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Engine Fuel System — LP/HP Pumps, FMU, Shut-off and Overspeed Protection

Thrust Management ends at the FMU; this article is the execution end of "how much fuel." It follows fuel from tank to nozzle — pressurised, heated, filtered, metered — and covers the FMU's three jobs, the ENG MASTER shut-off logic, and how overspeed protection works by cutting fuel. It is the hardware basis for start faults, the shutdown sequence, fuel faults and overspeed later in the chapter.

1. The fuel path

The fuel system supplies fuel to the combustion chamber at the required flow rate, pressure, and temperature. The fuel flows from the tank, via the Main Engine Pump and the fuel/oil heat exchanger, to the Fuel Metering Unit (FMU) and to the fuel nozzles.

Per DSC-70-40-10, fuel runs tank → main engine pump → fuel/oil heat exchanger → FMU → nozzles. The full schematic:

 FROM FUEL TANK
   │
 LP FUEL SHUT-OFF VALVE ◄── ENG MASTER (ON/OFF) + ENG FIRE pb  (dual command)
   │
 LP PUMP ─► FUEL/OIL HEAT EXCHANGER ─► FILTER (FADEC→ECAM)
   │
 HP PUMP (driven by the HP compressor shaft) ── splits ──┐
   │                                                       ├─► servovalve actuators
 FMV (metering valve) ─► HP FUEL FILTER                   │     (air/oil HX valve, VIGV/VSV ×2)
   │
 HP SHUT-OFF VALVE ◄── OVERSPEED PROTECTION (TOS + CPU)
   │
 FUEL FLOW TRANSMITTER (→ECAM)
   │
 MANIFOLD (12) ─► FUEL INJECTORS (24)

 bypass: SPILL VALVE / DUMP VALVE ─► DRAIN TANK ─► OVERFLOW VENT

Per the schematic, the engine has 12 manifolds and 24 fuel injectors.

2. Pumps and flow

The HP compressor shaft drives the HP fuel pump assembly. Fuel flows through the LP pump, then through the fuel/oil heat exchanger, a filter, and the HP pump. Fuel then divides into two flows, one for the servovalve actuators, and one for the FMU's metering valve.

Per DSC-70-40-20, the HP compressor shaft drives the HP fuel pump; fuel goes LP pump → fuel/oil heat exchanger → filter → HP pump, then splits between the servovalve actuators and the FMU's metering valve.

[!note]- The HP pump is driven by the HP shaft (N3) (integrative synthesis) Mechanically coupled to the HP shaft, the HP fuel pump delivers pressure that varies with N3. At start, low N3 means limited HP-pump pressure — one factor in the start logic (see Start Faults). This is a reading of the verbatim "HP shaft drives the HP pump."

3. The FMU — three functions

The FMU is controlled by the FADEC, and performs : ‐ Control of the fuel flow to the engine combustion chamber through the Fuel Metering Valve (FMV) ‐ Overspeed protection by closing the HP shutoff valve ‐ Actuation of the HP shutoff valve to start, or stop, the engine.

Per DSC-70-40-40, the FADEC-controlled FMU does three things: (1) meter fuel to the combustor via the FMV; (2) overspeed protection by closing the HP shut-off valve; (3) actuate the HP shut-off valve to start or stop the engine. Metering + protection + start/stop in one unit.

4. ENG MASTER shut-off and the dump valve

Setting the ENG MASTER lever to OFF directly commands the closing of the LP and HP fuel shutoff valves for that engine's fuel system. Upon closure of the HP fuel shutoff valve, the dump valve opens, which enables any remaining fuel from the burners to the drain tank to be purged. During engine start, fuel from the tank is returned to the LP fuel pump.

Per DSC-70-40-30, ENG MASTER OFF closes both the LP and HP shut-off valves; the moment the HP valve closes the dump valve opens to purge remaining burner fuel to the drain tank.

[!note]- Why a dump valve opens after shutdown (integrative synthesis) With the HP valve closed, fuel still sits in the nozzle/burner manifold. Opening the dump valve purges it to the drain tank, preventing residual fuel coking or abnormal burning at the hot end. This reads the verbatim "purge remaining fuel from the burners."

5. Overspeed protection — by cutting fuel

The FMU's overspeed protection works by closing the HP shut-off valve (DSC-70-40-40 verbatim). On the schematic the HP shut-off valve is annotated OVERSPEED PROTECTION (TOS + CPU) — overspeed detection triggers fuel cut. Full handling is in Overspeed / Severe Damage.

6. LP shut-off valve — a dual command

The LP fuel shut-off valve takes commands from both ENG MASTER and the ENG FIRE pb:

Two independent commands can both cut the engine's fuel — the meeting point of the fuel and fire systems.

7. Counterintuitive points

[!warning]- Overspeed protection cuts fuel, it does not "limit" speed The FMU protects against overspeed by closing the HP shut-off valve (DSC-70-40-40) — not by a speed governor. Cut the fuel, lose the fire.

[!warning]- Shutdown still purges fuel After ENG MASTER OFF closes the HP valve, the dump valve opens automatically to purge burner fuel to the drain tank (DSC-70-40-30). Shutdown is not just "cut fuel and done."

Self-test

[!note]- Q1. Trace fuel from tank to nozzle — and what drives the HP pump? Tank → LP shut-off valve → LP pump → fuel/oil heat exchanger → filter → HP pump (driven by the HP compressor shaft) → splits (servovalve actuators + FMU metering) → nozzles.

[!note]- Q2. The FMU's three functions? (1) FMV meters combustor fuel; (2) overspeed protection by closing the HP shut-off valve; (3) actuate the HP shut-off valve to start/stop the engine.

[!note]- Q3. What happens at ENG MASTER OFF, and why does the dump valve open? Closes the LP + HP shut-off valves; once the HP valve closes, the dump valve opens to purge burner fuel to the drain tank.

[!note]- Q4. How is overspeed protection achieved? The FMU closes the HP shut-off valve to cut fuel (schematic: OVERSPEED PROTECTION = TOS+CPU).

[!note]- Q5. What commands the LP shut-off valve, and how many injectors are there? ENG MASTER (OFF) and the ENG FIRE pb (dual command). 24 fuel injectors (12 manifolds).

Key takeaways

Point Detail
Path tank → LP s/o → LP pump → fuel/oil HX → filter → HP pump (N3-driven) → FMU → 24 injectors
FMU FMV metering + overspeed protection (close HP s/o) + start/stop (HP s/o)
ENG MASTER OFF close LP+HP s/o + dump valve purges burner fuel
LP shut-off dual command: ENG MASTER + ENG FIRE pb

References

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.

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