Fuel Systems and Controls
What this domain tests
This domain covers fuel metering from tank to nozzle, hydromechanical and electronic control, and the way FADEC or EEC schedules fuel while protecting limits.
Core rules
- Fuel flows from tanks through boost pumps, shut-off valve, filter and water separator, fuel/oil heat exchanger, fuel metering unit (FMU) and flow divider to the nozzles.
- A hydromechanical fuel control uses a governor, acceleration schedule and bellows to set fuel flow. An EEC or FADEC adds electronic scheduling, dual-channel redundancy and fault logging; it commands the FMU, which remains a hydraulic actuator.
- FADEC normally controls N1 (thrust) and protects N2, EGT and surge margin. It does not control everything: oil pressure, some bleed valves and thrust reverser interlocks may remain separate.
- Fuel types: Jet A-1 (common outside the US), Jet A (US) and JP-8 (military). Watch for water, microbial growth and ice; FSII is an anti-icing additive.
Worked example
At cruise, fuel flow is 2,500 lb/h and TAS is 450 kt. Fuel per nautical mile = 2,500 ÷ 450 = 5.56 lb/nm. If the flight plan expected 5.0 lb/nm, the aircraft is burning about 11% more than planned. Reasoning: specific range falls with headwind, lower altitude or bleed air use; check the cause before declaring a fuel problem.
Traps
- Thinking FADEC 'full authority' means it can override all limits.
- Confusing fuel flow (pph or kg/h) with fuel quantity.
- Ignoring a fuel filter bypass indication; contaminants reach the FMU.
- Assuming a dual-channel FADEC has no common failure modes; power supply and connectors matter.
Key points
- Fuel path: tank, boost pump, shut-off valve, filter, fuel/oil cooler, FMU, nozzles.
- Hydromechanical control uses a governor and acceleration schedule; FADEC adds electronic scheduling and protection.
- FADEC commands the FMU; it is not 'full authority' over every system.
- Jet A-1, Jet A and JP-8 are kerosene fuels; watch water and ice.
- Fuel per nautical mile = fuel flow ÷ TAS.