Originally Posted by
galaxy flyer
FADEC has its own Permanent Magnet Alternator to power it, so it runs if there’s NO other power available, it’s all internal to the FADEC. tdracer has covered this several times.
IF the RAT was deployed, flight controls are powered, there is no appreciable switchover time from the normal hydraulics. It deploys in a second. I’ve done in-flight RAT checks on three types and the transition is imperceptible. Note, much older designs, so it’s likely to be better in the 787.
The PMG is the primary source of power to the FADEC, ship power as a backup or during engine start / ground operations.