Originally Posted by
CaptainMongo
I hate to be ignorant but is there a FADEC capacitor which only obtains a charge from a FADEC PMA which gives the FADEC a 5 minute life? (I never really thought about it...)
It would need to be a heck of a capacitor - the power draw of the FADEC is significant - typically well over 100 watts (enough so that heat rejection is a concern under certain conditions).
Educated guess (and please feel free to correct me if my guess is all wet) - but I'm thinking Airbus may have a time delay relay in the aircraft power circuit that keeps the FADEC powered from the aircraft for five minutes after the fuel shutoff command. We did something like that on the 747-8 so that the engine monitoring equipment could finish downloading everything after shutdown before everything is depowered (except it's 15 minutes).
Easy way to tell if the FADEC is powered is to look at EGT - if EGT is being displayed, then the FADEC must be powered since there is no backup (rotor speeds often have analog backup)