Originally Posted by
LarryJ
The APU is not normally used inflight though it is required as a backup for ETOPS and overwater flights. An aircraft without a working APU would not be dispatched on such flights. You also couldn't do bleeds-off takeoffs or landings so there might be a performance penalty in hot/high conditions.
The real issue with inop APU is trying to maintain cabin temperature. They have to disconnect the ground pre-conditioned air (PCA) before we can do the air start. That leaves the cabin without any conditioned air until an engine is started. If there are problems with the air cart you're stuck without air while they try to fix it or find another air cart. Meanwhile, they can't reattached the PCA if the jetbridge has been retracted. If the outside air temperature is much above 50, or so, the cabin will get uncomfortably warm.
Once you have started one engine, you start the second engine using air pressure from the operating engine. You can't do that in the gate area because you have to increase thrust on the operating engine to get enough air pressure and that jet blast would create a hazard for the workers and equipment around the gates.
Interesting. I was on a 763 out of OGG last year where I was pretty sure we had an inop APU. From what I remember, they were reluctant to board but as the delay rolled, the captain came out and told us we were going to give it a shot, but there would be no cabin air while we were on the ground, so the plane would get hot and stuffy really quick (honestly wasn't
thatbad). They spun up one engine (I don't recall if there was a cart) and then we rolled out to a pad near the start of Rwy 20 to get the other engine going, before taxiing back to take off on 2. After that we took off and flew to DEN.
Could that have been something else, given we were allowed to fly overwater? Or does it have to do with the 767s having a RAT?