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Backup control in a true FADEC fuel system

Aviation Mechanic Powerplant (AMP) practice question 10 of 20 on Engine Fuel and Fuel Metering Systems. ACS element AM.III.I.K5 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONWhich statement is true of a true full authority digital engine control (FADEC) fuel system?

  1. AIt has no hydromechanical backup control.
  2. BIt uses a hydromechanical control as a backup.
  3. CIt meters fuel with a mechanical governor.
Show the answer and explanation

Correct answer: A. It has no hydromechanical backup control.

  • A — correct The handbook states that a true FADEC system has no hydromechanical fuel control backup system; the electronic engine control has complete authority over fuel flow.
  • B — incorrect That arrangement describes the earlier hydromechanical/electronic hybrid control, in which the hydromechanical portion takes over in manual mode if the electronic system fails.
  • C — incorrect In a FADEC system the metering valve simply reacts to commands from the electronic engine control; a mechanical speed governor belongs to hydromechanical designs.

BACKGROUND

Gas turbine fuel controls fall into three groups: hydromechanical, hydromechanical/electronic, and full authority digital engine control. The hybrid type uses a remotely located electronic engine control to adjust a hydromechanical fuel control, and the hydromechanical portion can take over in manual mode if the electronics fail. A true FADEC system has no hydromechanical backup. Electronic sensors feed engine parameter information to the electronic engine control, which determines the required fuel flow and transmits it to a fuel metering valve that simply reacts to the commands it receives. FADEC systems are used on many kinds of turbine engines, from auxiliary power units to the largest propulsion engines.

STUDY TIP

The word that separates the three control types is "backup." If losing the electronics still leaves a working mechanical mode, it is the hybrid type, not a true FADEC.

KEY POINTS

  • Turbine fuel controls are hydromechanical, hydromechanical/electronic, or FADEC.
  • A true FADEC has no hydromechanical backup fuel control.
  • In FADEC the electronic engine control computes fuel flow and the metering valve only reacts.

SOURCES

  • amp-powerplant/ch02.txt "Turbine Fuel Controls"
  • amp-powerplant/ch02.txt "FADEC Fuel Control Systems"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.