Really? I wasn't aware that it was like that. That just doesn't sound like a very good design idea to me, seems like it would be more common to have some sort of electrical problem than a bleed air problem. So are the engines started electrically as well I assume? I have got to find more to read about the design of this thing.
Yes, the engines are started electrically (each engine has two starter-generators). If you think about it, once the engine is running, an air turbine starter is useless weight, and yet another thing to fail ("Left start valve still open!"). A starter generator—not a new idea, mind—is not useless once the engine is running, for it ceases starting, and starts generating.
The details would make an electrical engineer very happy, and you can find some of them
here. "All electrical" is an exaggeration since it still has conventional engine driven hydraulics, and I think engine anti-ice is also done pneumatically, but environmental control/pressurization are not, and leading edge thermal anti- or de-icing is also done electrically. There are also no pneumatic air driven hydraulic pumps like on the 767 or 777. Remember that on an airplane, perfection is
not achieved when there is nothing left to add, but when there is nothing left to delete.
Here's the high level overview:
Each engine generator provides 235 volt variable frequency AC, connected directly to the engine. These generators are starter-generators, and are connected to units that convert this AC into whatever the loads demand (115V/400Hz for conventional aircraft loads, 270 volts DC for high demand, variable speed motors like cabin pressurization and hydraulic pumps, 28VDC for conventional airplane loads, and a handful of 235 volt AC loads).
External power is supplied at forward E&E for normal ops. The aft E&E external power supply is for when maintenance needs to run some of those variable speed motor loads.
Contrast most current jet airliners, where the generators are largely 115-volt, 400-cycle AC with constant speed drives (Boeing calls them integrated drive generators; whatever, the point is the N2 can spin at whatever RPM and the generator provides 400Hz), and the system provides 115V/400Hz to most airplane loads and 28VDC through transformer-rectifier units. Engine starting is done pneumatically as is cabin pressurization and anti-icing.
It's a pretty cool idea. Bleeds are actually complicated and siphon a considerable amount of thrust (1-2% in cruise) off of the engines, so eliminating them gives you correspondingly less fuel consumption in cruise, which compounded over the life of the airframe (or even a month of hard flying) really adds up. Plus, duct leaks are bad, and can be really bad, and all that extra ducting adds weight and maintenance.
The airplane is a radical departure from conventional airliner systems (in some ways this makes Airbus look positively backwards, which is not something you get to say about the traditionalists in Tukwila versus the French automators in Toulouse frequently) and the more I read about it, the more I like it.
(Type-rated 787 pilots, if I've blown some of this discussion, please point it out - but I think I got the 37,000-foot view.)
