FADEC, Full Authority Digital Engine Control

A FADEC has full authority: there is no hydromechanical backup that could run the engine if the electronics failed. The thrust lever is not connected to a fuel valve, it simply sends a demand to the electronic engine control, which decides what fuel flow will achieve it. The EEC takes inputs from the thrust lever, air data such as altitude and total air temperature, and engine sensors including N1, N2, EGT and various pressures and temperatures. It drives the fuel metering unit, the variable stator vanes and bleed valves, and the start and ignition sequence.

Because the lever commands thrust rather than fuel, the same lever angle gives the same rated thrust as altitude and temperature change, with the EEC compensating automatically. It also enforces limit protection: if achieving the commanded N1 would exceed the EGT, N1 or N2 limit, the control holds the limit instead of the demand. That protection is the reason an engine cannot simply be over-temped by pushing the lever forward.

Reliability comes from dual redundant channels. One channel is in control while the other is in standby, cross-monitoring it. On a fault the standby channel takes control automatically with no crew action, and the fault is recorded by the built-in test equipment for maintenance. Once running, the FADEC is powered by its own permanent-magnet alternator on the engine gearbox, so engine control is independent of the aircraft electrical system.

• Try this: Hold the thrust lever fixed and climb. Why does the fuel flow change while the commanded N1 stays the same?
• Try this: Set the outside air temperature very high and demand full thrust. Which limit does the EEC hold, and what does it do to the commanded N1?
• Try this: Press Fail channel A. What changes in the engine indications, and why is there no crew action required?