How to start engines on the Boeing 737. The process of starting a jet engine is a little more involved than starting a car, and rather than the engine starter motor being powered electrically, the aircraft uses compressed air to spin the engine before fuel and ignition are added. The normal start process uses bleed air from the APU (Auxiliary Power Unit) situated in the tail of the aircraft, to spin the inner compressor, known as the N2 rotor. We ensure that the APU bleed valve is open and once