Auxiliary Power Unit(APU)

Auxiliary Power Unit is the small engine located at the back hole of the aircraft.  APU is generally used for the starting of the main turbine engines before the flight with the assist of hydraulic accumulator or battery.
They provide electricity, compressed air, air conditioning, electric power and other aircraft systems.APU is also responsible for providing the backup electric power during flight.  It even has it´s own fire protection system which automatically activates an internal fire extinguisher reducing the risk of a fire.