Ignition system

The compressor begins building airflow.

Ignition and fuel are introduced in a controlled sequence.

The turbine then extracts enough energy from the hot gas to keep the compressor turning without starter help.

That change from an externally driven machine to a self-sustaining gas turbine is the central event of a normal start.

The engine reaches a self-sustaining condition.

The starter can disengage or be cut off according to the system's design.

Ignition may also be turned off after a stable start because continuous combustion can maintain the flame.

Aircraft can use continuous or automatic ignition in selected conditions, but that is a separate operating topic.