An auxiliary power unit (APU) is a small, self-contained power source that is used in land and rail vehicles as well as in ...

It's called the auxiliary power unit, and it's the smallest engine on the plane. But that doesn't mean it serves a small purpose. It provides the necessary power to start the plane's engines and ...

But there's another engine hidden int he back of the tail. Called the auxiliary power unit (APU) It's the smallest engine on a plane. But you could say it's the most important one of all.