

Economizer value.
Automatic mixture control.
Venturi.

If fuel is found running from the carburetor with the engine not running, the likely cause is that the

float needle valve is not seated properly.
float level is adjusted too low.
main air bleed is clogged.

Fuel is discharged for idling speeds on a float-type carburetor.

from the idle discharge nozzle.
in the venturi.
through the idle discharge air bleed.

When air passes through the venturi of a carburetor, what three changes occur?

Velocity increases, temperature increases, and pressure decreases.
Velocity decreases, temperature increases, and pressure increases.
Velocity increases, temperature decreases, and pressure decreases.

Where is the throttle valve located on a float-type carburetor?

Between the venturi and the discharge nozzle.
After the main discharge nozzle and venturi.
After the venturi and just before the main discharge nozzle.

An aircraft carburetor is equipped with a mixture control in order to prevent the mixture from becoming too

lean at high altitudes.
rich at high altitudes.
rich at high speeds.

Which of the following is NOT a function of the carburetor venturi?

proportions the fuel/air mixture.
Regulates the idle system.
Limits the airflow at full throttle.

Idle cutoff is accomplished on a carburetor equipped with a back-suction mixture control by

introducing low pressure (intake manifold) air into the float chamber.
turning the fuel selector valve to OFF.
the positive closing of a needle and seat.

One purpose of an air bleed in a float-type carburetor is to

increase fuel flow at altitude.
meter air to adjust the mixture.
decrease fuel density and destroy surface tension.

To determine the float level in a float-type carburetor, a measurement is usually made from the top of the fuel in the float chamber to the

parting surface of the carburetor.
top of the float.
centerline of the main discharge nozzle.

The throttle valve of float-type aircraft carburetors is located

ahead of the venturi and main discharge nozzle.
after the main discharge nozzle and ahead of the venturi.
between the venturi and the engine.

Why must a float-type carburetor supply a rich mixture during idle?

Engine operation at idle results in higher than normal volumetric efficiency.
Because at idling speeds the engine may not have enough airflow around the cylinders to provide proper cooling.
Because of reduced mechanical efficiency during idle.

What component is used to ensure fuel delivery during periods of rapid-engine acceleration?

Acceleration pump.
Engine-driven fuel pump.
Power enrichment unit.

The device that controls the ratio of the fuel/air mixture to the cylinders is called a

throttle valve.
mixture control.
metering jet.

The device that controls the volume of the fuel/air mixture to the cylinders is called a

mixture control.
metering jet.
throttle valve.

Which statement is correct regarding a continuous-flow fuel injection system used on many reciprocating engines?

Fuel is injected directly into each cylinder.
Fuel is injected at each cylinder intake port.
Two injection nozzles are used in the injection fuel system for various speeds.

During the operation of an aircraft engine, the pressure drop in the carburetor venturi depends primarily upon the

air temperature.
barometric pressure.
air velocity.

Which of the following causes a single diaphragm accelerator pump to discharge fuel?

An increase in venturi suction when the throttle valve is open.
An increase in manifold pressure that occurs when the throttle valve is opened.
A decrease in manifold pressure that occurs when the throttle valve is opened.

At what engine speed does the main metering jet actually function as a metering jet in a float-type carburetor?

All RPMs.
Cruising RPM only.
All RPMs above idle range.

An aircraft engine continuous cylinder fuel injection system normally discharges fuel during which stroke(s)?

Intake.
Intake and compression.
All (continuously).

What is the purpose of the carburetor accelerating system?

Supply and regulate the fuel required for engine speeds above idle.
Temporarily enrich the mixture when the throttle is suddenly opened.
Supply and regulate additional fuel required for engine speeds above cruising.

When troubleshooting an engine that will not idle, what would be a probable cause?

Economizer valve not operating correctly.
Mixture setting too rich.
Manifold valve is not operating properly.

What is the relationship between the accelerating pump and the enrichment valve in a pressure injection carburetor?

No relationship since they operate independently.
Unmetered fuel pressure affects both units.
The accelerating pump actuates the enrichment valve.

What is the relationship between the pressure existing within the throat of a venturi and the velocity of the air passing through the venturi?

