Forward Transfer — Bringing the Ballast Home

Aft transfer sends fuel to the tail once per flight; forward transfer brings it back — little and often. It maintains the CG against the slow forward drift of fuel burn, replenishes the inner tanks, and empties the trim tank before descent so the aircraft lands with a normal CG and no ballast stranded aft. This article covers the triggers, the destinations, the trim transfer pump (and the gravity fallback with its odd 3.4° rule), the two-step logic when the CG is already forward, and the gear-down isolation that brackets every flight.

1. Triggers — five ways to start

"The FCMC sends a forward fuel transfer signal, if one of the following conditions are met: The calculated CG = Target (stops when computed CG = Target -0.5%). The fuel contents of one of the two inner tanks decreases to 4 000 kg (forward transfer stops when content reaches 5 000 kg). The FMGES sends a time-to-destination signal below the threshold, or the aircraft descends below FL 245 (transfer is continuous but controlled by inner tank high levels to prevent overflow). Jettison is initiated. In electrical emergency configuration."

The second trigger is the rhythm-maker: a small top-up loop (4 000 → 5 000 kg) that repeats as the flight burns fuel. Aft transfer is one delivery; forward transfer is a drip-feed — the asymmetry introduced in FCMS computers. The third trigger is the endgame: from 35 minutes out (or FL245 in descent), transfer goes continuous until the trim tank is empty.

2. Destinations — and the balance bias

"A forward transfer is normally directed to the inner tanks. If there is an imbalance of 500 kg between the two inner tanks, the forward transfer is directed to the lighter inner tank... may be directed to the center tank, if it is not empty. In emergency electrical configuration, the forward transfer is always directed to the inner tanks."

Forward transfer tops up the lighter side (mirror image of aft transfer drawing from the heavier side); with fuel still in the centre tank it may simply return there; and in the emergency electrical configuration it goes straight to the inners — the shortest path to the engines. A receiving inner that reaches high level pauses (valve closes) and resumes at high level −2 000 kg; a receiving centre tank pauses at full and resumes at its underfull point (≈2 500 L below the high-level volume) — the same hover-cycle as main transfer.

3. The trim transfer pump — and life without it

The pump (in the trim tank, the only pump pointing forward) is built for its environment:

"The trim transfer pump has... an inducer, a main impeller, a reprime impeller, a flametrap, a shaft... The motor is star-connected, 3 phase 115VAC, each phase protected through a non-resettable thermal fuse... The reprime impeller causes fuel and any air to go through vent holes... This function de-aerates the fuel and helps the fast repriming of the pump."

Tail-tank fuel sloshes and entrains air; the reprime impeller continuously de-aerates so a gulp of air does not unprime the pump. Its operating envelope:

"The pump operates when all of 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 of fuel in trim tank or the trim tank lo-level sensor is wet. The pump stops when: the landing gear is extended; there is less than 200 kg and lo-level dry; the pressure-switch has low pressure for more than 10 minutes. The pump does not operate on the ground."

Note the third stop condition — ten minutes of low output pressure shuts the pump down automatically (self-protection against pumping nothing). The 200 kg floor matches the water-scavenge trigger in water scavenge, because this pump drives that system too.

Pump failed — gravity takes over, with an attitude rule:

"In the event of pump failure, forward transfer occurs by gravity, through the trim tank non-return valve. The FCMC inhibits the forward transfer by closing the TRIM PIPE ISOL VALVE, when the aircraft pitch exceeds 3.4° for more than one minute. Restarts the forward transfer, when the aircraft attitude is lower than 3.4° for more than 1 min."

[!warning]- Why 3.4 degrees Gravity transfer relies on the head between the trim-tank surface and the pipe outlet. Pitch the nose up far enough and the geometry stops cooperating — flow stalls or reverses. Rather than transfer unreliably, the FCMC freezes the process (1-minute persistence both ways, to ignore transients). It is also why a dead trim pump moves the CG-control endgame from 35 to 75 minutes before destination: gravity is slow and picky about attitude. The crew-facing version of this rule is the ABNORM MAN FWD XFR alert with its "above 270 kt and not in climb" gate (transfer faults). (Rationale integrative; values verbatim.)

4. Two-step forward transfer — when the CG is already forward

"If the center tank contains fuel and the CG is forward of 32% MAC, the transfer will be completed in two steps: When the center tank quantity reaches 17 000 kg, the trim tank is decreased to 2 400 kg. When the center tank is empty, the trim tank will be emptied."

