The engine start-up sequence begins with the auxiliary power unit, or APU. The APU is a miniature jet engine
with its own compressor, combustor, and turbine that provides electricity to the aircraft and compressed air for the air conditioning system while the aircraft is on the ground. Despite being a jet engine in design, the APU does not provide thrust to the aircraft. In addition to its other duties, the APU provides the first step in starting the jet’s main engines and causing its blades to rotate.
After passengers are onboard and buckled in, the APU begins to send compressed air to the jet’s main turbine engines. The compressed air passes through a small turbine on the outside of the engine, which causes it to spin. Attached to this turbine is a shaft which connects via gears to the main engine shaft
, which begins to spin as well.
Once the main engine shaft and its blades are spinning, the pilot adds fuel to the combustor section of the engine. An electric spark then ignites the mixture of fuel and air, and the exhaust passes from the combustor out through another turbine of blades, speeding the engine up to the point that it is self-sustaining. More fuel can then be added, which speeds the engine up even more, increasing its power output and eventually enabling flight.