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How Does an Aircraft APU Work?

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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) 更多...

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bayou
sal derosa 5
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…..
joekirbyjr
Carlen Kirby 2
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.
JetMech24
JetMech24 5
I think you got your planes mixed up. The 727 APU was in the main landing gear wheel well.
sparkie624
sparkie624 3
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.
JetMech24
JetMech24 1
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.
sparkie624
sparkie624 1
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.
joekirbyjr
Carlen Kirby 5
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!)
JetMech24
JetMech24 1
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.
ToddBaldwin3
ToddBaldwin3 1
I seem to recall the APU drew air from the wheel well and exhausted through a louvered vent on the right wing.
sparkie624
sparkie624 1
No, it was on the top of the wing... As a Mechanic, I have worked on quite a few of them.
ToddBaldwin3
ToddBaldwin3 1
Ok. It's been a long time, so my memory could be off. Thanks
sparkie624
sparkie624 1
You are from My error... Or close to it...
bayou
sal derosa 3
Retired USAF 1972-1992
Govt contractor in aircraft services
cahllc
P Turner 4
I just want to thank the older experienced guys for simplifying this article with straight forward explanations and insight as to the workings of the APU. Most people ether involved in or enthusiastic about aviation understand the APU’S but don’t always get a chance to hear from the guys that designed and worked on them and the stories are wonderful to hear! Thanks fellas for the input, please keep it coming!
cahllc
P Turner 1
Sorry for getting in on the comments so late, our internet has been poor for about a week now. Maybe some of you guys could go to work for Verizon and get things straightened out, they seem to only have the “sucked “ part down……..
joekirbyjr
Carlen Kirby 7
This article is rather simplified and probably does not interest most GA pilots. But it might cause someone to want to delve deeper into the subject. In my day job, I rely heavily on the APU. When no ground source of air pressure is available, it is very vital to have the APU up and running. The starters on my GE engines uses high volume compressed air to turn the engines to the proper RPM before fuel and ignition occur. For those interested, once our main engine is spooling up, the bleed air from that engine is directed to the second startup and the APU bleed is shut down. The APU is just another component down line during the startup procedure.
sparkie624
sparkie624 4
Actually the operation of an APU or a JET engine is remarkably simple... There is no real technology in the basic operation... Now days they add FADEC's to them to make them more efficient and different tail pipes to make them less noisy, but the bottom, Spin a shaft with some fans on it, Throw in fuel and a little fire and you have a Jet Engine!
bentwing60
bentwing60 4
to coin a phrase from MX., suck, squeeze, bang and blow with a generator on the downstream gearbox. quite simple actually! B-29's had gas powered APUs in later production models.

"The engines started, and the craft was ready for takeoff; however whilst moving towards the runway a gasoline leak occurred (stemming from the auxiliary power units fuel tank)."

https://www.warhistoryonline.com/whotube-2/b-29-kee-bird-frozen-time-watch.html?firefox=1

And Darryl Greenamyer was a True macho man! see his F104 exploits!
victorbravo77
victorbravo77 4
Called "Putt-Putts" as I recall.
briansfreeman
Brian Freeman 5
Sounds like an old girlfriend of mine...
sparkie624
sparkie624 2
From a Maintenance Prospective it is actually "suck, squeeze, bang and blow" machine that does the exact same thing as a Recip engine, where as there is only 1 difference. The Jet Engine (OR APU) does it all in different locations of the engine where the Recip does it all in the Same Location at different times... (The Perfect Lady so to speak)
xtoler
Larry Toler 4
I learned something new. The closest I got to MX was the MX job controller next to me when I worked ATOC at 313th APS or slammed some adult beverages after work. Recips were before my time, but I didn't realize they had an APU. I'm thinking like the C124's. Could one of the engines running at idle act as an APU?
That said, in the civilian world, I've seen Saabs run an engine; although it's a turboprop, as an APU. We flew J41's and some pilots we could do the same thing but company policy and safety issues prevented us from doing it. BTW, I have performed pax and cargo onload with engines turning on C130's and NATO C160's back in the day. They do have their own APU's but we moved people and cargo really quick that way.

Yeah, and I'm enjoying an adult beverage now, so forgive me if this post seems strange.
icwool
Chris Wool 2
Technically C130s had GTCs as they had no generators. Not sure about the J models.
heyeng
Chuck Blake 2
Beginning with the H1s, C-130s gained an APU.
sparkie624
sparkie624 2
From a Maintenance Prospective it is actually "suck, squeeze, bang and blow" machine that does the exact same thing as a recip engine, where as there is only 1 difference. The Jet Engine (OR APU) does it all in different locations of the engie where the Recip does it all in the Same Location at different times... (The Perfect Lady so to speak)
dcmarotta
Dan Marotta 3
So why does Flightaware keep posting articles on sites with paywalls?
TorstenHoff
Torsten Hoff 3
FlightAware has no control over what their users post. And the people who post the content may not be aware that what they share is behind a paywall -- for those who pay a subscription fee, the access is normally transparent.
ToddBaldwin3
ToddBaldwin3 1
In this case, they should know. The poster is one of the writers for Aeroexplorer. If younoook at his history, he had only posted articles from Aeroexplorer.
dcmarotta
Dan Marotta -2
I would have thought that the articles posted on FlightAware come from them, not from users. This site is getting too tedious to browse.
TimDyck
Tim Dyck 3
No one is forcing you to click on the articles your not interested in.
sparkie624
sparkie624 2
That is the way it has always been and I personally have not had any issues with it.
sparkie624
sparkie624 1
Just FYI, I have now been a member of FlightAware now since 2004 and feeding ADSB for the last 7+ years! (Since it first Started!

[This comment has been downvoted. Show anyway.]

xtoler
Larry Toler 14
Not everyone on this site are seasoned pilots like yourself.
augerin
Dave Mathes 7
...that's cold man, you might be surprised by how many general aviators don't have a clue....
TimDyck
Tim Dyck 12
Since I have experience working on both Jet engines and Turboprops I found this article to be pretty simplified. But I also respect the fact that most people have no clue that an APU exists let alone how it works so to them this is very educational. If you found the article uninteresting you can move on but maybe realize that to others this was a very interesting piece. This isn’t Facebook or Twitter were every post has to be followed by rude comments.

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