The FADEC alternate control mode meters fuel to maintain a reference.

In the FADEC (Full Authority Digital Engine Control) alternate control mode, the system meters fuel to maintain the N1, or the fan speed reference. N1 is critical because it represents the speed of the low-pressure compressor and is an essential parameter for engine performance. By maintaining a specific N1 target, the FADEC ensures that the engine operates efficiently and within safe operational limits, balancing performance and fuel consumption.

Maintaining N1 in alternate control mode is especially important in situations where primary control modes may not be available or inoperable, allowing for continued safe engine operations. The system can dynamically adjust fuel flow to maintain this target, which is vital for ensuring thrust consistency, optimizing fuel efficiency, and responding to changes in flight conditions.

Other parameters, like EPR (Engine Pressure Ratio), N2, and RPM, are relevant to engine performance but are not the primary targets in the FADEC alternate control mode. The system specifically focuses on N1 to ensure the most effective regulation of engine performance under these circumstances.