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How Does an Aircraft APU Work?Modern airplanes are outfitted with auxiliary power units (APUs) to meet the aircraft's large energy requirements while the engines are not running. This self-contained unit represents a constant-speed gas turbine engine located in the airplane's tail cone which is isolated in a titanium fireproof compartment from the rest of the aircraft. But how does it work? INFORMATIONAL (aeroxplorer.com) More...
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As a maintainer in retirement days, old reciprocating aircraft had an apu and many jet aircraft had a apu/gtc. Old school recips screwed their way and jets sucked and blew their way around the world! Just sayin…..
The first commercial airliner to use a tail mounted APU was the Boeing 727. That was my first aircraft as an ATP. Those were manufactured by Honeywell and were rather crude in the beginning. By the time I was on the flight deck all of the original auxiliary units had been replaced but were still made by Honeywell. Training for the Boeing 787 I was surprised to learn the APU has only one job and that is to generate electrical power. This is a first in APU manufacturing as the 787 requires far more electrical power than previous models.
I think you got your planes mixed up. The 727 APU was in the main landing gear wheel well.
Also, the APU was for ground Ops only. It could not be used in flight as a back up Generator, but with 3 Engines, they had 3 Generators to start with, so it was not really needed... The 727 also did not have automatic Phase control either as more modern planes do, so the Engineer had to do a lot more work to keep the Electrical loads adjusted and balanced.
That is all true. Funny how it is now required for ETOPS aircraft to be able to start and run the APU even at cruise altitude.
Not really... the reason is simple... It is there for a backup Generator Only incase 1 of the IDG's fail enroute. It is called failed Redundancy which is the same reason you have to have an ISIS or Stby Horizon.. Backup.
You are so right. My error! Should have said “737”. I was the flight engineer on the 727 when I started my flying career with PanAm. Watching tv and typing on my iPad. Not a good thing! The 727 has its own interesting history which includes the APU was an addition to the design when supposedly airlines requested it be made available. As you know, the engine configuration made the tail mounted APU a no-go. Boeing got creative and installed the rather small APU in the wheel well having to create an inlet located, if I remember correctly, in the right wing root(?) and exhausting in the opposite wing. The MD tri-engine had a similar problem. (I read this post carefully before selecting the post button!)
You were almost right, the intake was on top of the left wing at the root, and the exhaust was at the same location on the right wing. I got to meet the mechanic that came up with the idea of putting the APU in the MLG wheel well. He made the suggestion and the engineers took it from there. I attended a 737NG gen fam class that he was teaching.
I seem to recall the APU drew air from the wheel well and exhausted through a louvered vent on the right wing.
No, it was on the top of the wing... As a Mechanic, I have worked on quite a few of them.
Ok. It's been a long time, so my memory could be off. Thanks