Jim From United States of America, joined Jul 1999, 455 posts, RR: 1 Posted (7 years 5 months 1 week 20 hours ago) and read 9076 times:
Hey ya'll! Firstly, have a great Holiday!
Now to my question: I am having a 'heated' discussion with a co-worker regarding the 747-400 electrical system, specifically this -- I know that the APU cannot be started in flight. BUT, if it were to happen that the APU were running at take off, AND the flight crew de-selected all four IDGs, would the APU generators power the synch bus and thus the AC buses? I know that the APU generator AVAIL and ON lights would extinguish at rotation, due to air/ground input from truck tilt, but would the flight-deck 'go dark'?
Not that I expect anyone to try it and get back to me , but perhaps a 744 pilot may have done this in simulator training?
PhilSquares From , joined Dec 1969, posts, RR:
Reply 2, posted (7 years 5 months 1 week 8 hours ago) and read 8942 times:
As Zeke pointed out, the 744 APU can't be started in flight. While it was an option on the classic of in-flight start or not, I can't remember ever seeing it listed as an option on the 744.
The primary reason for the 744 APU being available on takeoff is for a bleeds off takeoff. Having the bleeds off gives a slight improvement on takeoff performance and by using the APU bleed, the packs can still run and provide cabin cooling.
The APU GENS will not power the sync bus when the aircraft is in flight.
Jim From United States of America, joined Jul 1999, 455 posts, RR: 1
Reply 3, posted (7 years 4 months 3 days 15 hours ago) and read 8057 times:
Zeke and PhilSquares,
Thanks for taking the time to answer my question. I have continued my research, and I simply CANNOT find any air/ground inputs to the APBs, and NONE to the AGCUs. So, can you tell me HOW an air/groiund transision causes the APBs to trip open?
Tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 78
Reply 6, posted (7 years 2 months 1 day 18 hours ago) and read 7010 times:
Quoting 777WT (Reply 5): Is there a reason why the APU on a 744 can't start in flight? by design?
By design. There's was no need for it (in theory), since the 747 is never supposed to be in a situation where all four engines stop providing electrical or pneumatic power. 747's have since had total electrical power loss events, but the appropriate fix there is to eliminate the common-mode failure, rather than an air-starting APU.
I'm not aware of any case of a 747 loosing all four engines in a fashion that wouldn't also render the APU useless.
Bridge From United States of America, joined Mar 2009, 23 posts, RR: 0
Reply 9, posted (7 years 1 month 4 weeks 15 hours ago) and read 6564 times:
I thought the APU was successfully started in the BA009 incident...I could be wrong, however. I tried to find some more info online but came up with nothing.
The ash encounter that knocked out the main engines was relatively brief...it was then simple to start the APU as normal. The aircraft was out of the ash cloud by then, albeit with severely damaged main engines.
As you said, though, had the APU been running, it surely would have been knocked out as well.
A bit off topic, but why have some classic 747s been modified to disallow the use of the APU in flight?
Lowrider From United States of America, joined Jun 2004, 3220 posts, RR: 9
Reply 10, posted (7 years 1 month 4 weeks ago) and read 6474 times:
Quoting Bridge (Reply 9): but why have some classic 747s been modified
I don't know of any that have been modified to prohibit the use of the APU in flight, but the classic came with 3 possible APU start options. 1. Inflight operation prohibited. 2. Inflight operation permitted only after starting on the ground. 3. Inflight start and operation permitted. The distingiushing feature was the design and operation of the intake door.
Quoting Tdscanuck (Reply 8): Of you've got enough ash to kill an engine, why do we think the APU would have kept running? The APU is considerably less tolerant of solids due to tighter clearances.
Assuming you are still in the ash cloud, the APU would probably not last long, but a few minutes of air to help crank ash impaired engines is preferable to a windmilling start.