Full Authority Digital Engine Control
FADEC stands for Full Authority Digital Engine Control. It is a computer, usually mounted on the engine itself, that controls everything the engine does: the fuel flow, the starting sequence, the valves and vanes that keep the compressor happy, and the limits that keep the engine in one piece. Full authority means exactly that: there is no mechanical fuel control behind it for the pilot to fall back on. To make that safe, a FADEC has two independent channels, each able to run the engine alone, with their own sensors and power.
Before FADEC, engines were run by hydromechanical fuel controls: clever assemblies of cams, springs and valves that metered the fuel. Pilots had to watch the gauges closely and move the levers gently. FADEC took that job over and does it more precisely, which saves fuel and makes engines last longer.
What it does
- Turns the lever into a target. The thrust lever does not open a fuel tap. It asks for a thrust setting, usually expressed as a fan speed (N1) or a pressure ratio, and the FADEC works out the fuel flow that gets there for today's air temperature, altitude and speed.
- Accelerates and decelerates safely. Push the lever forward and the FADEC feeds fuel in at a rate the rotor can follow, so the compressor does not surge. Pull it back and it does not cut so much fuel that the flame blows out.
- Guards the limits. It keeps the turbine below its temperature limit (see why jet engines don't melt) and the rotors below their maximum speed, whatever the pilot asks for.
- Starts the engine. It runs the whole start sequence and aborts it if the engine runs too hot (a hot start) or stops accelerating (a hung start).
- Keeps a record. It logs faults and exceedances so that maintenance crews know what the engine went through.
What FADEC does in the game
The game gives every engine a simplified FADEC that you can switch off from the top bar. With it on:
- The throttle lever sets a target rotor speed, from idle up to full power, and the FADEC adjusts the fuel to hold it. In the Design office you can reshape that schedule with the draggable throttle map.
- The fuel is limited so the compressor never runs closer than about 10 % from its surge line and the turbine stays about 25 K below its temperature limit. During a surge it drops the fuel to the minimum.
- In a manual start it aborts a hot start or a hung start before the turbine is hurt.
- On a turboprop it pulls the power back if the propeller overspeeds past 107 %.
- On a ramjet or scramjet it holds the inlet margin and the wall temperature, cuts the fuel when the tunnel is too slow, and relights the engine after a flameout.
Switch it off and the throttle sets the fuel flow directly, as on an early jet. The engine will do whatever you ask, including surge, overheat and overspeed. That is why several of the game's challenges, such as Overcooked, Surge survivor and Money shift, are played with FADEC off.
Things to try
- On the Training turbojet, slam the throttle from idle to full with FADEC on, then again with it off. Watch the surge margin reading both times.
- With FADEC off, hold full power and watch the TURBINE INLET temperature creep past the point where FADEC would have stopped it.
- On the free Turboprop at full power, shift the pitch down a few gears with FADEC on and see the overspeed governor step in.