There is no direct relationship between the pressure and the velocity.
The pressure is directly proportional to the velocity.
The pressure is inversely proportional to the velocity.

Which of the following is least likely to occur during operation of an engine equipped with a direct cylinder fuel injection system?

Afterfiring.
Kickback during start.
Backfiring.

What carburetor component limits the maximum airflow into the engine at full throttle?

Throttle valve.
Venturi.
Main metering jet.

On a carburetor without an automatic mixture control as you ascend to altitude, the mixture will

be enriched.
be leaned.
not be affected.

During engine operation, if carburetor heat is applied, it will

increase fuel/air ratio.
increase engine RPM.
decrease the air density to the carburetor.

The desired engine idle speed and mixture setting

is adjusted with engine warmed up and operating.
should give minimum RPM with maximum manifold pressure.
is usually adjusted in the following sequence: speed first, then mixture.

A nine-cylinder radial engine, using a multiple-point priming system with a central spider, will prime which cylinders?

1, 2, 3, 8, and 9.
All cylinders.
1, 3, 5, and 7.

What is a function of the idling air bleed in a float-type carburetor?

It provides a means for adjusting the mixture at idle speeds.
It vaporizes the fuel at idling speeds.
It aids in emulsifying/vaporizing the fuel at idle speeds.

If the volume of air passing through a carburetor venturi is reduced, the pressure at the venture throat will

decrease.
be equal to the pressure at the venturi outlet.
increase.

Refer to Figure 6. Which curve most nearly represents an aircraft engine’s fuel/air ratio throughout its operating range?

1.
3.
2.

What will occur if the vapor vent float in a pressure carburetor loses its buoyancy?

The amount of fuel returning to the fuel tank from the carburetor will be increased.
The engine will continue to run after the mixture control is placed in IDLE CUTOFF.
A rich mixture will occur at all engine speeds.

What method is ordinarily used to make idle speed adjustments on a float-type carburetor?

An adjustable throttle stop or linkage.
An orifice and adjustable tapered needle.
An adjustable needle in the drilled passageway which connects the airspace of the float chamber and the carburetor venturi.

For what primary purpose is a turbine engine fuel control unit trimmed?

To obtain maximum thrust output when desired.
To properly position the power levers.
To adjust the idle RPM.

Which type of fuel control is used on most of today’s turbine engines?

Electromechanical.
Mechanical.
Hydromechanical or electronic.

Under which of the following conditions will the trimming of a turbine engine be most accurate?

High wind and high moisture.
High moisture and low wind.
No wind and low moisture.

(1) The mixture used at rated power in air cooled reciprocating engines is richer than the mixture used through the normal cruising range.

(2) The mixture used at idle in air cooled reciprocating engines is richer than the mixture used at rated power.

Regarding the above statements,

only 1 is true.
only 2 is true.
both 1 and 2 are true.

Under which of the following conditions would an engine run lean even though there is a normal amount of fuel present?

The use of too high an octane rating fuel.
Incomplete fuel vaporization.
The carburetor air heater valve in the HOT position.

During the mixture adjustments, which of the following is normally observed to determine when the correct mixture has been achieved?

Changes in fuel/air pressure ratio.
Fuel flowmeter.
Changes in RPM or manifold pressure.

An indication that the optimum idle mixture has been obtained occurs when the mixture control is moved to IDLE CUTOFF and manifold pressure.

decreases momentarily and RPM drops slightly before the engine ceases to fire.
increases momentarily and RPM drops slightly before the engine ceases to fire.
decreases and RPM increases momentarily before the engine ceases to fire.

The use of less than normal throttle opening during starting will cause

a rich mixture.
a lean mixture.
backfire due to lean fuel/air ratio.

When checking the idle mixture on a carburetor, the engine should be idling normally, then pull the mixture control toward the IDLE CUTOFF position. A correct idling mixture will be indicated by

an immediate decrease in RPM.
a decrease of 20 to 30 RPM before quitting.
an increase of 10 to 50 RPM before decreasing.

