Check out any average commercial jetliner. If you look close enough, you’ll noticed that it has a hole on the back of the tail.
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 lift off the ground. Without the APU, there’s not enough energy to start the plane’s main engines.
The main engines require so much energy that they need the APU to give them a little boost in the beginning.
Once in the air, the APU is shut off and the main engines take over.
Video courtesy of Airbus.
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