How are the engines started on a large aircraft, eg Boeing 777.?
Most commercial jets have an APU which has been discussed on here already. What it does is use the thrust to power a compressor mounted shaft, similar to turbine helecopters using the power to spin something rather than to just provide thrust. The shaft drives a generator to provide AC power to the aircraft for lighting, instruments, start switches etc. Some APU’s use a shaft that drives an air compressor to provide bleed air for airconditioning and to provide bleed air to spin the N2 compressor. While some extract air from the compressor section of the APU. Most startup procedures require starting the APU which requires DC power from the batteries or ground source. The APU doesn’t need bleed air from another source making it self reliant, however it does require a minimum of 23 volts from a source to start the APU (737-300). When the APU has achieved starter cutout speed a blue light indicates that APU generator power is available, so switching the APU generator switches transfers pow
