Water Scavenge

Fuel always carries some water — condensation on cold tank walls, moisture delivered with the uplift. Water is heavier than fuel, so it settles to the tank floor, where it threatens three things: ice that clogs strainers and lines at low temperature, corrosion of the structure, and microbiological growth (the chromate tank coating is the other half of that fight).

The A330's in-flight answer is counter-intuitive: it does not drain the water away. It stirs it back in:

"If water gets into the fuel in the trim tank, the trim-tank water scavenge system keeps that water in suspension."

"In suspension" means jet pumps lift the settled water off the tank floor and mix it back into the fuel — fine enough to be burned through the engines with the fuel, never given the chance to pool and freeze. Draining water overboard is a separate, ground-maintenance function (the water drain valves in tanks and storage). Don't conflate the two: drain removes water on the ground; scavenge burns it in flight.

1. Three scavenge systems

"The water scavenge system keeps in suspension, the water that gets into the fuel in: the trim tank, the fuel collector cells (in the wings), the wing tank."

 ┌────────────── three water-scavenge systems ───────────────┐
 │                                                            │
 │ ① trim tank:       2 × jet pump  ← driven by the trim      │
 │                                    transfer pump           │
 │ ② collector cell:  jet pump tap  ← driven by the MAIN      │
 │                                    pumps (when running)    │
 │ ③ wing tank:       jet pump      ← driven by the STANDBY   │
 │                                    pump                    │
 │                                                            │
 │ common principle: high-pressure fuel through a nozzle →    │
 │ venturi suction → floor water/fuel mixture swept up and    │
 │ sprayed back into the tank, held in suspension             │
 └────────────────────────────────────────────────────────────┘

2. The jet pump — suction with no moving parts

"The jet pump has two fuel inlets, the pressure inlet and the suction inlet. When fuel from the pressure inlet goes through the jet nozzle, it causes a suction at the suction inlet. The mixed flow of fuel/water goes through and out of the jet pump outlet. The non-return valve prevents an opposite flow through the jet pump."

   pressure fuel from a main/transfer/standby pump
        │
        ▼  pressure inlet
   ════►【jet nozzle】═════════►  outlet (mixture sprayed back
            ╲  ▲ venturi: high     into the tank)
             ╲ │ velocity → low
        suction inlet  pressure → suction
        (floor water/fuel mixture; non-return valve
         blocks reverse flow)

[!warning]- A jet pump is not a pump No motor, no impeller — it borrows another pump's high-velocity flow and uses the venturi effect to lift the floor sediment. No moving parts means nothing to wear out. Same physics as a carburettor or a water-jet aspirator.

3. Who drives what — and the conditions

① Trim tank (driven by the trim transfer pump):

"The trim tank water scavenge system uses the trim transfer pump and two jet pumps, which operate when these conditions occur at the same time: ‐ the aircraft is in flight and the landing gear and slats are retracted ‐ there is more than 200 kg (441 lb) of fuel in trim tank (or the trim tank lo-level sensor is wet) and the trim pipe is not isolated."

(The FCMS chapter of the AMM quotes a 700 kg figure for the same trigger — an internal inconsistency between AMM chapters; the scavenge chapter's own 200 kg is used here, and the storage overview chapter independently also states 200 kg.)

② Collector cell (driven by the main pumps):

"The collector-cell water scavenge system operates when a related main fuel pump is in operation. The standby pump does not operate the collector-cell water scavenge system."

[!warning]- Standby-only operation leaves the collector cell unscavenged The cell's jet pump rides on the main pumps. In a degraded case where engines are fed by the standby pumps alone, collector-cell water scavenge stops — harmless for a short period, but a boundary worth knowing.

③ Wing tank (driven by the standby pump):

"The wing-tank water scavenge system uses the standby fuel pump to operate its related jet pump."

4. Control, indication — and a relay interlock

• The trim system is sequenced by the FCMS (through the trim transfer pump); the collector-cell and wing-tank systems simply ride their host pumps.
• There is no cockpit indication that scavenge is running. The only crew-visible trace is indirect: a failed trim transfer pump is reported to the maintenance system and appears as FUEL TRIM PUMP in the post-flight report.
• The electrical schematic for the wing-side scavenge shows the timing is not purely software: a chain of flight-lock and control-lock relays, gated by the nose-gear ground signal and the standby-pump pressure switches, sits between the FCMS and the pumps — the computer requests, the hard-wired interlock qualifies. The same "software proposes, hardware disposes" pattern recurs across the fuel system.

Self-test

[!note]- Q1. How does the A330 deal with water in the tanks in flight? It does not drain it — jet pumps keep the water in suspension, mixed into the fuel, so it burns through the engines. Draining overboard is a ground-maintenance task via the water drain valves.

[!note]- Q2. What makes a jet pump suck, and what can wear out in it? Venturi effect — a host pump's high-velocity fuel through a nozzle creates suction at a second inlet. Nothing can wear out: it has no moving parts (a non-return valve guards reverse flow).

[!note]- Q3. What must all be true for trim-tank scavenge to run? In flight, gear and slats retracted, more than 200 kg in the trim tank (or lo-level sensor wet), and the trim pipe not isolated.

[!note]- Q4. Engines feeding from standby pumps only — which scavenge stops? The collector-cell scavenge (it rides the main pumps). The wing-tank scavenge, driven by the standby pump, keeps working.

[!note]- Q5. How would the crew ever learn the trim scavenge's pump failed? Only indirectly: FUEL TRIM PUMP in the post-flight maintenance report. There is no in-flight running indication for scavenge.

Key takeaways

References

• AMM 28-17-00 Description and Operation (scavenge systems, jet pump, operating conditions, indication).
• AMM 28-10-00 (storage overview corroborating the 200 kg trigger); AMM 28-51-00 (FCMS chapter; 700 kg variant noted as an inter-chapter inconsistency).
• ASM 28-21-05 (RIB3 water scavenge relay schematic).
• The "stir and burn" framing and the interlock pattern remark are integrative synthesis.

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.