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Auxiliary Power Unit. A device for providing alternate power to an aircraft, usually located in the aircraft's tail.

11 votes

In what conditions is the APU used in midair?

In case of an emergency in flight, they can be used to start the engines (Note: The engines can be started in mid air using either cross bleed air or windmilling) Another thing is that even if the APU
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6 votes

Can airliners operate without an operating APU?

In most airline operations, APU is not mandatory. In ETOPS operations, FAA requires that the APU be present if it is required for the aircraft for complying with ETOPS requirements. … Also, APU should have in-flight starting capability, whether it is required for operations or not as long as APU is required by aircraft type certification. …
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33 votes

What is this open panel at the rear of this airplane?

It is the Auxiliary Power Unit (APU) inlet door. Image from Boeing Aero Magazine …
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