"The power source for APU starting may be the airplane battery, a ground power source, or an engine-driven generator. The power source for engine starting may be the APU generators, engine-driven generators on the opposite side engine, or two forward 115 VAC ground power sources. The aft external power receptacles may be used for a faster start, if desired."
So the battery seems to be very important when you're somewhere with no ground generator. The battery starts the APU, then the power from the APU starts the engines.
That's what I was wondering, but does this happen? And if the other engines can start it as Joezydeco suggests, does not having them matter? Surely the number or airports without decent facilities to help start them must be small?
I was wondering about that. They could probably fly the things without APU start batteries, as long as they avoid having to do a cold start at a grass strip. So, new AD, no landing 787's at podunk facilities with no ground carts, or, electrical service.
http://www.boeing.com/commercial/aeromagazine/articles/qtr_4...
Page 4 says this:
"The power source for APU starting may be the airplane battery, a ground power source, or an engine-driven generator. The power source for engine starting may be the APU generators, engine-driven generators on the opposite side engine, or two forward 115 VAC ground power sources. The aft external power receptacles may be used for a faster start, if desired."
So the battery seems to be very important when you're somewhere with no ground generator. The battery starts the APU, then the power from the APU starts the engines.