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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.

This article is part of a collection that originally appeared on Patrick Smith, 38, is an erstwhile airline pilot, retired punk rocker and air travel columnist. His book, Ask the Pilot (Riverhead) was voted “Best Travel Book of 2004” by Patrick has traveled to more than 55 countries and always asks for a window seat. He lives near Boston.