Auxiliary Power Unit. Its primary purpose is to provide power to start the main engines of the aircraft. An APU powers up the aircraft in two stages. First, the APU is started by an electric or hydraulic motor, with power supplied by a battery, accumulator, or external power source (ground power unit). After the APU accelerates to full speed, it can provide enough power to start the aircraft's main engines, either by turning an electrical generator or a hydraulic pump, or by providing compressed air to the air turbine of the starter motor.