With a forward CG and a wet centre tank, dumping all the tail ballast forward at once would push the CG further forward while the centre tank has no room anyway. So: step one takes the trim tank down only to 2 400 kg, step two (centre empty) finishes the job.

[!warning]- Three different "two-step" criteria live in the manuals — keep them apart

1. Normal-operations logic (this article): CG forward of 32 % + centre fuel → steps at centre 17 000 kg / trim 2 400 kg.
2. Manual forward transfer after FCMS failures (AMM): ZFCG forward of 23 % + centre fuel → first step ends when the centre tank is 50 % full.
3. FUEL LO TEMP procedure: CG forward of 26 % is a monitoring line during forward transfer (not a two-step rule at all). Same flavour, three different scenarios and numbers — never mix them across procedures.

5. Emptying the tank — the closing sequence

When the trim tank runs dry (lo-level sensor dry), the system closes up shop in a fixed order:

"The trim pipe isolation valve (W) closes immediately; the auxiliary forward transfer valve (V) closes immediately; and after 60 seconds the trim tank isolation valve (T) closes."

W and V slam shut at once; T waits 60 seconds — long enough for the trim pipe's contents to drain forward before the door closes. (The pipe holds 75 litres, 60 of them usable — counted in FOB but in no tank's figure; fuel quantity indication.)

6. The cockpit controls — and the gear-down guillotine

Two overhead controls govern the trim line:

• T TANK MODE (AUTO / FWD): FWD stops CG control and forces a forward transfer — into the centre tank on six-tank aircraft, into the inners on five-tank aircraft. Release back to AUTO returns control to the FCMS (aft transfer may resume).
• T TANK FEED (AUTO / ISOL / OPEN): AUTO gives the FCMS the trim line; ISOL closes the four trim-line valves (T, L, V, W) — the "lock the pipe" position used throughout the abnormal procedures; OPEN holds the forward-transfer valves open so the trim tank and pipe drain completely — with the APU caveat that fully draining the line starves the APU's normal source (APU fuel feed).

And the automatic bracket around every flight:

"During flight the trim transfer pipe is automatically isolated when the landing gear control lever is set to DOWN."

Gear down = trim line isolated (W and T closed) — no CG transfers in the landing phase. The release is stricter than the trigger: the valves cannot reopen until the gear and the slats both indicate retracted and the aircraft is above 25 500 ft — though T TANK MODE FWD can still force the trim-tank isolation valve open as the manual rescue path. After a go-around, CG control does not resume at gear-up; it waits for the full set.

7. Dispatch

Some operators' MEL treats the forward-transfer chain by function: items that lose only the automatic scheduling (dispatch with manual forward transfer per a defined schedule, or with the trim tank empty) versus items touching the isolation function, which inherit the strict standard — an unisolatable trim line is a leak path and an APU question at once.

Self-test

[!note]- Q1. What keeps forward transfer "little and often" in cruise? The inner-tank loop: any inner falling to 4 000 kg triggers a transfer that stops at 5 000 kg — repeating as fuel burns, against the one-shot aft transfer.

[!note]- Q2. The trim pump fails in cruise. What changes about the descent plan? Forward transfer continues by gravity (non-return valve) but freezes whenever pitch exceeds 3.4° for a minute; CG-control end moves from 35 to 75 minutes before destination to allow for the slower, attitude-limited transfer.

[!note]- Q3. Why does the trim-tank isolation valve wait 60 seconds after W and V close? To let the trim pipe's contents (75 L, 60 usable) drain forward before the door shuts — otherwise that fuel is stranded in the pipe.

[!note]- Q4. Quote the three "two-step/monitor" CG numbers and their owners. 32 % — normal two-step forward transfer (centre 17 000 kg / trim 2 400 kg); 23 % — AMM manual transfer after FCMS failure (centre 50 % full); 26 % — LO TEMP procedure monitoring line. Different scenarios; never interchange.

[!note]- Q5. After a go-around, when can automatic CG transfers resume? Only when gear AND slats both indicate retracted AND the aircraft is above 25 500 ft. Gear-up alone is not enough.

Key takeaways

References

• FCOM DSC-28-10-90 (triggers, destinations, gravity rule, two-step, controls).
• AMM 28-27-00 Description and Operation §6.H/§7 (pump construction and envelope, closing sequence, gear isolation and release, manual-transfer criteria).
• Some operators' MEL (forward-transfer and isolation items).
• The 3.4° rationale and the "drip-feed" framing 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.