Aft CG Transfer — Fuel as Ballast

This is the signature idea of the A330 fuel system: in cruise, the aircraft pumps fuel into the tailplane to move its own centre of gravity aft — and flies measurably cheaper for it. This article explains the physics (why an aft CG cuts drag), the target (how AFT CG TARGET is set and when it is automatically pulled forward), the schedule (when the transfer runs and why usually only once), the plumbing (where the fuel comes from), and the protections (an independent watchdog and a balance guard).

The return journey — getting the fuel back before descent — is forward transfer.

1. The physics — trim drag and why aft is cheaper

For steady flight every moment must balance. The CG normally sits ahead of the wing's centre of pressure, so wing lift generates a nose-down moment — which the tailplane must oppose with a downward force.

        wing lift L ↑
          │
   ┌──────┴────────────────────────────┐
   │  CG        CP                      │═══► tail download D ↓
   │  ●─────────●                       │     (trim force)
   └────────────────────────────────────┘
   nose-down moment = L × (CP–CG arm)   →  tail must push DOWN

   cost ①: the download itself creates drag (trim drag)
   cost ②: the wing must lift (L + D) → higher angle of attack
           → more induced drag

Move the CG aft (toward the CP) and the nose-down arm shrinks; the tail download shrinks with it; both costs fall together. The A330's tool for moving the CG is the trim tank inside the THS — the longest moment arm on the airframe, so a few tonnes of fuel buy a large CG shift. The FCOM states the purpose directly:

"The trim tank transfer system controls the aircraft's Center of Gravity (CG)... When the aircraft is in cruise, the system optimizes the CG position to increase fuel economy by reducing drag... The FCMC calculates the aircraft's CG and compares it to a target value."

The boundary is equally physical: too far aft erodes longitudinal stability margin. So the system aims at a target with safety margin built in, and puts an independent watchdog behind it (Section 3).

2. The target — and the automatic forward corrections

"The FCMC calculates the aircraft's CG and compares it to a target value. (This target depends on the aircraft's actual weight.)"

The target is pulled forward — more conservative — whenever confidence drops, and the corrections stack:

"If the FMGEC detects a CG that is too far aft, then the target will automatically be moved forward by 3%. The target also moves forward 1.5% in the case of FQI data degradation, or if ZFW/ZFWCG have not been entered or need to be reinitialized in flight. The above-mentioned CG target alterations should be added together."

Uncertain data means a deliberately less-optimal, safer CG — the fuel saving is the first thing traded away. This is the systems-level reason a correct ZFW/ZFCG entry earns real money: enter it wrong (or not at all) and the aircraft quietly flies with part of its trim-drag benefit surrendered.

3. The watchdog — FMGEC, twice

"The FMGEC independently monitors the CG. If it detects that the CG is too far aft, it moves the target CG forward 3.0% MAC. If it detects the problem a second time, it... stops the CG control and shows a warning to the crew. The crew then manually set a forward transfer."

Strike one: target pulled forward 3 %. Strike two: automatic CG control is stopped, the crew is warned (the EXCESS AFT CG alert family — triggered when the CG sits more than 2.5 % behind target on the second detection), and CG management goes manual. A second, independent computer holding veto power over the first is the certification answer to "what if the FCMC mismanages the ballast."

4. The schedule

"Automatic CG control begins during climb to FL255. Ends at descent to FL245, or when the FMGES time to destination is less than 35 min (or less than 75 min in the case of trim tank forward transfer pump failure)."

CG optimisation is a cruise-altitude activity: starts climbing through FL255, ends descending through FL245 or at 35 minutes to destination — pushed out to 75 minutes if the trim pump is dead, because gravity-only forward transfer is slower and attitude-limited (forward transfer). At low altitude (takeoff, landing) the trim line is kept isolated entirely.

Start conditions — all simultaneously:

"In flight, the FCMC only starts an AFT fuel transfer, when all of the following conditions are met: Landing gear retracted; Slats retracted; Trim tank is not full; Inner tank's fuel quantity is above 6250 kg; Aircraft is above FL255; Aircraft CG is not on target."

