Master Facts: "What is the APU?"

If you know what the APU is, feel free to give me extra info on what it does:) For all that don’t, then fear no more!

History of the APU:
The first commercial jetliner with an APU was the Boeing 727, which debuted in 1964. Before that, planes used APU’s that were attached to trucks, making the engine starting process longer.

Let’s back up a bit. APU stands for the Auxiliary Power Unit. The APU is a small jet engine under the tail that supplies electricity and air conditioning when both of the plane’s engines are not running. All types of modern airliners have an APU. Besides providing electricity and air conditioning in emergency situations, it also provides high-pressure air needed for starting the main engines. This is needed because the internal batteries on larger aircraft aren’t powerful enough to initially rotate the engine’s compressors. It also provides comfort to passengers boarding when the engines aren’t on. Some airports limit the use of APU’s for sound purposes, and use ground power to start the engine. The APU is critical because it gives backup power in emergency situations, making planes slightly safer. So whenever you hear a hissing noise like 1000 hairdryers while boarding the plane, don’t fear, the APU is here.



Great informative paragraph for those that need it! Nice topic

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APU on aircraft have been used as early as WWI on airships /Zeppelins, used to power radio sets and similar. During WWII a lot of bombers and early jets had them as well, but again more for internal power rather than starting the engines.

As the OP stated in 1963 the first gas turbine APU was fitted to the B727.


And the APU was one of the things (a smaller thing) that made it possible for Sully to land on the hudson.

With fuel left, the APU will run (unless in some cases of elec fault or similiar). 1549 had fuel left and despite that the APU item was very late in the QRH for water ditching Sully turned that on when he felt the engines go. One of the most important things this does is keep the aircraft in normal law and keeping it within the airbus flight envelope providing the 5 utmost protections such as bank angle and speed protections (just to name two of them).


CaptainJoe on YouTube has an awesome video explaining this if you want even further details…


Thanks for that! Great info!

APU means auxiliary power unit which is located in the tail of the Aircraft back door and it’s used to run engines run air conditioning while the aircraft is on ground… and sometimes hydraulic and pneumatic systems…etc… I’m actually going to college for Aviation maintenance technology and so far this is what I know. Thanks for reading and wish me luck

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Bonus fact:

The reason why the cabin ventilation shuts off right before the engines start, is because they need all available pneumatic air possible for starting the engines. Otherwise when you’re at the gate, they redirect the pneumatic air to the cabin, so you won’t freeze or sweat to death. :)

Bonus bonus fact:

Same thing happens under takeoff. The engines need all the air they can get for maximum performance (since it isn’t enough air when the aircraft is stationary) so they shut the packs (ventilation) off then as well. :)


I found out what it was like 3 weeks ago:-)

Not all the time. It really depends whether a packs & bleeds off take-off is needed. If the aircraft is

  • very heavy
  • you have a short runway
  • it is very hot
  • you are very high
  • you have anti ice on

a packs & bleeds may very well be required. But if none of these factors are relevant then the engines will have sufficient performance to supply sufficient thrust for take-off and for packs & engine bleeds to remain on.

Also many airports have limits on how soon you can turn the APU on. Often you can’t turn it on until around 15 minutes before your expected pushback time.


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