Exhaust

Q: I notice a hole up under the tail of most airplanes that seems to some kind of exhaust. What is this?

That's the exhaust for the auxiliary power unit (APU), which is a small jet engine used for electricity, heating, cooling, etc., when the main engines are not running, or to supplement them. It also provides the compressed air for starting the main engines. All modern jets and even some turboprops have these, and they are typically, but not always, located under the tail.

Article added on September 20, 2005

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