Section 15.11

Fuel Systems

Fuel metering and scheduling, the FCU and FADEC, distribution and surge/flame-out protection.

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Main ideas

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.

Solved examples

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.

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