Fuel Imbalance

Lateral imbalance is the most rehearsed fuel abnormal — and the one whose first step is most often skipped in the rush to fix it. The procedure's opening caution is not about balancing at all: prove it isn't a leak first. This article covers the detection thresholds, the two cautions that gate the procedure, the main balancing method (crossfeed + selective pump-off), the gravity backup when the crossfeed won't open, and the certification context that keeps the whole subject calm.

1. Detection — what the aeroplane does and doesn't tell you

The system already fights imbalance silently: forward transfers bias toward the lighter inner, aft transfers draw from the heavier (forward / aft) — but those biases only act while a transfer is running. Plain asymmetric burn (one engine thirstier, an APU on one side's supply) accumulates untouched between transfers.

The crew-facing cues:

• SD FUEL page quantities flash when the wings differ by more than 3 000 kg — the attention-getter, deliberately set at the certification limit's full-tank region, not an early warning;
• the LIM-FUEL ceiling it points to: maximum imbalance 2 900 kg with inners full / 1 480 kg with outers full, varying linearly — and no limit at all below 7 500 kg (inner) / 1 730 kg (outer);
• your own scan: trend between the wing totals during the half-hourly fuel check (fuel management) catches an imbalance long before anything flashes.

2. The two gates before balancing

[!warning]- Gate 1 — is the fuel actually missing? FOB + fuel used against the departure figure, before touching anything. A significant or growing shortfall means the "imbalance" is a leak wearing a disguise — and the leak procedure explicitly forbids the imbalance procedure in that case (balancing feeds the good side's fuel toward the hole). Divert to fuel leak.

[!warning]- Gate 2 — feedline burst on the heavy side's engine? After a FEEDLINE BURST with the engine shut down, the crossfeed must not be opened toward that side. The imbalance procedure's backup path (below) exists partly for this case.

3. Main method — crossfeed and starve the light side

FOB ................................. CHECK   (gate 1)
WING X FEED ............................ ON
◆ valve confirmed open (in-line green):
   LIGHT side STBY PUMP ............... OFF
   LIGHT side MAIN PUMP 1 ............. OFF
   LIGHT side MAIN PUMP 2 ............. OFF
      (disregard the resulting FUEL WING PUMPS LO PR)
   ▸ when quantities converge:
   MAIN PUMPS 1+2, STBY ................ ON
   WING X FEED ....................... AUTO

Both engines drink from the heavy side; the light side rests. Two craft points: the expected WING PUMPS LO PR for the deliberately-stopped pumps is disregarded by procedure (a rare explicit licence to ignore an ECAM); and restore the pumps as the quantities converge, not after — flow keeps moving while valves travel, and the overshoot becomes tomorrow's mirror-image imbalance.

4. Backup — the gallery and a wing-down trickle

Crossfeed failed closed (or forbidden):

OUTR TK XFR ............................ ON     ← inners linked via the refuel gallery
STRAIGHT FLIGHT .................. MAINTAIN
AP .................................... OFF
BANK ANGLE ............ 3° WING DOWN ON LIGHTER SIDE
USE RUDDER TO KEEP HDG AND NEUTRAL STICK
USE RUDDER TRIM
▸ when transferred:
OUTR TK XFR .......................... AUTO · resume normal trim · AP as required

Physics instead of plumbing: the gallery connects the two inner tanks; a steady 3° bank toward the lighter wing makes fuel run downhill across it. It is a hand-flown sideslip exercise — rudder holds heading, stick stays neutral, and the flow is in hundreds of kilograms per minute: patience is part of the procedure.

5. The calm context

LIM-FUEL closes the subject with the sentence that frames every imbalance discussion:

"In exceptional conditions (i.e. fuel system failure), the above-mentioned values for maximum fuel imbalance may be exceeded without significant effect to the aircraft handling qualities. The aircraft remains fully controllable in all flight phases."

And the leak procedure goes further: with a wing leak, one inner full and the other empty is an acceptable approach-and-landing state. The limits are a certification envelope; exceeding them under failure conditions costs trim and tidiness, not control. The dangerous error in this chapter is not a big imbalance — it is balancing into a leak.

Self-test

[!note]- Q1. Why is the flashing FUEL page a poor early warning? It triggers at 3 000 kg — already at the full-tank limit region. The half-hourly trend check is the early warning; the flash is the missed-it alarm.

[!note]- Q2. What two checks gate the balancing procedure? Conservation (FOB + used vs departure — leak test) and the feedline-burst history of the heavy side (crossfeed prohibition).

[!note]- Q3. During balancing, a WING PUMPS LO PR appears for the light side. Action? None — the procedure says disregard it; you caused it by switching those pumps off deliberately.

[!note]- Q4. Describe the crossfeed-failed backup in one sentence. Link the inners through the refuel gallery (OUTR TK XFR ON) and hold a 3° bank toward the light wing with rudder for heading, letting gravity walk the fuel across.

[!note]- Q5. Is an out-of-limits imbalance a controllability emergency? No — per LIM-FUEL the aircraft remains fully controllable in all phases even beyond the limits under failure conditions; a full/empty wing pair is an accepted landing state in the leak procedure.

Key takeaways

References

• QRH fuel imbalance procedure + FCOM PRO-ABN-FUEL expanded notes (cautions, both methods).
• FCOM LIM-FUEL (limits, controllability statement); DSC-28-10-130 (flash logic).
• FCTM fuel-check discipline.
• The "silent rebalancing only during transfers" observation is integrative synthesis of the transfer logic.

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.