Fuel System Overview

The A330 fuel system does far more than store fuel and feed two engines. It is a computer-managed system that uses the fuel itself as a movable ballast — pumping fuel aft into the tailplane in cruise to shift the centre of gravity rearward and cut trim drag. That single idea separates the A330 from most transport aircraft and runs through this entire chapter.

This article is the chapter's master map: tank layout and capacities, the two tank configurations, what each subsystem does, what still works on emergency power, and the certified limitations a crew must know before every departure.

1. What the fuel system does — seven functions

The FCOM defines the system in seven lines:

"The fuel system: ‐ Stores fuel. ‐ Controls and monitors the correct quantity of fuel. ‐ Supplies fuel to the engines and the Auxiliary Power Unit (APU). ‐ Controls the transfer of fuel to maintain the Center of Gravity (CG) within limits. ‐ Maintains fuel in the outer tanks for wing bending relief. ‐ Allows fuel jettison for rapid weight reduction. ‐ Controls refueling and defueling."

Re-ordered as the life of the fuel — store → breathe → feed → transfer for CG → measure → jettison → refuel/defuel — these seven functions are the skeleton of the thirty articles that follow.

Note the configuration dependency: the jettison line only appears in the FCOM version applicable to aircraft fitted with a centre tank and the jettison option. Not every A330 can jettison fuel (see Section 3).

2. Tank layout and capacities

2.1 The geography

                         WING PLAN VIEW (storage)
  wing tip                                                    wing tip
 ┌──────┐   OUTER TANK   ┌──── INNER TANK ────┐  OUTER TANK ┌──────┐
 │ VENT │◄── (outboard,──┤  ▓▓ collector ▓▓   ├──(bending ─►│ VENT │
 │SURGE │   bending      │  ▓▓   cell    ▓▓   │   relief)   │SURGE │
 └──────┘   relief)      │  ▲ SPLIT valve ▲   │             └──────┘
                         └──(inner division)──┘

                    TRIMMABLE HORIZONTAL STABILISER
                 ┌───────────  TRIM TANK  ───────────┐
                 │ (inside the THS — the CG ballast)  │
                 └────────────────┬───────────────────┘
                          THS vent surge tank (RH side)

Three facts the diagram carries:

• Outer tanks sit at the wing tips, inner tanks at the roots, vent surge tanks outboard of everything. The outboard position of the outer-tank fuel is deliberate — its weight bends the wing down against lift, relieving root bending loads. That is why outer fuel is used last (Section 3.2).
• Each inner tank hides a collector cell — a compartment kept full at roughly 1 000 kg that houses the two main pumps and provides negative-g protection:

"Each inner tank contains one collector cell that: ‐ Maintains a fuel reservoir for the fuel booster pumps and provides negative 'g' protection to feed the engines. ‐ Is maintained full and contains about 1 000 kg (2 200 lb) of fuel."

• The trim tank lives inside the trimmable horizontal stabiliser — the longest moment arm on the aircraft, which is exactly what makes a few tonnes of fuel an effective CG tool.

2.2 Usable fuel

Two design margins worth remembering: total unusable fuel is below 0.23 % of capacity, and a full tank can absorb a 2 % volume expansion (a 20 °C temperature rise) without spilling. Vent surge tanks hold at least five times the volume of their vent pipes before any fuel can go overboard.

2.3 Configuration splits — more than one switch

A330 fleets mix two tank configurations: aircraft with a centre tank (six tanks, 109 186 kg) and aircraft without (five tanks, 76 561 kg). The FCOM marks the split by FSN/MSN effectivity ranges in every affected section — the difference comes from build batches, not from the -200/-300 designation, so two outwardly identical airframes in one fleet can differ by some 32.6 tonnes of capacity.

The centre tank is not the only option switch. The FCOM Aircraft Configuration Summary lists three further yes/no fuel options per aircraft:

1. FTIS — the Fuel Tank Inerting System (nitrogen-enriched air into the centre tank);
2. Jettison — including the JETT GW function and the JETTISON ARM/ACTIVE pushbuttons;
3. Refuel panel in cockpit — the FUEL/REFUEL pushbutton with its CKPT-light logic.

