Me
#881,
"If the APU and/or its 2 gennies (starter/generators) fail, perhaps catastrophically, there must remain a power supply to control the APU shutdown, and trigger the fire bottle(s) if necessary. The APU may have a FADEC generating its own power during normal running, but the APU also needs some kind of battery back-up, IMO. (Must admit I'm "winging" this one!)"
inetdog #887,
"...Which also makes it interesting to see that the battery failure "triggered an APU shutdown". Possibly once the fuel was cut off, the fully functional APU managed to generate enough power while coasting down to complete any required sequence. There was no other power source on the plane at the time, but it does sound like the shutdown would have occurred regardless of what other power was available."
Good point. However, isn't it a sort of chicken-and-egg situation? Normally, the APU genny trips off line almost immediately. And, as
USMCProbe now says:
"I believe on most airliners, if the batteries are shut off (offline) one way or another, the APU shuts down due to the APU ECU being unpowered."
So, what protects an APU if it catches fire during an auto-shutdown, with no personnel present? (Must emphasise this is not a 787-specific question.)