Engine Fuel System

"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. Control of fuel supply is made by the FADEC via the FMU. High pressure fuel is also used to provide pressure for some engine component actuators."

That single FCOM paragraph declares fuel's three identities on this engine: it is the energy source (flow rate), it is a heat-transfer medium (temperature — in the fuel/oil heat exchanger, fuel and oil do each other a favour), and it is the hydraulic muscle for engine actuators (pressure — the VSVs and the oil-cooler valve are fuel-powered, as article 03 showed). This article follows a drop of fuel from the LP shut-off valve to the spray nozzles and answers three questions along the way: who pressurises it, who meters it, and who can cut it off. Inside the answer to the last question lives the hardest-edged safety design on the whole engine — the execution end of overspeed protection.

The aircraft-side fuel system, from the tanks to the LP valve, belongs to ATA-28.

1. The full chain

 tank (ATA-28) ──► LP FUEL SHUT-OFF VALVE ◄┄┄ [ENG FIRE PUSH] + [ENG MASTER]
                        │
                   LP PUMP (centrifugal)
                        │
              FUEL/OIL HEAT EXCHANGER (FOHE)
                        │
                   FILTER ── clog → FADEC → ECAM
                        │  ⓟ pressure → ECAM
                   HP PUMP (gear type, HP-spool driven)
                        │            ┌── servo fuel ──► AOHE valve actuator
                   ┌────┴────┐       ├───────────────► VIGV/VSV actuators ×2
                   │  FMU    │───────┘
                   │ FMV + SPILL VALVE (constant ΔP)        ┌─ TOS ─┐
                   │ HP SHUT-OFF VALVE ◄────────────────────┤        │ OVERSPEED
                   │ DUMP VALVE ──► DRAIN TANK              └─ OPU ─┘ PROTECTION
                   └────┬────┘        (overflow vent)   ◄┄┄ [ENG MASTER]
                        │
              FUEL FLOW TRANSMITTER ── FF → ECAM
                        │
                  HP FUEL FILTER
                        │
                 MANIFOLD (12 feed pipes) ──► FUEL SPRAY NOZZLES (24)

Four reading points from the FCOM schematic. First, the two shut-off valves answer to different masters: the LP valve's command lines come from the ENG FIRE pushbutton and the ENG MASTER; the HP valve's from the ENG MASTER and the overspeed protections (TOS and OPU). Fire closes the outer gate; overspeed protection squeezes the inner one — the division developed in §4/§5. Second, the legend's three line styles (fuel feed / hydraulic control / drained fuel) literally draw the three identities — the servo branch leaves after the HP pump to feed the oil-cooler valve and both VIGV/VSV actuators. Third, the fuel-flow transmitter sits downstream of the HP shut-off valve — the FF you read on ECAM is the fuel actually entering the nozzles, excluding the servo branch. Fourth, the dump-valve → drain-tank → overflow chain on the left is the shutdown purge path, whose recovery loop article 02 already traced.

2. Two pump stages: centrifugal to underwrite, gear to meter

"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."

The division of labour is a textbook pairing. The LP pump is a single-stage centrifugal machine with an axial inducer — its job is not high pressure but a guaranteed inlet head for the HP pump, preventing cavitation (vapour bubbles would let a gear pump churn emptily). The HP pump is a positive-displacement spur-gear pump — flow strictly proportional to speed, indifferent to back-pressure: exactly the steady supplier a metering system wants. Three supporting details: a 1600-psid relief valve at the HP pump outlet catches overpressure; the LP pump's drive passes through the HP pump's shaft (the two pumps share one drive line); and the pump-to-gearbox interface carries the dry cavity and drain arrangement that makes any shaft-seal leak immediately visible at the drains mast (article 02).

3. The FMU: a constant pressure drop makes position equal flow

The FCOM gives the metering valve in three pilot-level sentences:

"The Fuel Metering Valve (FMV) transforms FADEC orders through a torque motor and servovalve into fuel flow to the engine fuel nozzles. The FMV resolver generates a feedback signal proportional to the FMV position. The bypass valve maintains a constant pressure drop across the FMV to ensure that the metered fuel flow is proportional to the FMV position."