Centre tank, FTIS, jettison and the cockpit refuel button are four independent configuration switches. Check each against the aircraft's effectivity; never infer one from another.

3. The subsystems at a glance

3.1 Storage

The AMM gives storage six responsibilities:

"The storage system: ‐ contains the fuel for the engines and APU ‐ helps to protect the fuel system against fire ‐ gets heated fuel from the Integrated Drive Generators and returns it to the inner fuel tanks ‐ keeps the air pressure in the system near to the ambient air pressure ‐ keeps the trim pipe shroud drained ‐ does not let water collect in the trim tank."

Each of those maps to its own article: tanks and structure, fire prevention, IDG heat-exchange return, venting, shroud drainage, water scavenge.

3.2 Why outer fuel is kept — wing bending relief

In flight the wing bends upward under lift. Fuel weight at the tips produces a downward moment that opposes the bending and unloads the root structure. The burn order is therefore not arbitrary — outer fuel is preserved until last, and the main transfer system exists to move centre and outer fuel inboard only when the inner tanks need it.

3.3 Distribution

"The distribution system makes sure that the fuel: ‐ is supplied to all the engines during all flight conditions ‐ is supplied to the APU ‐ can be isolated from an engine or the APU when necessary ‐ is in the correct (and safe) configuration for flight ‐ can be moved from tank to tank as necessary ‐ is moved forward or rearward to give control of the aircraft CG in flight."

The headline redundancies: two main pumps per collector cell with a standby pump that starts automatically if a main pump fails or is switched off; a crossfeed valve, normally closed, that splits the system into two halves and — when open — lets any pump feed any engine; and one LP valve per engine, commanded by the ENG MASTER or forced closed by the engine FIRE pushbutton.

3.4 Trim transfer — the smart core

"The trim transfer system controls the CG of the aircraft. When the aircraft is above 25500 ft. the system keeps the aircraft in a position that: ‐ reduces the drag on the aircraft ‐ increases the fuel economy. For this function the system moves the fuel from the wings to the trim tank (aft transfer) or from the trim tank to the wings (forward transfer)... The system operates automatically but the crew can manually set a forward transfer."

Moving fuel aft moves the CG aft; the tailplane download needed for trim shrinks, and with it both trim drag and the induced-drag penalty of carrying that download. The performance benefit is free — the price is rigorous monitoring of the aft limit, which is why an independent FMGEC watchdog backs up the FCMC (covered in the CG articles).

3.5 Jettison (optional fit)

On aircraft so equipped, jettison dumps fuel at approximately 1 080 kg per minute (system rated flow). The crew starts it manually; it stops when the crew stops it, when the FCMS reaches a preset fuel value, or when an inner-tank low-level sensor runs dry.

3.6 Indicating — four systems

"The indicating systems are: ‐ the Fuel Quantity Indicating (FQI)... ‐ the Manual Magnetic Indicators (MMI)... ‐ the tank level sensing... ‐ the fuel temperature measurement..."

FQI computes mass (volume × density) continuously for ECAM and the refuel panel; MMIs are unpowered dipsticks for ground cross-checks; level sensing provides the hard-wired high/low/overflow detections; temperature is measured in the LH outer, both inners, and the trim tank — the RH outer tank has no temperature probe.

3.7 The brain — FCMS, and what it does not control

"The FCMS has two computers... FCMC1 and FCMC2. Usually FCMC1 controls the FCMS unless conditions occur to cause FCMC2 to control the system. Although only one computer sends the discrete command signals, each computer monitors the system all the time."

The FCMS runs the intelligence: water scavenge, automatic refuelling and its high-level protection, main transfer, CG computation and trim transfer, quantity and temperature computation, data to other systems, fault reporting — and, where fitted, stopping a jettison.

