At what engine condition should fuel be switched on during startup?

The procedure for starting a turbine engine involves a carefully managed sequence to ensure safe and optimal operation. Fuel should be switched on when the engine reaches a certain percentage of its maximum RPM, which is typically identified as 12-16%. This timing is crucial because at these RPM levels, the engine is better prepared to receive fuel without stalling or causing excessive startup wear. Introducing fuel too soon, before reaching this threshold, can lead to combustion instability and potential engine damage.

Starting the fuel flow too early, such as immediately upon ignition, can lead to issues with fuel wash on the hot components or instability in the combustion process. If the engine is not yet at a sufficient RPM to ensure proper airflow and combustion efficiency, premature fuel introduction could cause the engine to flood or experience a flame-out.

Moreover, delaying fuel flow until after the engine reaches maximum temperature is inappropriate since the engine will not be operational and could result in failure to start. Stabilizing the engine RPM above 25% is generally relevant for monitoring performance after ignition, but switching on fuel at that later stage does not align with common best practices for starting procedures. Therefore, initiating fuel flow at 12-16% RPM optimally balances the need for safe operation with engine responsiveness during the start sequence