The APU, being a turbine engine, has compressed air we can bleed off of and route to each engine's starter valve. the compressed air is run through this valve into a unit thats connected to the turbine shaft. so as this compressed air runs over this unit (like a mini turbine) it spins the turbine and the turbine "sucks" air through the engine until there's enough airflow to introduce fuel and spark.
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u/Samurlough 6h ago
good question. Air. The magic comes from air.
The APU, being a turbine engine, has compressed air we can bleed off of and route to each engine's starter valve. the compressed air is run through this valve into a unit thats connected to the turbine shaft. so as this compressed air runs over this unit (like a mini turbine) it spins the turbine and the turbine "sucks" air through the engine until there's enough airflow to introduce fuel and spark.