For the first time last night, I had an APU fail to start on a B73NG. We had a FAULT light on the overhead. This was after it had spun up and the EGT rolled back. It started after a second attempt.
When we chocked in, the CA spoke with the engineer/mechanic and he said that you should hold the APU switch to ON for 3 seconds before releasing.
I smiled and said, "Oh, I see."
That's the second time I've heard this, the first time being before entering ETOPS at cruise and the CA told me to move the switch slowly since we were so high.
Now, before I add any more emoticons, could there be any
truth to this theory? To me, it's not like you're priming a recip engine. You're just sending a signal to a computer. I suppose there could be a solenoid/RCCB somewhere that needs just a second to latch, but I doubt it. If the APU tried to start, then it has nothing to do with the switch.