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After a successful start of a high bypass turbofan engine, the air starter:
  • A
    is disengaged by using a clutch that is controlled by the main engine control.
  • B
    is disengaged by the coupling between engine N1 spool and starter by the use of centrifugal forces.
  • C
    remains engaged and rotates continuously with the N2 spool.
  • D
    is disengaged by the coupling between engine N2 spool and starter by the use of centrifugal forces.
The typical start sequence of a gas turbine engine on the ground would be the following:
  • Start selected: the start valve admits air supplied by the pneumatic system to operate the starter. On nowadays commercial aircraft, the FADEC controls the start valve electrically and operates it pneumatically with bleed air.
  • Ignition ON: the igniters are selected ON and are ready to light the mixture.
  • High-pressure fuel ON: after fuel is delivered by the high-pressure shut-off valve (HPSOV) it is sprayed into the combustion chamber by fuel nozzles under high-pressure where it is mixed with compressed air and lights up.
  • Light-up: the mixture starts burning and the flame spreads all along with the combustion chamber (since it is interconnected). The flame becomes self-sustainable.
  • Self-sustaining speed: all the engine’s spools have reached a self-sustaining speed where assistance from the air starter is not needed anymore. The air starter is disengaged by the coupling between engine N2 spool and starter by the use of centrifugal forces.
  • Starter circuit canceled.

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