Hi Chris,
A little info for you on the B787 APU. It was designed and built by Hamilton-Sundstrand, now part of United Technologies. The model is the APS5000. It weighs 540 lbs. and develops 1,100 shaft horsepower. It is a variable speed APU, speed determined based on temperature and altitude. It is 50% quieter and has 10% lower emissions compared to a B767 APU. This makes it less objectionable to being run on the ground at a gate during a relatively short turn around situation. If it is shut off during flight, it is capable of being started at any altitude during flight up to 43,100 feet which is remarkable for a turbine engine, usually they don't like to start very well at high altitudes.
My understanding is that for a B787 ETOPs flight to take place, the APU and associated generators must be operable or the plane can't fly an ETOPs route.
sb-sfo pointed out:
There is a limitation on the APU that once it is shut down, it requires 25 minutes cooldown due to shaft bowing before it can be restarted.
This is surprising to me in that the usual problem with small turbines is core lock or near core lock. This is because the tolerances between the rotating parts and the outside diameter casing are tighter to prevent leakage and efficiency losses, much more important on a small engine verses a large engine. The down side is the casing cools much faster and shrinks around the internal rotating components. Since the engine is so short forward to aft, I would think the shaft would not bow.
and Lyman, Pratt & Whitney doesn't start either GE or RR engines on the B787, UTC does, P&W is a separate division entirely...
TD