The constant pressure drop is the soul of the metering system (synthesis): flow through an orifice is a function of area times pressure differential — pin the differential, and flow depends on area alone. Commanding valve position is commanding flow, and one calibration holds for life. (The 50-psi constant-ΔP regulator inside the VSV control unit, article 03, is the same philosophy in miniature.) The AMM completes the execution detail: the metering plunger is stroked by primary/secondary servo pressures from the torque-motor flapper; the constant differential is held by the pressure-drop and spill valve, whose loading spring carries a stack of bimetallic discs that automatically re-trim the differential with fuel temperature — cold, viscous fuel and hot, thin fuel are compensated mechanically, with no software in the loop. Whatever the gear pump over-delivers, the spill valve returns to the LP side: a positive-displacement pump never under-delivers; the surplus simply goes back.

What the FADEC aims this machinery at is EPR, not a spool speed:

"The FADEC computes the fuel flow that will maintain the target EPR. As the FADEC maintains this EPR, it allows N3 to vary while remaining between N3 minimum and N3 maximum."

N3 floats. At the same thrust, today's N3 and last month's N3 may differ — engine deterioration and bleed configuration both move it, and the FADEC additionally applies a bleed-dependent N3 correction. Judging thrust by memorising an N3 number is a trap; the thrust yardstick is EPR (article 07).

4. The shutdown chain: one MASTER movement, three events

"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."

Three events: LP valve closes (outer gate) → HP valve closes (inner gate) → dump valve opens (purge). "Directly commands" has a precise electrical meaning: the MASTER's manual shutdown path is hardwired to the HP-valve torque motor, bypassing the EEC and EIVMU entirely — energised, it cuts fuel within 0.5 second and latches off after 5 seconds, and the design rule is that fuel-off always has priority over fuel-on. Even with every computer dead, this one switch still shuts the engine down (article 16 details the six-stage switch and dual-torque-motor architecture).

Why the purge matters is stated bluntly in the nozzle chapter:

"When the engine is shutdown, the fuel in the manifold is drained to a level below the bottom fuel spray nozzles. This makes sure that fuel cannot continue to flow into the combustion system and subsequently cause a fire."

The purged fuel goes to the drain collector tank and is drawn back into the system on the next start by the ejector (article 02) — nothing wasted, no fire risk left dripping in the combustor. Note also what the schematic shows about the FIRE pushbutton: its command line runs to the LP valve only — the fire procedure locks the fuel out at the wing side, while MASTER's civilised shutdown does the full double-gate-plus-purge sequence. The two-key nature of the LP valve (FIRE pushbutton or MASTER) becomes operationally decisive in the abnormal-shutdown procedures of article 24.

5. The execution end of overspeed protection: 110 and 117

The FCOM places the overspeed action thresholds in the fuel chapter — among the most important numbers in this entire series:

"TURBINE OVERSPEED PROTECTION (TOS): The FADEC provides protection against LP turbine overspeed, due to LP shaft breakage between compressor and turbine. If it occurs, the engine is shut down by closing the HP shut-off valve. OVERSPEED PROTECTION UNIT (OPU): Independent of the FADEC, the engine is fitted with the N1/N2 overspeed protection unit. In the event of overspeed condition detected by the OPU, which occurs when N1 reaches 110 % or N2 reaches 117 %, the OPU controls the engine shutdown by closing the HP shut-off valve."

Set these against the red lines of article 00 and the threshold layering becomes unmistakable — two different event classes that must never be merged in memory:

The roughly ten-point buffer between red line and action point (synthesis) is there for three reasons: it gives the EEC's red-line limiters room to work first, it tolerates normal transient overshoot, and it allows time for the OPU's dual-ASIC vote (article 04) — a protection must always act later than a limitation, or routine transients would be executed as emergencies. And note where both protections converge: each one closes the same HP shut-off valve — the OPU through the FMU's overspeed valve, the TOS through the shut-off valve's overspeed torque motor. Two independent electrical paths, one hydraulic component.

6. The three idles, FCOM edition

The fuel chapter gives the selection conditions for the three idle levels — complementing the design-purpose view of article 05:

"Modulated idle ‐ Is regulated according to: Bleed system demand / Oil temperature / Mach number. ‐ Is selected: In flight, when the flaps are retracted and the gear is up. / On ground, provided reverse is not selected. Approach idle ‐ Is regulated according to aircraft altitude, regardless of bleed system demand. ‐ Selected in flight, when the FLAPS are extended to FLAP 2, FLAP 3, or FULL, or when the landing gear is down. ‐ Allows the engine to rapidly accelerate from idle to go-around thrust. Reverse idle ‐ Selected on ground, when reverse idle thrust is selected. ‐ Slightly higher than forward idle thrust."

