Trim-Tank Fuel Unusable

The trim tank is the A330's fuel-economy trick — until its plumbing fails and the tonnes in the tail become dead weight on the longest moment arm of the aircraft. Worse than dead: as the flight burns fuel and the aircraft lightens, the same trapped mass pushes the CG further and further aft. This article maps the three roads into the trap (the FUEL TRIM LINE FAULT family), then the endgame procedure with its two CG weapons, its four-hour clock, and its landing technique.

 roads into the trap                          ECAM face
 ├─ ① aft transfer valve stuck open          T TANK XFR FAULT branch
 │     (forward transfer recirculates)         ([transfer faults](./ata-28-22-transfer-faults.md))
 ├─ ② trim-line isolation valve stuck open   TRIM LINE FAULT — scenario A
 │     + centre tank not empty                 (uncommanded aft transfer)
 ├─ ③ trim-tank isolation failed             TRIM LINE FAULT — scenario B
 │     (line must be locked → fuel sealed in)
 └─ aggravator: trim pump failed             TRIM LINE FAULT — FUEL TRIM PUMP
       (forward transfer on gravity only)
 all converge on → TRIM TANK FUEL UNUSABLE (Section 3)

1. Scenario A — the door that won't shut, with pressure behind it

Trigger: the trim line isolation valve is failed open and the centre tank is not empty. Why the second condition matters: a stuck-open door is harmless until something pushes — and the centre pumps push continuously. Their pressure through the open valve is an uncommanded aft transfer, walking the CG aft with nobody at the controls. (Centre empty = no pressure source = same stuck valve, no alert.)

JETTISON ................... OFF      ← if dumping, the centre pumps are
L + R CTR PUMPS ............ OFF        the jettison muscle — stop that first
T TANK MODE ................ FWD      ← reclaim what was pushed aft
   (trim pump failed → gate: WHEN SPD > 270 KT AND NOT IN CLIMB)
▸ if CG forward of 32 % — two steps:
   WHEN T TANK < 2.4 T:  T TANK FEED ISOL · CTR PUMPS ON
   WHEN CTR EMPTY:       CTR PUMPS OFF · FEED AUTO
▸ WHEN T TANK EMPTY: MODE AUTO · CTR PUMPS ON · JETTISON AS RQRD
Note 1: the caution recalls at each step / tank-empty — scheduled, not a relapse
Note 2: unsuccessful, fuel trapped → TRIM TANK FUEL UNUSABLE (Section 3)

The craft detail: in the two-step pause, T TANK FEED ISOL substitutes for the door that won't close — the line's other three valves lock the pipe while the centre pumps go back to work. STATUS bills: aft transfer inoperative, APU inoperative whenever the trim line is isolated (APU fuel feed).

2. Scenario B — the isolation that can't be trusted

Trigger: the trim tank isolation is failed. The response sounds paradoxical — isolate it yourself:

TRIM TANK FEED ............. ISOL     ← lock the whole line deliberately
APU MASTER SW ............... OFF     ← the line was the APU's normal source
▸ IF T TK ISOL AFTER T.O. (manual isolation had been selected on ground):
   TRIM TANK FEED .......... AUTO     ← the tank comes back

An isolation function that cannot be relied on means the line's integrity is unproven — better a deliberately locked pipe (and its known costs: trim fuel sealed in, APU off) than an unverifiable one threaded through the feed network. The third line is the distinction that saves a tank: isolation selected by people on the ground (maintenance, dispatch) is reversible after takeoff; isolation declared by failure is not.

3. The aggravator — FUEL TRIM PUMP

The TRIM LINE FAULT family's third member (the AMM names the sub-failure FUEL TRIM PUMP — "the trim transfer pump has had a failure"): T TANK MODE to AUTO, and forward transfer by gravity only — the 3.4° / 270 kt geometry rules apply (forward transfer). It traps nothing by itself; it makes every reclaim slower and attitude-fussy — the straw on the camel's back when stacked on A or B. Two quantifications from the same AMM section: the EXCESS AFT CG sub-alert fires when the CG sits more than 2.5 % behind target on the FMGEC's second detection (aft-CG transfer); and a trim-tank isolation valve stuck closed adds about 450 litres to the tank's unusable fuel.

