15.11 Engine fuel systems — fuel metering and FADEC

Scope

A level 1, B1 level 2: operation of engine control and fuel-metering systems including electronic engine control (FADEC) and electronic power augmentation; system layout and components.

Requirements of the engine fuel control

The fuel control must deliver the right mass of fuel for the thrust demanded while keeping the engine within its limits:

• correct fuel for starting, idle, acceleration and deceleration without surge (too much fuel on acceleration), flame-out (too little on deceleration — "rich/lean blow-out") or over-temperature;
• automatic compensation for inlet pressure and temperature (altitude, airspeed, ambient temperature);
• limiting of rotor speeds (N1/N2 governors, overspeed protection), turbine temperature and compressor delivery pressure;
• control of compressor variable geometry (VSVs, bleed valves) on many engines;
• quick shut-off (HP shut-off valve).

Typical engine fuel system layout

Aircraft tanks → aircraft boost pumps → engine LP (spar) shut-off valve → engine LP pump (centrifugal, raises pressure to prevent cavitation in the HP pump) → fuel/oil heat exchanger (fuel-cooled oil cooler — heats the fuel to prevent ice crystals blocking the filter) → LP fuel filter (with bypass and ΔP indication) → HP pump (positive-displacement gear pump; output proportional to speed; relief valve) → fuel metering unit (FMU) / hydromechanical unit (HMU) → fuel flow transmitter → flow divider / pressurising valve → fuel manifolds → fuel nozzles (spray nozzles) in the combustor.

Excess HP pump flow not needed by the engine is bypassed back to the pump inlet (bypass/spill valve in the FMU). Some fuel (servo fuel) is used as hydraulic fluid to move engine actuators (VSV actuators, bleed valves, active clearance control valves).