The auxiliary power unit

The auxiliary power unit (APU) on a commercial airliner is mounted in the tail at the rear of the aircraft, this turbines runs when the aircraft is on the ground.

The purpose of the APU is to provide power for the aircraft running electrical systems and most importantly for passengers - the air conditioning system. This was made very clear to me on a series of flights when the APU had failed, sitting in the aircraft sweating away certainly made us aware of this.

I was always aware of the APU but become more interested in its role after this problem. I had questions such as is the unit shut down during flight. What is the size of the unit? What power source is used to crank the turbine to start.

I am aware a start cart is sometimes used to crank the turbines with the power generated by the running turbine to crank the rest of the turbines. The running APU has enough power to crank the turbines without the need of a start cart so I was guessing heavy duty cabling runs from the APU to the turbines.

From some research, I have found out the APU is small enough to be started via electrical means with the APU then providing a pneumatic start using an air bleed from the running APU to fire up the turbines. All in all, it is a fairly effective and cost efficient method to crank commercial jet airline turbines.