Q&A | Aircraft Engine Fuel Metering Systems

Multiple Choice Questions

1. What factor is not used in the operation of an aircraft gas turbine engine fuel control unit?
a. Compressor inlet air temperature.
b. Mixture control position.
c. Power lever position.

2. 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
a. increase direction.
b. decrease direction.
c. decrease direction after over-adjustment.

3. When trimming a turbine engine, the fuel control is adjusted to
a. produce as much power as the engine is capable of producing.
b. set idle RPM and maximum speed or EPR.
c. allow the engine to produce maximum RPM without regard to power output.

4. A supervisory electronic engine control (EEC) is a system that receives engine operating information and
a. adjusts a standard hydromechanical fuel control unit to obtain the most effective engine operation.
b. develops the commands to various actuators to control engine parameters.
c. controls engine operation according to ambient temperature, pressure, and humidity.

5. A full-authority electronic engine control (EEC) is a system that receives all the necessary data for engine operation and
a. adjusts a standard hydromechanical fuel control unit to obtain the most effective engine operation.
b. develops the commands to various actuators to control engine parameters.
c. controls engine operation according to ambient temperature, pressure, and humidity.

6. In a supervisory EEC system, any fault in the EEC that adversely affects engine operation
a. causes redundant or backup units to take over and continue normal operation.
b. usually degrades performance to the extent that continued operation can cause damage to the engine.
c. causes an immediate reversion to control by the hydromechanical fuel control unit.

7. The active clearance control (ACC) portion of an EEC system aids turbine engine efficiency by
a. adjusting stator vane position according to operating conditions and power requirements.
b. ensuring turbine blade to engine case clearances are kept to a minimum by controlling case temperatures.
c. automatically adjusting engine speed to maintain a desired EPR.

8. What should be checked/changed to ensure the validity of a turbine engine performance check if an alternate fuel is to be used?
a. Fuel specific gravity setting.
b. Maximum RPM adjustment.
c. EPR gauge calibration.

9. The generally acceptable way to obtain accurate onsite temperature prior to performing engine trimming is to
a. call the control tower to obtain field temperature.
b. observe the reading on the aircraft Outside Air Temperature (OAT) gauge.
c. hang a thermometer in the shade of the nose wheel-well until the temperature reading stabilizes.

10. 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
a. false low exhaust gas temperature reading.
b. trim setting resulting in engine overspeed.
c. false high compression and turbine discharge pressure, and a subsequent low trim.

11. Generally, the practice when trimming an engine is to
a. turn all accessory bleed air off.
b. turn all accessory bleed air on.
c. make adjustments (as necessary) for all engines on the same aircraft with accessory bleed air settings the same-either on or off.

12. A reciprocating engine automatic mixture control responds to changes in air density caused by changes in
a. altitude or humidity.
b. altitude only.
c. altitude or temperature.

13. On a float-type carburetor, the purpose of the economizer valve is to
a. provide extra fuel for sudden acceleration of the engine.
b. maintain the leanest mixture possible during cruising best power.
c. provide a richer mixture and cooling at maximum power output.

14. 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
a. acting on the fuel in the float chamber.
b. of the fuel as it enters the carburetor.
c. of the air as it enters the venturi (impact pressure).

15. If the main air bleed of a float-type carburetor becomes clogged, the engine will run
a. lean at rated power.
b. rich at rated power.
c. rich at idling.

16. Which method is commonly used to adjust the level of a float in a float-type carburetor?
a. Lengthening or shortening the float shaft.
b. Add or remove shims under the needle-valve seat.
c. Change the angle of the float arm pivot.

17. What is the possible cause of an engine running rich at full throttle if it is equipped with a float-type carburetor?
a. Float level too low.
b. Clogged main air bleed.
c. Clogged atmospheric vent.

18. One of the things a metering orifice in a main air bleed helps to accomplish (at a given altitude) in a carburetor is
a. pressure in the float chamber to increase as airflow through the carburetor increases.
b. a progressively richer mixture as airflow through the carburetor increases.
c. better fuel vaporization and control of fuel discharge, especially at lower engine speeds.

19. A punctured float in a float-type carburetor will cause the fuel level to
a. lower, and enrich the mixture.
b. rise, and enrich the mixture.
c. rise, and lean the mixture.

20. The back-suction mixture control system operates by
a. varying the pressure within the venturi section.
b. varying the pressure acting on the fuel in the float chamber.
c. changing the effective cross-sectional area of the main metering orifice jet).