When a new carburetor is installed on an engine,

warm up the engine and adjust the float level.
do not adjust the idle mixture setting; this was accomplished on the flow bench.
and the engine is warmed up to normal temperatures, adjust the idle mixture, then the idle speed.

The purpose of the back-suction mixture control in a float-type carburetor is to adjust the mixture by

regulating the pressure drop at the venturi.
regulating the pressure on the fuel in the float chamber.
regulating the suction on the mixture from behind the throttle valve.

Reciprocating engine power will be decreased at all altitudes if the

air density is increased.
humidity is increased.
manifold pressure is increased.

If the idling jet becomes clogged in a float-type carburetor, the

engine operation will not be affected at any RPM.
engine will not die.
idle mixture becomes richer.

An aircraft engine equipped with a pressure-type carburetor is started with the

primer while the mixture control is positioned at IDLE CUTOFF.
mixture control in the FULL RICH position.
primer while the mixture control is positioned at the FULL LEAN position.

One of the best ways to increase engine power and control detonation and preignition is to

enrich the fuel/air mixture.
use water injection.
lean the fuel/air mixture.

An excessively lean fuel/air mixture may cause

an increase in cylinder head temperature.
high oil pressure.
backfiring through the exhaust.

The density of air is very important when mixing fuel and air to obtain a correct fuel-to-air ratio. Which of the following weighs the most?

75 parts of dry air and 25 [arts pf water vapor.
100 parts of dry air.
50 parts of dry air and 50 parts of water vapor.

A mixture ratio of 11:1 normally refers to

a stoichiometric mixture.
1 part air to 11 parts fuel.
1 part fuel to 11 parts air.

The economizer system in a float-type carburetor

keeps the fuel/air ratio constant.
functions only at cruise and idle speeds.
increases the fuel/air ratio at high power settings.

A carburetor is prevented from leaning out during quick acceleration by the

power enrichment system.
mixture control system.
accelerating system.

In turbine engines that utilize a pressurization and dump valve, the dump portion of the valve

cuts off fuel flow to the engine fuel manifold and dumps the manifold fuel into the combustion to burn just before the engine shuts down.
drains the engine manifold lines to prevent fuel boiling and subsequent deposits in the lines as a result of residual engine heat (at engine shutdown).
dumps extra fuel into the engine in order to provide for quick engine acceleration during rapid throttle advancement.

What effect does high atmospheric humidity have on the operation of a jet engine?

Decreases engine pressure ratio.
Decreases compressor and turbine RPM.
Has little or no effect.

What are the positions of the pressurization valve and the dump valve in a jet engine fuel system when the engine is shut down?

Pressurization valve closed, dump valve open.
Pressurization valve open, dump valve open.
Pressurization valve closed, dump valve closed.

What could cause a lean mixture and high cylinder head temperature at sea level or low altitudes?

Mixture control valve fully closed.
Selective accelreating system.
Automatic mixture control stuck in the extended position.

Which of the following is NOT an input parameter for a turbine engine fuel control unit?

Compressor inlet pressure.
Compressor inlet temperature.
Ambient humidity.

Detonation occurs when the fuel/air mixture

burns too fast.
ignites before the time of normal ignition.
is too rich.

What corrective action should be taken when a carburetor is found to be leaking fuel from the discharge nozzle?

Replace the needle valve and seat.
Adjust the mixture control.
Adjust throttle linkage.

A major difference between the Teledyne-Continental and RSA (Precision Airmotive or Bendix) continuous flow fuel injection systems in fuel metering is that the

RSA system uses air pressure only as a metering force.
Continental system utilizes airflow as a metering force.
Continental system uses fuel pressure only s a metering force.

The function of the altitude compensating, or aneroid valve used with the Teledyne-Continental fuel injections system on many turbocharged engines is to

prevent an overly rich mixture during sudden acceleration.
prevent detonation at high altitudes.
provide a means of enriching the mixture during sudden acceleration.

The primary purpose of the air bleed openings used with continuous flow fuel injector nozzles is to

provide for automatic mixture control.
maintain constant fuel pressure.
aid iin proper fuel vaporization.

PP Fuel Metering Systems

Jon K

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