Just a few engine and APU questions!
I'm curious to know how exactly turbofan engines are started. Are all engines turned initially by compressed air or do some use an electric motor to get the engine rotating to start speed? Does the power (be it air or electric) to start the engines always come from the APU or do airports have facilities to start airliner engines without the APU running? Is this even possible? Are all engines (in the case of a 747 for example) started in the same way, or does one running engine supply power to start the other three? Are turboprops started in a similar way?
Also, if an engine shuts down in flight, how fast does the aircraft need to be travelling to re-light the engine? (This question stems from the 1989 Kegworth disaster when the only working engine was mistakenly shut down, but the aircraft was gliding too slow to re-light it).
I have a few APU questions as well. Firstly, how is the APU started? If it's a small jet engine presumably the starting procedure is similar to that of the main engines. Are they electrically turned by a battery driven motor? Why are the APU's on some airliners not startable in flight?
Secondly, how many of the aircrafts systems can the APU run? Is it able to run ALL the electrics or just the critical components (i.e. Not running the galleys, all the cabin lighting, etc).
And thirdly, am I correct in thinking the APU on a twin jet (777, A330, F100) has to be both startable in flight and be able to run all the aircrafts hydraulic systems? If this is the case for twins, can the APU on 3 or 4 engined aircraft also power the hydraulics (assuming it can be started in flight).
If anyone could possibly take the time to answer these questions, or point me in the direction of a website that could be of some use I'd be most grateful!