APU, Auxiliary power unit, a little turbine powered generator located in the tail to furnish electrical power on the ground when the main engines are shut down. We had 'em on military transports, so we could have aircraft power for lights and electronics and cargo door motors while on the ground in remote dirt strips with no ground power. Also used for engine starting. I was surprised to learn that a civilian airliner like the 787 had an APU. 787 is intended to operate from real airports that have ground power equipment.
And, Boeing and / or Pratt & Whitney seems to have screwed up the design of the 787 APU. Shutting down the APU on the ground automatically closes the APU intake door, cutting off airflow and overheating the APU. Heat build up is so bad it warps the APU drive shaft out of true after about 20 minutes. It does eventually cool down and straighten out. But if the APU is restarted before cooldown (2 hours) bad things happen. United Airlines, a 787 operator, says "Bowed rotor shaft can cause turbine rub and significant damage". Like a new APU for God Only Knows how much money.
Adding insult to injury, Boeing says that leaving the APU in "run" will keep the air intake door open, but, this only works on ground power. The APU battery only has enough juice to hold the door open for 15 minutes, where as it takes a hour of open intake door to keep the APU from over heating. Which is weird, The APU battery is good for 30-40 ampere hours, which ought to be enough to keep a door solenoid powered for a whole day. If this is true, it means the APU battery barely has enough juice to start the APU, and any drain, say from keeping the intake door solenoid power for a couple of hours, means the APU battery won't have enough charge to start the APU. Which is a nice way of saying that the APU will fail. Bad scene.
Good luck Boeing, you are gonna need it.
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