21. If an aircraft engine is equipped with a carburetor that is not compensated for altitude and temperature variations, the fuellair mixture will become
a. leaner as either the altitude or temperature increases.
b. richer as the altitude increases and leaner as the temperature increases.
c. richer as either the altitude or temperature increases.

22. Float-type carburetors which are equipped with economizers are normally set for
a. their richest mixture delivery and leaned by means of the economizer system.
b. the economizer system to supplement the main system supply at all engine speeds above idling.
c. their leanest practical mixture delivery at cruising speeds and enriched by means of the economizer system at higher power settings.

23. If a float-type carburetor becomes flooded, the condition is most likely caused by
a. a leaking needle valve and seat assembly.
b. the accelerating pump shaft being stuck.
c. a clogged back-suction line.

24. 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
a. main air bleed.
b. back-suction line.
c. atmospheric vent line.

25. What occurs when a back-suction type mixture control is placed in IDLE CUTOFF?
a. The fuel passages to the main and idle jets will be closed by a valve.
b. The float chamber will be vented to a negative pressure area.
c. The fuel passage to the idle jet will be closed by a valve.

26. Which of the following best describes the function of an altitude mixture control?
a. Regulates the richness of the fuel/air charge entering the engine.
b. Regulates the air pressure above the fuel in the float chamber.
c. Regulates the air pressure in the venturi.

27. Select the correct statement concerning the idle system of a conventional float-type carburetor.
a. The low-pressure area created in the throat of the venturi pulls the fuel from the idle passage.
b. Climactic conditions have very little effect on idle mixture requirements.
c. The low pressure between the edges of the throttle valve and the throttle body pulls the fuel from the idle passage.

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

29. The economizer system of a float-type carburetor performs which of the following functions?
a. It supplies and regulates the fuel required for all engine speeds.
b. It supplies and regulates the additional fuel required for all engine speeds above cruising.
c. It regulates the fuel required for all engine speeds and all altitudes.

30. 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?
a. It will become leaner.
b. No change will occur until the altitude changes.
c. It will become richer.

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ANSWERS

1. b
Automatic fuel control units sense power lever position, engine rpm, compressor inlet air temperature and density, and burner pressure or discharge pressure. Since turbine engine aircraft do not utilize a mixture control, it is not a factor in the operation of the fuel control unit.
2. a
In order to stabilize internal components and ensure consistent results, most engine manufacturers specify that all trim adjustments be made in the increase direction. In other words, the engine will idle at a value just above the minimum idle speed and maximum thrust will be obtained slightly before the power levers reach the full forward position.
3. b
Field adjustments, or trimming adjustments made to turbine engine fuel controls are limited to idle rpm and maximum speed adjustments.
4. a
A supervisory electronic engine control (EEC) includes a computer that monitors several engine operating criteria and uses this information to adjust a standard hydromechanical fuel control unit (FCU) to obtain a constant thrust for a given power lever position.
5. b
A full-authority electronic engine control (EEC) performs all of the functions required to operate an engine. In other words, it receives data from the aircraft and engine systems and then issues commands to various actuators that control engine operating parameters.
6. c
Any fault in a supervisory electronic engine control (EEC) automatically causes the EEC to relinquish engine control to the hydromechanical control unit. At the same time, the EEC sends a signal to the cockpit that illuminates an annunciator light to inform the flight crew of the change in operating mode.
7. b
The active clearance control (ACC) portion of an electronic engine control (EEC) system controls turbine blade-to-engine case clearances by controlling the amount of air that is directed through the engine case. By keeping clearances to a minimum, pressure losses caused by air leakage at the blade tips is minimized.
8. a
Turbine engines are designed to operate using a specific type of fuel with a given BTU value and specific gravity. Therefore, if a performance check is made on an engine using an alternate fuel, the specific gravity setting should be checked and changed as necessary on the fuel control unit to ensure proper performance. Although an alternate fuel is likely to cause an engine to perform differently.
9. c
Part of the procedure used for trimming an engine includes measuring engine inlet barometric pressure and ambient temperature. To ensure a temperature value that is accurate, it is common practice to hang a thermometer in the shade of the nose wheel-well.
10. c
Facing an engine into a strong wind produces the same effect as improving the efficiency of the engine's compressor. Therefore, if an engine is trimmed while facing into an excessive wind, a false high compression and turbine discharge pressure, as well as a subsequent low trim are likely to occur.

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