Why usually once:

"Normally, only one aft fuel transfer occurs per flight. However, if the CG in cruise is ahead of the target by more than 2%, and the trim tank quantity is below 3000 kg, an additional aft transfer will occur."

One delivery to the tail per flight; afterwards the CG is maintained by the drip-feed of forward transfers as fuel burns. A second aft transfer needs both a real need (CG >2 % ahead of target) and room to act (trim below 3 000 kg).

The transfer also stops for: trim tank full, CG on target, an inner tank falling below 6 250 kg, the crew selecting T TANK MODE FWD, manual main-transfer buttons (OUTR TK XFR — and on six-tank aircraft CTR TANK XFR — set to manual), and on jettison-equipped aircraft, jettison set to ON (the jettison drill commandeers the same aft transfer valves).

[!warning]- A manuals discrepancy worth knowing The AMM's general overview quotes ">4 000 kg" for the inner-tank start condition where its detailed operation section and the FCOM both say 6 250 kg. The detailed/FCOM value is used here; treat the overview figure as suspect.

5. The plumbing — two sources, one destination

The trim tank's own pump never does the aft transfer — it points the other way (forward). Aft transfer is the wing pushing fuel to the tail:

"Fuel for trim tank aft transfer is provided by the center TK, when it contains fuel, or by the inner tanks when the center tank is empty."

• Centre tank has fuel (six-tank aircraft): the centre transfer pumps push fuel through the trim-pipe isolation valve (W) and trim-tank inlet valve (L) — the aft transfer valves (B/D) stay shut. The stop point carries a deliberate undershoot: the transfer ends when calculated CG = target − 0.5 %, leaving margin for the fuel still in the pipe.
• Centre empty (or five-tank aircraft): the main pumps push inner-tank fuel through the aft transfer valves (B/D) into the gallery and on to the trim tank.

Balance guard: if the inner tanks differ by more than 500 kg, the transfer draws from the heavier side only — the FCMS closes the lighter side's aft transfer valve, waits for the quantities to equalise, then reopens it and continues from both. The CG system tidies lateral balance as a side effect.

When the transfer completes, the inlet (L) and aft transfer valves (B/D) close — and the trim-tank isolation valve (T) is opened, pre-positioned for the forward transfers to come.

6. Dispatch

Some operators' MEL covers the aft-transfer chain as three families: the aft transfer valves, the trim-pipe valves, and the FCMS CG function itself (via the FCMC and ZFW-data items in FCMS computers). The common pattern: failures that merely lose the fuel saving are dispatchable with the trim tank kept empty or its schedule pre-bought (manual forward transfer at cruise start); failures that could trap fuel aft or defeat the isolation are held to the strict standard.

Self-test

[!note]- Q1. Why does an aft CG reduce fuel burn? Smaller nose-down arm → smaller tail download → less trim drag, and the wing no longer lifts the extra download → less induced drag. The trim tank's long moment arm makes fuel an efficient ballast.

[!note]- Q2. What pulls the AFT CG TARGET forward, and by how much? FMGEC detecting CG too far aft: 3 %. FQI degradation or missing/re-initialised ZFW–ZFCG: 1.5 %. The corrections add.

[!note]- Q3. What happens on the FMGEC's second "too far aft" detection? Automatic CG control stops, the crew is warned (EXCESS AFT CG family — CG more than 2.5 % behind target, second detection), and forward transfer is set manually.

[!note]- Q4. Where does aft-transfer fuel come from with a full centre tank — and which valves stay shut? The centre transfer pumps push it via the trim-pipe isolation (W) and trim inlet (L) valves; the aft transfer valves (B/D) remain closed. Stop at CG = target − 0.5 %.

[!note]- Q5. The inners are 600 kg apart when aft transfer starts. What does the system do? Draws from the heavier side only (lighter side's B/D valve closed) until balanced, then resumes from both — lateral balance maintained as a by-product.

Key takeaways

References

• FCOM DSC-28-10-90 (purpose, target, corrections, schedule, conditions, sources).
• AMM 28-27-00 Description and Operation §7.A (FMGEC monitor, source plumbing, balance guard, −0.5 % stop, stop-condition lists).
• Some operators' MEL (aft-transfer chain items).
• The trim-drag physics exposition is integrative synthesis consistent with the cited sources.

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.