As I'm currently with the Royal Norwegian Air Force at a Norwegian AFB, I regularly am in contact with different equipment used on fighters while they are on the ground.
One such equipment is the GTC (Gas Turbine Compressor) which is a small jet engine used to provide air to fighters that don't have a internal APU. (So as you probably have understood, its a mobile APU).
since our airport also is a civil airport, 734,5,7,8 and MD80 are present every day together with prop aircrafts and GA aircrafts.
Sometimes we are called out to provide start air to the big jets. One weekend several jets, needed our support. Since the GTC "replace" the APU I'll guess that those a/c had inop. APU's.
My question actually regards the safety in operating aircrafts with inop. APU.
As long as the airport have the required ground facilities it would not be any problem starting the engines. But what if the aircrafts loose all engines while inflight. I know its not common, but it do have happened. (BA 747 that flew into volcano ash)
If only one of the engines stop, I assume that the other engine will provide the air needed for a restart, or is the velocity of the outside air enough to provide the required N speeds to ignite a engine?
If not, it will be troublesome to restart a plane with no running engines.
As I'm writing this the Air Ontario F28 disaster back in the early 1990s also comes to my mind. Off course I'm not an expert on that tragic accident, but after reading Air Disaster Vol.3 you got some ideas why the accident occurred.
It was actually several independent actions or lack of them in both the airline management and on that particular day.
Anyway the F28's APU was inop. and after landing at Dryden, an airport without the required facilities to provide air ) the a/c had to leave one engine running during the turn around.
That day it was snowing, but both Fokker and Air Ontario forbid de icing of aircraft with running engines.
I don't know why, but the captain did decide to take off, but ice had accumulated on the wings and the aircraft went down less than a minute after take off ,killing several of the passengers.
I'm not saying this accident wouldn't have occurred if the APU had been operative, but I guess it was one factor that lead to the accident.
So do it exist any rules about flying with inop. APUs ?