Originally Posted by alanh
The only time the APU would be needed in flight would be a double engine failure, or at least a failure of all the electrical generators and their backups.
I was once on a UA flight from DEN to YVR (737, I think) and about 45 minutes out the pilot turned around and flew back to DEN.
He said the APU had thrown up a warning light and although it wasn't a safety issue he was required to return to DEN and have it fixed. Presumably they didn't want to fix it in Vancouver.
Luckily DEN's a major hub and they were able to come up with another aircraft to put us on.
Cheers,
Geoff Glave
Vancouver, Canada