What is the APU?

by Captain Joe

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 are 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 its 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 APUs are a single spool turbine which power an electrical generator and air compressor.

The APU has its 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 its own fire protection system which automatically activates an internal fire extinguisher reducing the risk of a fire.






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