Ever noticed what this tiny hole is for?

A commercial aircraft is an awesome machine. It is paked with cutting-edge techs, many of which are still unknown to us. If you look closely, you will find a small hole in the back of a plane's tail. Its an engine called the Auxilary Power Unit, the samllest but the important engine in a plane. The APU generate the necessary power to atart the main engines and to run many on board system.