Quantity and Level Faults

This family is where the level sensors (hard wet/dry lines) and the quantity computation (the FQI) each take their turn failing — or telling the truth at the worst moment. Four situations: a tank genuinely running low (the fuel-grab), fuel climbing into the vent system (overflow), the collector cell not staying full (the feed reservoir alert), and the gauges lying high (overread). The common thread: know which witness you are trusting — the FQI and the level sensing fail independently (fuel quantity indication), and the procedures exploit that.

1. FUEL L(R) WING TK LO LVL — the fuel-grab

Triggers when both low-level sensors in one inner tank are dry for 60 s — fuel in that tank somewhere in the 1 100–2 520 kg band (attitude-dependent). The drill pulls every reachable reserve toward the hungry side:

(centre not empty:) CTR TANK XFR ....... MAN     ← centre's fuel, now
OUTR TK XFR ............................. ON     ← outers' reserve, now
If TRIM TK not empty: T TANK MODE ...... FWD     ← tail's fuel, now (+~1 % burn)
▸ IF NO FUEL LEAK and IF FUEL IMBALANCE:
   WING X FEED .......................... ON
   STBY + MAIN PUMPS (low side) ........ OFF     ← feed that engine from the
                                                   healthy side; rest the low tank

Three manual transfers stacked in one procedure — the system stops scheduling and grabs everything. The second block is the imbalance method recycled with the usual gate (no leak — fuel leak); and the cross-side guard holds here too: if the other side's main pumps are already dead, the ECAM will not ask you to switch these off.

2. FUEL L + R WING TK LO LVL — the final notice

All four sensors dry 60 s. LAND ASAP, the same three transfers — and one reversal:

WING PUMPS .............................. ON      ← all of them
▸ IF NO FUEL LEAK: WING X FEED ......... ON

[!warning]- Single low: pumps OFF. Double low: pumps ON. With one side low, the healthy side carries the load and the low tank rests. With both sides low there is nobody to lean on — every pump runs to squeeze the last usable fuel from every corner. Opposite actions, same logic: maximise what reaches the engines.

3. FUEL CELL NOT FULL — the reservoir alert

The collector cell — the always-full well the main pumps drink from (tanks and storage) — reports under 500 kg with its side's pumps running:

JETTISON (if installed and operating) ... OFF    ← dumping competes for the fuel
                                                   that should be refilling the cell
▸ IF NO FUEL LEAK: WING X FEED .......... ON
▸ a collector cell keeps falling (crossfeed not fully open),
  or both cells falling:
   AVOID NEGATIVE G FACTOR                       ← the cell IS the negative-g
                                                   protection; a part-empty cell
                                                   protects less

A cell that won't stay full means the flap valves aren't replenishing it as fast as the pumps drain it — low tank, jettison competition, or cell damage. The negative-g line is the quiet teeth: the protection margin is the fuel in the cell.

4. FUEL WING TK OVERFLOW — fuel in the airway

A surge-tank sensor wet for 25 s: some transfer has overfilled a tank and fuel is climbing the vent pipes (tank venting). The fix is always stop the filling — never the vents:

five-tank:  T TANK FEED ........ ISOL            ← forward transfer was the filler
            ▸ WHEN EITHER INR < 17 T: FEED AUTO
six-tank:   L + R CTR PUMPS ..... OFF            ← centre transfer was the filler
            T TANK MODE (only if CTR empty) FWD  ← reroute trim fuel to the
            ▸ WHEN EITHER INR < 17 T: PUMPS ON     centre instead

The 17 t gate yet again — resume the transfer once the receiving tank has room. On the ground, the same sensors light the refuel panel's amber OVERFLOW lights and stop the refuel.

5. FUEL OVERREAD — when the fuel "grows"

Fuel cannot multiply in flight. FOB + fuel used above the departure figure, or rising — any "gain" — means the measurement is lying high. The QRH:

FOB / F.USED ................ CHECK
◆ discrepancy confirmed:
   FUEL QTY UNRELIABLE                ← even with no amber dashes anywhere
   DISREGARD FMS FUEL PREDICTIONS
   COMPUTE FOB FROM: INITIAL FOB − F.USED
   FUEL LO LVL ALERTS REMAIN RELIABLE ← the independent witness
   Maintenance action due before next flight

The two memorable lines: the gauge is condemned even if it looks healthy (no dashes, no XX — overread is precisely the failure the display grammar cannot flag); and the low-level alerts stay trustworthy because they are a different system entirely. From there the flight runs on arithmetic: fuel used (engine flowmeters) against the known departure FOB, updated at every waypoint.

The FWC runs the same conservation sum continuously and raises FUEL F.USED/FOB DISAGREE when the books differ by more than 3 500 kg — its procedure is a two-way signpost: leak detected → the leak drill; overread confirmed (on the ground) → this one. Loss or lie: the two directions the same arithmetic can fail.

Self-test

[!note]- Q1. What exactly has happened when WING TK LO LVL triggers, hardware-wise? Both low-level sensors in that inner tank dry for 60 seconds — a discrete, FQI-independent detection; tank contents are in the 1 100–2 520 kg band depending on attitude.

[!note]- Q2. Why pumps OFF in single low level but ON in double? Single: the healthy side can feed both engines (crossfeed), so the low side rests. Double: no healthy side exists — every pump squeezes the remainder.

[!note]- Q3. What makes CELL NOT FULL more than a quantity nuisance? The collector cell is the negative-g feed protection; a part-empty cell shortens that protection — hence AVOID NEGATIVE G, and jettison (a competitor for the same fuel) goes off.

[!note]- Q4. Why does the OVERFLOW procedure never touch the vent system? The vents are doing their job (catching the spill); the fault is a transfer overfilling a tank. Stop or reroute the transfer; resume below 17 t.

[!note]- Q5. After declaring FUEL QTY UNRELIABLE, what still protects you from running a tank dry? The low-level alerts — discrete sensors independent of the FQI chain — plus the manual conservation arithmetic (initial FOB − fuel used).

Key takeaways

References

• FCOM PRO-ABN-FUEL (LO LVL single/double, CELL NOT FULL, WING TK OVERFLOW, F.USED/FOB DISAGREE) and QRH FUEL OVERREAD.
• AMM 28-46-00 (level sensing); FCOM DSC-28-10-130 (display grammar limits).
• The witness-independence framing is integrative synthesis of the cited systems.

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.