What it does not own matters just as much: the engine LP valves answer to the ENG MASTER and FIRE pushbuttons, and the APU LP valve to the APU FIRE pushbutton, without passing through the FCMS. Even with both FCMCs dead, the ability to cut fuel to an engine or the APU survives.

4. What still works on emergency power

When only emergency power is available, the AMM bus-by-bus inventory shows the fuel system keeps:

• crossfeed valve (motor 1), intertank transfer valves, auxiliary forward-transfer valve, trim-tank isolation valve — on DC ESS;
• left and right No. 2 main pumps — on AC ESS SHED;
• engine and APU LP valves, emergency isolation valves — on hot battery and DC ESS SHED buses.

Three conclusions for the crew: one wing's pump can still feed both engines through the crossfeed; both engines still have a powered pump each; and every isolation/cut-off capability survives. The system loses its comfort, not its spine.

5. Limitations (LIM-FUEL) — the pre-departure hard lines

Two verbatim anchors:

"Minimum fuel quantity for takeoff ... 5 200 kg (11 461 lb). The ECAM alerts that are related to fuel low level in the wing tanks (FUEL WING TK LO LVL, etc.) must not appear for takeoff."

"In exceptional conditions (i.e. fuel system failure), the above-mentioned values for maximum fuel imbalance may be exceeded without significant effect to the aircraft handling qualities. The aircraft remains fully controllable in all flight phases."

That last sentence frames every imbalance discussion in this chapter: the limits are a certification envelope, not a loss-of-control cliff.

6. The monitoring discipline

"Fuel checks should be performed when the aircraft overflies a waypoint, or at least every 30 min. Any difference should alert the flight crew... For any message or alert related to the fuel quantity or imbalance, the flight crew should consider a fuel leak as a possible cause."

Two habits follow the aircraft through this chapter: the half-hourly conservation check (FOB + fuel used against departure FOB), and the reflex of treating any quantity or imbalance anomaly as a possible leak until proven otherwise. On the dispatch side, some operators' MEL cross-refers every fuel ECAM alert to its relief item in a dedicated mapping table — the fastest route from an alert on the ground to the applicable dispatch conditions.

Self-test

[!note]- Q1. Two airframes of the same A330 variant differ in total fuel capacity by over 30 tonnes. How? One has the centre tank (six-tank configuration, 109 186 kg total), the other does not (five-tank, 76 561 kg). The split follows build-batch effectivity, not the type designation. Centre tank, FTIS, jettison and the cockpit refuel button are four independent options — check each.

[!note]- Q2. Why does the A330 keep fuel in the outer tanks until last? Tip fuel weight bends the wing down against lift, relieving wing-root bending loads (wing bending relief). The burn order is engineered, not incidental.

[!note]- Q3. With both FCMCs failed, can the crew still shut off fuel to an engine? Yes. The engine LP valves are commanded by the ENG MASTER / FIRE pushbutton circuits, which do not pass through the FCMS. The same applies to the APU LP valve.

[!note]- Q4. On emergency power only, how many fuel pumps still run? One No. 2 main pump per side (AC ESS SHED). With the crossfeed valve (DC ESS, motor 1) the surviving pumps can feed both engines.

[!note]- Q5. What are the two fuel hard lines before every takeoff? Minimum 5 200 kg fuel on board, and no wing-tank low-level ECAM alert displayed. Imbalance and fuel temperature must also be within LIM-FUEL values.

Key takeaways

References

• FCOM DSC-28-10-10 (system functions), DSC-28-10-20 (tank arrangement, collector cell), DSC-28-10-100 (APU feed), LIM-FUEL (limitations), GEN Aircraft Configuration Summary (option fits).
• AMM 28-00-00 Description and Operation (storage/distribution/indicating/FCMS responsibilities; emergency power inventory; jettison rate).
• FCTM PR-AEP-FUEL (fuel checks and leak-first discipline).
• Sections flagged as reasoning (the burn-order rationale, the emergency-power conclusions) are integrative synthesis from the cited sources.

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.