When starting a turbojet engine, when should the starter be disengaged?

The starter should be disengaged when the engine reaches idle RPM because at this point, the engine has successfully started and is self-sustaining. Idle RPM indicates that the engine has accelerated sufficiently to allow combustion and maintain operation without the aid of the starter. This ensures that the engine can continue to run reliably and prevents any unnecessary wear or strain on the starter mechanism, which is designed for initial engine spooling rather than extended operation.

Disengaging the starter at idle RPM also ensures that the system operates within its designed parameters, promoting safety and longevity of the engine components. Starting the engine and disengaging the starter before this threshold can lead to stall or failure to start, while waiting until the engine reaches full throttle or during the takeoff roll may create unnecessary complications and potential for failure in the starting sequence.