FCMC and Inerting Faults

When the fuel system's brain falters, the muscles all keep working — pumps pump, valves answer their hard-wired masters. What dies is the scheduling. This article covers the computer-failure ladder from one FCMC to both (and the "human FCMC timetable" that replaces them), the weight-data alerts that feed the CG computation, and the one fault in the chapter that changes nothing at all.

1. FUEL FCMC 1(2) FAULT — one brain down

Procedure: crew awareness. The other computer takes over through the truth-table changeover (FCMS computers); functionally the flight continues unchanged. The asymmetry lives on the ground: FCMC1 inoperative is a no-go in some operators' MEL, while FCMC2 inoperative dispatches with the trim-temperature and FQI-degradation penalties — the master channel gets zero tolerance.

2. FUEL FCMC 1+2 FAULT — the human FCMC timetable

No data from the FCMS at all. The fuel system reverts to manual control, and the procedure is the FCMC's scheduling job rewritten as a crew timetable:

FCMC 1+2 ................. RESET            ← first, try to get the brain back
WEIGHT/CG ........... INITIALIZE
   (reset succeeds → this alert is REPLACED by
    FUEL NO WEIGHT/CG DATA — that exchange is good news:
    computers alive, they just need their numbers again)
FUEL TK XFR: MAN ONLY
▸ IF CG AFT OF 32 % (read CG on the MCDU FUEL PRED page —
                     the SD figure is gone):
   T TANK MODE ............ FWD             ← bring tail fuel forward
      (≈1 % burn; trim pump failed → the 270 kt gate)
▸ WHEN FOB BELOW 60 T:
   (newer standard: CTR TANK XFR MAN)
   OUTR TK XFR ............. ON             ← release the reserves manually
▸ WHEN FL < 250 IN DESCENT:
   T TANK MODE ............ FWD             ← the descent forward transfer,
                                              now on the crew's clock

Each line replaces an automatic behaviour: the 32 % branch stands in for the FMGEC-supervised CG control; the 60 t line stands in for the automatic outer-tank release; the FL250 line stands in for the descent forward transfer. The crew becomes the scheduler; the timetable is three numbers: 32 % — 60 t — FL250.

The bills on STATUS: fuel transfers manual only; on aircraft with the GA SOFT option, go-around thrust reverts to TOGA only (the soft go-around function leans on FCMS weight data); on the applicable build standard, jettison inoperative (fuel jettison) — the weight-reduction plan must not count on it.

[!warning]- Where did the CG number go? The SD FUEL page's CG was the FCMC's computation. With both dead, read the CG on the MCDU FUEL PRED page (FMS-side estimate). Different source, same decision threshold.

3. The weight-data pair

FUEL NO WEIGHT/CG DATA — the computers have no ZFW/ZFCG (never entered, or lost in a reset):

WEIGHT/CG ........... INITIALIZE     ← re-enter against the loadsheet

FUEL ZFW ZFCG DISAGREE — the two FMGECs are feeding different numbers:

FMGEC VALUES ........... CONFIRM     ← both FMGECs' ZFW/ZFCG, ECAM GW,
                                       ECAM CG — line by line vs loadsheet
◆ ECAM CG within 2 % of loadsheet CG → clear the alert
◆ beyond 2 % → the QRH ZFW/ZFCG DISAGREE procedure

Both alerts guard the same dependency: every CG decision in this chapter — targets, transfers, the 32 % branch above — computes from the crew-entered zero-fuel data. Garbage in, ballast misplaced.

4. FUEL INERTING SYS FAULT — the fault that changes nothing

On FTIS-equipped aircraft: the inerting layer has failed. No fuel-handling change follows — the aircraft reverts to the baseline ignition-source defences that protect every non-FTIS A330 anyway (inerting and fire prevention). The alert's value is maintenance awareness, not crew action. It is worth knowing precisely because the correct response is so easy to over-do: there is no transfer to make, no tank to avoid, no profile to fly.

Self-test

[!note]- Q1. Both FCMCs reset successfully and the alert changes to FUEL NO WEIGHT/CG DATA. Good or bad? Good — the exchange means the computers are alive again and merely need their ZFW/ZFCG re-sent (the data does not survive a cold start). Initialise and carry on.

[!note]- Q2. Recite the human-FCMC timetable. CG aft of 32 % (MCDU-read) → T TANK MODE FWD; FOB below 60 t → OUTR TK XFR ON (+ CTR MAN on the newer standard); below FL250 in descent → T TANK MODE FWD again.

[!note]- Q3. Why does a double FCMC failure touch go-around thrust on some aircraft? The GA SOFT (reduced go-around) function depends on FCMS weight data; without it, go-around reverts to TOGA only — brief the change before approach.

[!note]- Q4. The ECAM CG and the loadsheet differ by 1.5 % under a ZFW ZFCG DISAGREE. Action? Confirm the FMGEC values line by line; within 2 % the alert may be cleared. Beyond 2 % the QRH procedure applies.

[!note]- Q5. What fuel-handling changes follow FUEL INERTING SYS FAULT? None. The centre tank loses its oxygen-depletion layer and stands on the same ignition-prevention design as every non-FTIS aircraft. The alert informs; it does not task.

Key takeaways

Point	Value
One FCMC	awareness; changeover covers it; dispatch asymmetry on the ground
Both FCMCs	reset → re-initialise weight → manual timetable: 32 % / 60 t / FL250
CG source	SD figure gone; read the MCDU FUEL PRED page
Side effects	GA SOFT → TOGA only (option); jettison inoperative (build standard)
Weight pair	NO DATA → initialise; DISAGREE → confirm, 2 % decision line
Inerting fault	changes nothing — baseline defences remain

References

FCOM PRO-ABN-FUEL (FCMC 1(2) / FCMC 1+2 / NO WEIGHT-CG DATA / ZFW ZFCG DISAGREE / INERTING SYS FAULT, per build standard).
FCOM DSC-28 (FCMS reversion); some operators' MEL (FCMC asymmetry).
The "human FCMC" framing and the alert-exchange reading 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.