Why do commercial jet aircraft have a hole in their tail?

That's the vent for the exhaust from the auxiliary power unit (APU), a small turbine that sits in the tail cone.

The APU provides electrical power for the aircraft's electronics, airconditioning, cabin lighting and other electrical systems when the aircraft is on the ground.

A parked aircraft can also draw power from the electric grid or from a portable ground power unit (GPU) but when that's not available the APU is called into action.

By Michael Gebicki.

Full story at Traveller.