Originally Posted by AllanJ
The APU on many jet planes contains a little jet engine. So a 747 has five engines and a 737, three. The exhaust comes through the hole in the rear which in turn accumulates discolored stains.
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Technically, an APU is a system that includes a gas turbine that powers a generator and provides compressed air to start the engines. It's very similar to what a train locomotive or an M1A1 battle tank uses, albeit on a smaller scale.