View Full Version : FADEC and battery

25th Jan 2008, 07:20
Had a query relevant to loss of all power in B744. After the battery drops out, apart from loss of standby instruments etc does the FADEC and hence engine control drop out as well?

25th Jan 2008, 08:05
FADEC's (or EEC's) are normally powered by a dedicated little generator, driven by their own engine.

Do not mistake that little power source for the normal engine driven generator!

The EEC generator (or alternator) will deliver enough power already at very low engine RPM (so also during windmilling airstarts). It will usually have separate coils for the two EEC channels. Often the electrics needed to carry the signal from thust lever to EEC are also powered from that generator.
In other words, even with the most complete electrical power failure on board, the engine control should still be unhampered.

Only as a backup to the EEC alternators will there be a connection from aircraft electrical system to EEC; on the ground, before engine is turned by the starter, could be an example of aircraft electrics powering the EECs.

25th Jan 2008, 08:12
After the battery drops out, apart from loss of standby instruments etc does the FADEC and hence engine control drop out as well?

Nope.. runs on indefinitely.. or, rather, until the engines run out of fuel...
As long as the engine core rpms (N2) remain above 9% (CF6) or N3 remains above 8% (RB211), the ECU/EEC's are powered (courtesy of engine-driven permanent magnet alternators).

But if the rpms go this low, you have probably fallen out the sky already... and you can probably assume that the engine has flamed out.. and without battery power, you won't be able to relight.


25th Jan 2008, 14:27
On B744 CF6-80C2B1F the Control Alternator can give sufficient power for ECU at > 11% N2.:ok:

25th Jan 2008, 20:12
"The control alternator meets all EEC power requirements when N2 increases above 11%. It continues to meet the requirements until N2 decreases below 9%. If one phase of either or both windings fails, the control alternator continues to meet all EEC power requirements if N2 is above 45%" :}

25th Jan 2008, 20:31
These small separate AC generators have been used in the past on large piston types as well...those aircraft equipped with CurtisElectric propellors, for example.
Total electric failure from the aircrafts electrical system/ships battery...props still controllable.
A CAR4b requirement.

25th Jan 2008, 22:05
Thanks all.

Loose rivets
26th Jan 2008, 05:37
What powers/fuels the 'Little engines". ?

26th Jan 2008, 06:38
The EEC's permanent magnet "dedicated alternator" is mechanically driven (via a gearbox) by the spinning "N3 rotor" of the aircraft's engine: The Rolls Royce Trent on the Boeing 777 has 3 rotors (N1, N2 and N3).

For beginners: A rotor is basically a shaft (solid or hollow) with "stages" of blades at intervals along it. The only rotor you can really see is the big N1 compressor "fan" at the front of the engine. If you looked in the back of the engine, you would see the N1 "turbine" blades. The N2 and N3 rotors are nestled inside the engine (between the N1 compressor and turbine blades).

If the engine is turning, either by normal means.. or by windmilling (at flight speeds), the N3 will be turning.