hey there, good question. If we have dispatched with 2 normally working CSDs and the engine comes back online, it is normal practice, at least from what i've seen, to leave the APU on for a little while, just incase the engine craps out again. All that will depend on the conditions for failure...if it was a harmless flameout i,e a situation that permits a restart, then the APU can be switched off fairly soon. Depending on the a/c the APU can come online in a range from 30 seconds, and sometimes it takes up to 1.5 minutes! just depends on how she feels LOL.
At night it is common practice at our airline to switch the APU on, during final approach, in case we go around and have an engine failure, we have a crucial element...electricity! Even in the sim it's nerve wrecking when electrical faults start coming at you, eventually causing u to fly on DC stanby power with only a handful of instruments working....
nothing is a gurantee in this world....you can lose one engine, thus losing the generator, then you can lose the generator in the other engine.... anything can happen, so the APU consumes little fulel at altitude, about 17kg/hour and on the ground somewhere around 150 kg/hour...basically nothing much
please send any other questions i can be of assistance with
The only time there is too much fuel onboard, is when you're on fire!