Fuel Jettison

On equipped aircraft, jettison is the fast way down to landing weight: roughly a tonne a minute overboard through nozzles at the trailing edge. It is an option — not every A330 has it — and where fitted it is a whole-system mobilisation: centre pumps as the main muscle, the aft transfer valves opened to add inner-tank fuel, and a set of guards (CG sentries, low-level stops, a preset gross weight) standing between "dump fuel" and "dump too much." This article covers the plumbing, the normal automatic sequence, the stop logic, and the environmental discipline.

The failure modes — jettison unavailable, and the valve that won't close — are in jettison faults.

1. The mobilisation

  CENTRE TANK ──[CTR pumps]──┐
                              ├──► refuel/jettison gallery ──► [JETTISON VALVES X/Y]
  INNER TANKS ──[main pumps]─┘                                  (outlets at the
        via AFT TRANSFER VALVES (B/D, opened for jettison)       flap-track fairings)
                                                                      │
  TRIM TANK ── forward transfer joins automatically                   ▼
  (jettison initiation is itself a forward-transfer trigger)      overboard

Jettison borrows the transfer system's hardware wholesale: the gallery, the centre pumps, the aft transfer valves. That is why initiating jettison is one of the forward-transfer triggers (forward transfer) — the tail's fuel is brought forward into the stream — and why an active jettison suspends aft transfer (aft-CG transfer): the same valves cannot serve two masters. Rated flow is approximately 1 080 kg per minute (planning figure: about 1 000 kg/min average).

2. Normal sequence — automatic mode

From the QRH/FCOM procedure:

FMS FUEL PRED page ........................ SELECT
FINAL GW (JETT GW key) .................... ENTER     ← the target weight
T TANK MODE ........................... CHECK AUTO
JETTISON ARM .................................. ON
JETTISON ACTIVE ............................... ON     ← fuel goes overboard
   (a FUEL T TANK XFR FAULT appearing during jettison:
    procedure says disregard that ECAM)
...at the MCDU-entered gross weight → automatic stop
JETTISON ACTIVE / ARM ........................ OFF

The two-switch (ARM then ACTIVE) design is deliberate friction — no single action dumps fuel. While fuel flows, the procedure posts a CG sentry:

[!warning]- The CG caution during jettison Monitor the CG throughout; reaching either the forward or the aft limit for takeoff/landing means stop the jettison. And with the landing gear down there is no automatic trim transfer — if the trim tank still holds fuel in that state, CG management is manual. Jettison changes the fuel distribution faster than any other normal process; the CG can walk while you watch.

Automatic stop — three independent sources:

1. the crew switches ACTIVE/ARM off;
2. the FCMS reaches the MCDU-entered JETT GW (re-entering a lower GW can restart the jettison);
3. the low-fuel floor: inner tanks down to a combined 10 000 kg (on ETOPS-configured sensing, the stop can come earlier when the dedicated sensors run dry) — jettison may never bite into final-reserve territory.

Manual mode: clear the JETT GW from the MCDU and run ARM/ACTIVE the same way — but then nothing stops the jettison except you. No target, no automatic floor logic backstopping a distracted crew; manual mode is for when the automatic target logic is unusable, not for convenience.

3. Environmental and airmanship discipline

From the FCOM expanded procedure:

• minimum 5 000 ft AGL for dispersal (the fuel must evaporate, not rain);
• no jettison in thunderstorm conditions;
• avoid flying through the jettisoned plume — it descends at roughly 500 ft/min: turning back into your own track below the dump altitude is the classic self-contamination error;
• use cases per the FCOM: meeting approach/landing climb gradient requirements (note — a FLAP 3 landing may remove the need), restoring normal MLW, or preserving the flight profile after an en-route engine failure.

Coordinate with ATC for an area and altitude; the planning arithmetic is straightforward — tonnes to lose ÷ ~1 t/min.

4. Configuration and dispatch

Jettison is one of the four independent option switches (overview) — confirm the fit per airframe, never assume from the tank configuration. Where fitted:

• electrical dependencies run deep: several electrical emergency configurations list jettison as inoperative — the weight-reduction plan for a major electrical failure cannot count on it;
• some operators' MEL carries the jettison valves and system as dispatchable items (low category, with the operational consequence pre-declared: rapid weight reduction unavailable — think overweight-landing or hold-to-burn planning instead);
• the refuel logic is jettison-aware: an open or unreporting jettison valve is a refuel stop condition (refuelling).

Self-test

[!note]- Q1. Where does jettisoned fuel come from, in what order of muscle? Centre pumps push centre fuel as the main source; the aft transfer valves open so main pumps add inner-tank fuel; the trim tank joins via the automatic forward transfer that jettison initiation triggers.

[!note]- Q2. Name the three automatic-stop sources in automatic mode. Crew switches off; FCMS reaches the MCDU-entered JETT GW; the inner-tank floor (combined 10 000 kg, or earlier via ETOPS-configured sensors).

[!note]- Q3. What is the standing caution while fuel is going overboard? Watch the CG; stop the jettison at either takeoff/landing CG limit — and remember gear-down means no automatic trim transfer, so tail fuel needs manual handling in that state.

[!note]- Q4. What changes in manual mode? No JETT GW target — and with it, no automatic stop at a weight. Only the crew (and the low-level floor logic) stands between the aircraft and over-dumping.

[!note]- Q5. Minimum altitude and the plume rule? 5 000 ft AGL minimum, no thunderstorms, and never re-enter your own descending plume (~500 ft/min sink).

Key takeaways

References

• FCOM DSC-28-10-80 (system), PRO-ABN/QRH fuel jettison procedure (sequence, cautions, environmental notes).
• AMM 28-31-00 Description and Operation (valves, plumbing, stop logic).
• PRO-ABN-ELEC (electrical configurations listing jettison inoperative); some operators' MEL 28-31 items.
• Planning framing is integrative synthesis from the cited figures.

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.