What is a APU? Explained by 'CAPTAIN' Joe

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Dear friends and followers, today´s question is another absolute classic, „Joe could you explain, what is the APU?“
The APU is a very important topic, so let´s get started !

I´m pretty sure many of you have seen and heard the APU before. Okay let´s imagine you´re in an bus driving you towards your airplane which is parked at an outside position.
As you get off the bus and onto one of the mobile stairs you can hear this sound? (APU sound) That my dear friends is the sound of an running APU.
APU stands for Auxiliary Power Unit. On most commercial jets the APU is placed at the far end of the fuselage and it´s main purpose on ground is to make the plane self sustainable providing it with electrical power and bleed air pressure for the air-conditioning system and especially for engine start.

Most APU`s are a single spool turbine which power an electrical generator and air compressor. The APU has it´s own electronic control box which monitors the start sequence, the turbine speed and exhaust temperature, oil and fuel supply and the bleed air outlet and in case of a failure it controls an automatic shutdown to prevent any further damage. It even has it´s own fire protection system which automatically activates an internal fire extinguisher reducing the risk of a fire.

[jeffreymorris]

Captain Joe, bringing aviation knowledge to the fans of flying.
Thank you!

[AlonsoRules]

The APU played probably the most important role in the Hudson River Miracle - its early deployment was a masterstroke, despite not being on a checklist

[Chuckish]

On a military aircraft like the C-130 Hercules, we didn't start the APU until just before engine start, we had external ground electrical power to power the electronics and hydraulics, after starting engines 3 & 4, the APU was usually shut down as hthere was enough air from the two started engines to start the third engine and then air from the 3 started engines could start the 4th engine. On an older airliner like the Boeing 707-342C used by the RCAF up until the got their Airbus aircraft, the APU was in a start cart attached to the aircraft and used to start all 4 engines, then shut down, the external ground power stayed connected until all 4 generators were brought online. then both were hauled away by a mule as the final checks were being done.

[raff-raffr]

Please don't forget make a video about the engine start sequence

[bloodridegaming]

Watch this 1.25x speed Thank me later : )

[enochsharma]

Cpt. Joe! A video on the air conditioning system especially the bleed air and packs would be really helpful.

[sorcererCermet]

I know all of this stuff is essentially hardwired in your brain at this time, but I have never personally heard of an APU and always used to think the noise was just somehow the main engines making noise. I had never even thought about how power could be supplied to the cabin during docking, even though I had seen the APU exhaust port on many planes.

Thank you for your super insightful video and I look forward to learning about the main engine start sequence :)

[ElizabethSwims]

Couple of questions. If the jet engine requires compressed air to run where does it get it when it’s at such a high altitude? Second question. Is it possible to start a jet engine by diving and using wind speed kind of like they do with prop planes or push starting a car?

[cancelanime]

“Getting off the bus onto the mobile stairs”

American airports: am i a joke to you?

[flyingrico]

Why do you always start up the right engine (engine 2) first?

[Zfx]

I was 16 years old when I first saw this and other videos by captain Joe, I had a dream of flying since childhood and these just made it even deeper. Now I am 23 and came back to watch these videos while doing my type rating for A330-200, 300 as a second officer!

Thank you Captain Joe!

[nj]

My friend who flies for AA says he also uses the APU to assist during take off from high temp / high altitude airports, especially while flying the 321. The APU runs the a/c and he has max thrust available for takeoff. He said that he's only over temped the engines 3 times, all at Phoenix Arizona. Love your video's CJ :-)

[colonelccccc]

You are THE CAPTAIN ! Thank you for all your videos !!!!

[aayushkumar-spzy]

thank you sir for making my requested video.this was very helpful.
Plz make a video on RNAV, NAV, NAV*, LOC, LOC* etc. and there buttons

[rodneylackey]

Very helpful for a non-mechanical senior citizen!! Many thanks!

[grahamrothphotography]

Good video! Here in YEG (Edmonton) when its cold out (-15C or colder) a few airlines wants APU fuel to run the APU overnight the keep the plane from freezing.

[MalaysianAviator-]

How do turboprop planes use reverse thrust without the reverser doors? eg.ATR72 ATR42 and the DASH8

[patrickjohnson]

Another fantastic video Joe. BTW have a nice christmas

[EtaminGaming]

Thank you very much Joe for explaining how the APU works!

[nmanbamboo]

Man!! I always wanted to know about the exhaust port at the tail section and now got someone giving a simple and effective explanation about just what it does. Thanks a lot for this.