Main Transfer — the Burn Order

Engines only ever drink from the inner tanks' collector cells. Every other tank is a warehouse, and main transfer is the logistics that moves stock to the shop floor: centre fuel pushed inboard by pumps, outer fuel released inboard by gravity, in a fixed order that ends with the bending-relief fuel last. This article covers the sequence, the two transfer mechanisms, their automatic triggers, and the manual overrides.

1. The burn order

   six-tank aircraft:                     five-tank aircraft:
   ① TRIM (CG schedule — own logic)       ① TRIM (CG schedule)
   ② CENTRE ──pump──► INNER               ② INNER burns
   ③ INNER burns                          ③ OUTER ──gravity──► INNER (last)
   ④ OUTER ──gravity──► INNER (last)

Two rules generate the whole table: the inner tanks are kept topped up from the centre while it lasts, and outer fuel — the wing-bending-relief ballast — is released only when the inners are nearly down to it (overview). Trim-tank fuel rides its own CG timetable (aft-CG transfer / forward transfer).

2. Centre → inner: pumped, in a top-up cycle (six-tank aircraft)

Two transfer pumps in the centre tank push fuel into the inner tanks through the inner-tank inlet valves. The FCMS runs it as a hover-around-full cycle: fill an inner to its high level, pause, resume when it has burned down about 2 000 kg below high level, repeat. The pause threshold on the receiving side is the underfull point — set approximately 2 500 litres below the volume that wets the high-level sensors (referenced to a level aircraft). Net effect: while the centre tank has fuel, the inner tanks stay essentially full and all the burn is logically "centre fuel".

Manual layer: the CTR TANK XFR pushbutton set to MAN commands the transfer regardless of the automatic schedule — the procedures use it when level-sensing or FCMS logic is suspect, typically gated on "WHEN EITHER INR < 17 T" (17 tonnes = the system's universal "is there room in the inner tank" gate, recurring across the abnormal chapter). Setting it to MAN also stops any aft transfer in progress — manual main-transfer intervention and automatic CG scheduling do not run together.

The pumps protect themselves by logic rather than endurance: centre empty + pump still running = FUEL CTR TK XFR FAULT, whose procedure switches them off before the dry-run thermal fuses decide the matter permanently (main fuel pumps, transfer faults).

3. Outer → inner: gravity through latched valves

The outer tanks sit outboard and slightly higher; their fuel needs no pump — only permission. Each side has an intertank transfer valve (codes O/P) that the FCMS opens when the inner side needs the reserve:

• automatic trigger: an inner tank's quantity falling to the transfer threshold (the same 3 500 kg figure the pressure-defuel logic uses for the outers' automatic release — defuelling) or the inner low-level sensing;
• once open, the valves stay open — the transfer is latched to completion; outer fuel is a one-way release, not a modulated top-up;
• both sides open together (symmetry preserved), and the fuel simply falls inboard.

Manual layer: OUTR TK XFR pushbutton ON opens the transfer valves (and the related inlet valves) immediately — the "release the reserves now" button. The abnormal procedures lean on it constantly: it is the second line of the low-level fuel-grab, the CG-forward tool in the trim-fuel-unusable drill, and the gallery detour when the crossfeed is failed closed. Like CTR TANK XFR MAN, setting it ON suspends automatic aft transfer.

[!warning]- The two transfers are different machines Centre→inner is pumped, cyclic, modulated; outer→inner is gravity, latched, all-or-nothing. That is why centre fuel can be "kept" while inners burn, but outer fuel, once released, is committed — and why procedures that need CG or balance effects from outer fuel (forward CG shift, gallery detour) reach for OUTR TK XFR rather than anything centre-side.

4. Dispatch

Some operators' MEL carries the main-transfer items along the architecture's lines: the outer-to-inner transfer function (intertank valves) and the centre transfer pumps each have their own items — a single centre transfer pump inoperative is typically dispatchable (the other pump covers, with conditions), while failures that remove a whole transfer path bring fuel-management and loading conditions. One indexing quirk worth knowing: the centre-tank pumps are filed under the transfer system (28-26), not the engine-feed pump chapter — the MEL classifies them by what they do, not what they look like.

Self-test

[!note]- Q1. Recite the burn order on a six-tank aircraft. Trim per its CG schedule; centre fuel pumped to keep the inners full; inners burn; outers released by gravity last (bending relief preserved longest).

[!note]- Q2. How does the centre→inner transfer decide to pause and resume? Fill to high level, pause, resume when the inner has burned ~2 000 kg below high level; the underfull point is ≈2 500 L below the high-level-sensor volume. A hover-around-full cycle.

[!note]- Q3. What is special about the intertank transfer valves once they open? They latch — outer→inner transfer runs to completion. Gravity does the moving; the valve only grants permission.

[!note]- Q4. What side effect do CTR TANK XFR MAN and OUTR TK XFR ON share? Both suspend automatic aft (CG) transfer — manual main-transfer intervention overrides the CG schedule.

[!note]- Q5. What does the recurring "17 t" gate mean in transfer procedures? It is the system's standard check that an inner tank has receiving room — manual centre-transfer steps wait for an inner below 17 tonnes before pushing more fuel in.

Key takeaways

References

• FCOM DSC-28-10-50/-60 (transfer sequence, triggers, manual controls).
• AMM 28-26-00 Description and Operation (transfer pumps, intertank valves, cycle logic); AMM 28-25-00 (underfull definition; 3 500 kg outer-release in defuel logic).
• Some operators' MEL 28-26 items.
• The "warehouse/logistics" framing and the two-machines contrast 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.