FAA Aviation Mechanic Powerplant (AMP)Turbine EnginesEasy
A hydromechanical fuel control unit (FCU) on a turbine engine schedules fuel flow to the engine. Which combination of inputs does the FCU primarily use to compute the correct fuel flow?
- AOutside air temperature and indicated airspeed only
- BOil pressure and exhaust gas temperature only
- CN2 speed and fuel tank quantity
- DCompressor inlet air temperature and pressure, engine speed, and throttle (power lever) position
Show answer & explanationAnswer & explanation
Correct answer: D. Compressor inlet air temperature and pressure, engine speed, and throttle (power lever) position
A hydromechanical FCU meters fuel based on compressor inlet pressure (Pt2) and temperature (Tt2), engine rotor speed, and throttle lever position, allowing it to schedule the correct fuel flow for acceleration, deceleration, and steady-state operation while protecting against compressor stalls and overtemperature.
Why the other options are wrong
- A. Airspeed and OAT are not direct FCU inputs; compressor inlet conditions are used instead.
- B. Oil pressure and EGT are monitored parameters, not primary FCU scheduling inputs.
- C. Fuel tank quantity has no role in metering fuel flow to the combustor.
Hydromechanical Fuel Control Unit
A mechanical/hydraulic device that meters fuel flow to a turbine engine based on compressor inlet conditions, rotor speed, and throttle position.
- Uses Pt2/Tt2, N speed, and throttle position
- Prevents overtemperature and compressor stall during transients
- Being replaced in modern engines by FADEC
Memory trick: Think 'PETS' - Pressure, Engine speed, Temperature, Stick (throttle) for FCU inputs.