4. The endgame — TRIM TANK FUEL UNUSABLE

Whichever road led here:

T TANK MODE ................ FWD
T TANK FEED ............... OPEN      ← last attempt — tank and pipe together
◆ IF TRIM TANK FUEL UNUSABLE confirmed:
   OUTR TK XFR .............. ON      — "The CG moves forward"
   L CTR TK PUMP ........... OFF ┐    — keep fuel in the centre tank to
   R CTR TK PUMP ........... OFF ┘      offset the trapped tail fuel
   MAXIMUM FLIGHT TIME: 4 HOURS
      — after 4 h, depending on fuel distribution,
        the aft CG limit may be reached

The two CG weapons, both about geography. The outer tanks sit aft-outboard on a swept wing: draining them into the inners moves wing fuel one notch forward (at the price of the bending-relief ballast — failure management is a market of trade-offs). The centre tank is the most forward large tank on the aircraft: the normal logic empties it ASAP; this procedure does the opposite — keeps fuel there as nose ballast, and says why in plain text ("reduce the effect of unusable trim tank fuel on the aircraft's center of gravity") — one of the rare procedure lines that carries its own explanation.

The 4-hour hourglass. The trapped mass's moment is constant, but the aircraft keeps burning fuel and lightening — the same tail moment divided by a shrinking weight walks the CG percentage aft. Four hours is the conservative budget to the aft limit. Operational reading: the diversion decision starts now, the SD CG figure joins the scan (the page's only red-capable number — controls and ECAM), and fuel planning runs on FOB minus the tail (the half-boxed FOB does the subtraction visually).

Landing beyond the aft limit (where the CG table — or the 39 % line in the five-tank version — says so):

APPR SPD: VLS + 10 kt
LDG DIST PROC ............. APPLY
FOR ROLLOUT: APPLY CONTINUOUS MANUAL BRAKING

Ten knots buys back pitch authority and buffet margin at an aft CG; the distance procedure pays for those ten knots; continuous manual braking keeps the deceleration's pitch coupling in the crew's hands rather than the autobrake's schedule. (The technique rationales are integrative; the three lines are verbatim procedure.)

Self-test

[!note]- Q1. Why does scenario A need the centre tank non-empty to trigger? The stuck-open valve only matters with a pressure source behind it — the centre pumps. Their pressure through the open door is an uncommanded aft transfer; an empty centre has nothing to push with.

[!note]- Q2. Why does scenario A start with JETTISON OFF? A jettison in progress runs on the centre pumps — the very pressure source feeding the runaway aft transfer. Stop the dump before securing the pumps.

[!note]- Q3. "Isolation failed → isolate it" — resolve the paradox. The automatic isolation function is unreliable; T TANK FEED ISOL locks the line with the remaining valves deliberately. A known-locked pipe with known costs beats an unverifiable one.

[!note]- Q4. Explain the 4-hour limit from first principles. Constant tail moment ÷ decreasing aircraft weight = CG percentage drifting aft as fuel burns. Four hours is the budgeted time before the aft limit may be reached — a diversion clock, not a comfort margin.

[!note]- Q5. Name the two CG weapons and their costs. OUTR TK XFR ON — moves outer fuel inboard/forward; costs the wing-bending-relief ballast. Centre pumps OFF — keeps the most forward tank full as ballast; costs the normal burn order and that fuel's availability.

Key takeaways

References

• FCOM PRO-ABN-FUEL TRIM LINE FAULT (three scenarios) and TRIM TANK FUEL UNUSABLE / QRH equivalent (endgame, CG note, landing rows).
• AMM 28-27-00 §7.E/§7.F (FUEL TRIM PUMP naming, EXCESS AFT CG quantification, 450 L figure).
• The hourglass arithmetic and landing-technique rationales 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.