True that every commercial plane must be able to start the APU in the air (or at least the ETOPS planes)?

Question

True that every commercial plane must be able to start the APU in the air (or at least the ETOPS planes)?

Answer 1

ETOPS for sure (and there is certain criteria for starting it in the air).

Most twin engined planes (like the non ETOPS 737s) require the APU to start in the air for electrical systems redundancy.  The 737 has a split bus electrical system so one generator on one side can't power the other generator's bus if one generator fails. However, APU can power the dead bus.

Many old planes like the 727 can't start the APU in the air (I've heard stories that pilots HAVE started it in the air, but is not part of the SOPs).  In this case the APU isn't needed for redundancy, just for power and air on the ground.

Answer 2

You must only be able to start the APU in flight if the APU generator is required as part of a redundancy system.  If your aircraft has more than 2 generators already installed, there is no requirement to be able to start it in flight.  However, there are not many aircraft flying today that have APUs that cannot be started in flight, its simply easier and safer to design it that way.

Answer 3

Not every commercial airplane can start its APU in flight,but the majority can for electrical redundancy and also to supply bleed air. ETOPS airplanes must have an inflight rated APU for that very reason.

Answer 4

Starting the APU in flight is only required on ETOPS A/C for it to keep its ETOPS rating. If it has to be defered than it is no longer able to fly as an ETOPS A/C. On all other airliners it can be defered without any problems unless one of the generaters on an engine is out of service then you need to have the APU running for safety.