Fuel Systems (metering, control, FADEC)
What this domain tests
Fuel Systems questions cover metering, control, and the hand-off between hydro-mechanical and FADEC control. Expect fuel scheduling, fuel-oil heat exchange, nozzle supply, and FADEC redundancy.
Core rules
- The fuel system must deliver filtered, pressurized fuel at the correct flow for starting, acceleration, and steady state. The fuel metering valve sets flow; the pressurizing valve ensures minimum nozzle pressure.
- Hydro-mechanical controls use a governor, acceleration schedule, and limiters. FADEC replaces many functions with digital schedules and provides envelope protection.
- FADEC uses dual channels, cross-channel data, and built-in tests. It controls fuel flow, ignition, variable geometry, and bleed valves on many engines.
- Fuel-oil heat exchangers use fuel to cool oil and oil to heat fuel, improving combustion and preventing ice.
Worked example
A FADEC commands 1,800 kg/h during climb. Fuel density is 0.80 kg/L, so volumetric flow is 1,800/0.80 = 2,250 L/h = 37.5 L/min. If the fuel metering valve is calibrated in pph and the aircraft uses lb/h, convert: 1,800 kg/h x 2.2046 = 3,968 lb/h. The reasoning: unit conversion errors are a common source of wrong answers, and FADEC schedules are usually in engineering units defined by the manufacturer.
Traps
Assuming FADEC controls only fuel; confusing the metering valve with the shutoff valve; ignoring fuel temperature limits; forgetting that fuel is also a heat sink; treating all FADEC channels as active.