Three pilot-level details beyond the AMM version: modulated idle's regulators include oil temperature (a cold engine idles a little faster to warm itself); approach idle's trigger is FLAP 2/3/FULL or gear down — which is why the idle visibly rises by itself when you select FLAP 2; and reverse idle runs slightly higher than forward idle.

7. The nozzle end: twenty-four mouths fed equally

"The filtered HP fuel is supplied to a fuel manifold which supplies fuel equally to 24 fuel spray nozzles."

"Equally" is engineered twice over. The manifold's twelve feed pipes serve one pair of nozzles each, with a calibrated diameter reduction between the first and second nozzle of each pair to balance the split. And inside every nozzle sits a tungsten-carbide distributor weight: at low flow, nozzles at different clock positions would otherwise feed unevenly under gravity — the weight's own gravity component compensates in exactly the opposite sense, so the 12-o'clock and 6-o'clock nozzles drink alike even at minimum flow (a spring seats the weight closed when fuel stops). The nozzles themselves are air-blast atomisers: P30 air through two counter-rotating swirler rows shears the fuel film into a combustible mist — the upstream interface of the combustor described in article 01.

8. Where the fuel system meets the failure chapters

Self-test

[!note]- Q1. Why a centrifugal LP pump and a gear-type HP pump? The centrifugal stage exists to give the HP pump a cavitation-proof inlet head; the gear stage is positive-displacement — flow proportional to speed, indifferent to back-pressure — the steady supplier metering requires. Surplus delivery returns through the spill valve; a 1600-psid relief valve backstops the outlet.

[!note]- Q2. What makes "flow proportional to FMV position" true — and what happens when fuel temperature changes? The bypass/spill valve holds a constant pressure drop across the FMV, so orifice area alone determines flow. The differential's set-point spring carries bimetallic discs that re-trim it with fuel temperature — viscosity compensation done in metal, not software.

[!note]- Q3. Name the three events triggered the instant the MASTER goes OFF. LP shut-off valve closes, HP shut-off valve closes, dump valve opens — purging the manifold below the lowest nozzle level (fire prevention), the fuel going to the drain tank for recovery on the next start. The manual path is hardwired: 0.5 s to fuel-off, latched after 5 s, fuel-off priority.

[!note]- Q4. The N1 red line is 99 % — why does the OPU wait until 110 %? Layering: 99 is the operating limitation (the EEC's limiters keep you below it); 110/117 are protection action points — the last gate when limitation fails. The buffer accommodates the limiters, normal transients and the dual-ASIC vote, so protections never execute routine excursions.

[!note]- Q5. Idle rises by itself when FLAP 2 is selected. Normal? Yes — approach idle is selected at FLAP 2/3/FULL or gear down, regulated on altitude regardless of bleed demand, sized so the engine can accelerate rapidly from idle to go-around thrust.

Key takeaways

References

• FCOM DSC-70 (engine fuel system, general/operation/control) — system role and three identities, pump sequence, MASTER shutdown chain and start-time recovery, FMV three-sentence description, EPR target with floating N3, TOS and OPU action thresholds, idle selection conditions.
• AMM 73-12 (fuel pumps, D/O) — centrifugal/gear pairing, relief valve, shared drive, dry-cavity drains.
• AMM 73-13 (fuel distribution, D/O) — manifold pipe pairing, distributor weights, air-blast swirlers, shutdown purge rationale.
• AMM 73-21 (FADEC, D/O — FMU section) — plunger servo mechanisation, pressure-drop and spill valve with bimetallic discs.
• AMM 76-12 (engine master control, D/O) — hardwired manual shutdown path, 0.5 s/5 s latch, fuel-off priority.
• FCOM fuel-system schematic (read directly) — valve command lines, line-style legend, FF transmitter position, dump-drain chain.
• Integrative synthesis (marked in text): the underwrite/meter division; the constant-ΔP soul argument; the N3-floats trap; the three-reason buffer between red line and action point.

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.