Fuel Systems
Fuel metering and scheduling, the FCU and FADEC, distribution and surge/flame-out protection.
Fuel systems, summary notes
- The engine fuel system meters exactly the right fuel flow for the power demanded, at all altitudes, speeds and temperatures, while protecting the engine from surge, overspeed, over-temperature and flame-out.
- A high-pressure (HP) pump (usually gear-type) supplies more fuel than needed; the fuel control unit (FCU) meters the required amount and spills the excess back, scheduling fuel against throttle position, RPM, compressor pressure and inlet temperature.
- Modern engines use FADEC (Full Authority Digital Engine Control): an electronic engine controller reads the sensors and commands a hydromechanical fuel-metering unit, giving precise scheduling, automatic limit protection and reduced crew workload.
- Downstream, a flow divider and fuel manifold distribute fuel to the burners; a pressurising-and-dump valve ensures a minimum pressure for correct spray and drains the manifold at shutdown to prevent carbon build-up.
- Acceleration and deceleration schedules keep the fuel flow inside the surge and flame-out limits during throttle changes; an HP shut-off (cock) stops fuel to shut the engine down.
- ⚠ Exam trap: the FCU/FADEC schedules fuel to stay within the surge line on acceleration and above the weak-extinction (flame-out) limit on deceleration, it is not simply proportional to throttle; excess pump fuel is spilled back, not burned.
Why can't the throttle simply be connected directly to a valve that sets fuel flow in proportion to lever angle?
Raw proportional fuel would over-fuel on rapid acceleration (driving the compressor into surge and spiking EGT) and under-fuel on deceleration (risking flame-out). The FCU/FADEC schedules fuel against RPM, pressures and temperature so acceleration stays below the surge line and deceleration stays above weak extinction, while protecting RPM and temperature limits.
Fuel systems concept map
Fuel Systems
Fuel systems quiz
Fuel Systems, quiz
1. The fuel control unit of a gas turbine schedules fuel to:
Match engine demand while avoiding surge and over-temperatureKeep the fuel tanks fullControl the cabin pressure2. The fuel control unit (FCU) primarily:
Meters the correct fuel flow and schedules itIgnites the fuelCools the oil3. FADEC stands for:
Full Authority Digital Engine ControlFuel And Direct Engine CoolingFan And Duct Exhaust Controller4. On rapid acceleration the fuel schedule must stay:
Below the surge lineAbove the maximumAt idle5. On rapid deceleration the fuel schedule must stay:
Above the weak-extinction (flame-out) limitBelow zeroAt full flow6. The HP fuel pump usually delivers:
More fuel than needed, with the excess spilled backExactly the burned amountLess than needed7. A pressurising-and-dump valve:
Ensures spray pressure and drains the manifold at shutdownCools the turbine by metering a controlled bleed of fuel through the blade root passagesMeters oil to the bearings in proportion to the fuel flow demanded by the control unit8. The engine is shut down by:
Closing the HP fuel shut-off cockOpening the bleed valvesIncreasing ignition