At what percentage of N2 should the EFC come on during engine start?

The correct percentage at which the Engine Fuel Control (EFC) system is expected to activate during engine start is 38% N2. This activation point is critical because it ensures that the engine receives the appropriate amount of fuel to achieve optimal ignition and combustion conditions as the compressor reaches a sufficient speed. At this point, the engine has built enough airflow due to the rotating speed of the N2 spool, allowing the EFC to begin injecting fuel properly.

This helps establish stable engine parameters and enhances the likelihood of a successful start, minimizing the risk of flameout or excessive starter engagement time. Understanding the operational characteristics of the engine, including the significance of timing for EFC activation, is essential for safe and effective engine management during start-up procedures.