APU Fuel Feed

The APU is fed from the same tanks as the engines — but along a path of its own, with two pumps in two different places and an automatic source-switch the crew rarely notices. The path matters operationally because it runs through the trim pipe: whenever a procedure isolates the trim line, the APU's normal supply goes with it. This article maps the two sources, the valve, and the failure modes that put an altitude number on APU availability.

1. Two sources, one consumer

  NORMAL SOURCE                              BACKUP SOURCE
  trim pipe (aft fuel network)               LH collector cell
        │                                          │
  ● APU AFT fuel pump                        ● APU FWD fuel pump
  (pressurises the trim pipe                 (in the left collector
   toward the APU line)                       cell, beside the mains)
        │                                          │
        └────────────► [APU isolation valve] ◄─────┘
                              │
                       [APU LP VALVE]
                              │
                            APU

• Normal: the aft APU pump supplies the APU from the trim-pipe network — geometrically sensible, since the APU lives in the tail cone and the trim pipe already runs there.
• Backup: when the trim pipe is isolated or unpressurised, the forward APU pump in the left collector cell takes over, feeding through the APU isolation valve.

The switch is automatic. The cleanest illustration is in the refuelling chapter: while the trim tank is being refuelled, gallery pressure can drop below what the APU line needs — the refuel-gallery low-pressure switch then closes the trim-line valves and the APU "is then supplied from the APU forward fuel pump through the APU isolation valve" with no crew involvement (refuelling).

[!warning]- Every trim-line isolation starves the APU's normal source T TANK FEED to ISOL, the TRIM LINE FAULT procedures, gear-down auto-isolation of the trim pipe — all of them cut the aft path. Usually the forward pump covers it silently; but when a procedure isolates the trim pipe and the forward path is not available, the STATUS page says it plainly: APU INOP when trim tank isolated, and the TRIM TANK FEED-failure procedure orders APU MASTER SW OFF (trim-tank fuel unusable).

2. The APU LP valve — same doctrine as the engines

The APU's last fuel gate follows the engine pattern exactly: commanded by the APU MASTER switch, slammed shut by the APU FIRE pushbutton, hard-wired around the FCMS, dual-supplied with one leg on a battery-backed bus. Fuel cut-off to the APU survives any computer failure — and works on batteries alone, which is precisely the scenario (battery start of the APU) where it is most needed.

Position-versus-command disagreement raises FUEL APU LP VALVE FAULT — Crew awareness, read through the SD valve symbol: the real content is whether you still hold the APU's isolation capability.

3. The altitude number — AFT pump failure

FUEL APU AFT PUMP FAULT removes the normal source. The consequence is not "APU lost" but an envelope cut:

• with fuel in the trim tank, the APU remains available only up to FL250 — the forward path and line geometry support APU operation in the lower envelope only;
• the alert itself is Crew awareness plus the STATUS limitation; planning is where it bites (an APU you counted on for an ETOPS or relief scenario above FL250 is no longer on the menu).

Dispatch follows the same two components: some operators' MEL carries items for the aft APU pump (dispatchable with the altitude-type limitation reflected in operations) and for the APU LP valve (the isolation function held to the strict standard of the engine LP valves).

4. Where this shows up in other procedures

The APU thread runs through the whole abnormal chapter — worth collecting in one place:

Self-test

[!note]- Q1. Where do the APU's two pumps sit, and which is normal? Aft APU pump on the trim-pipe network (normal source); forward APU pump in the LH collector cell (backup through the APU isolation valve). The switch between them is automatic.

[!note]- Q2. During trim-tank refuelling the gallery pressure sags. What keeps the APU running? The refuel-gallery low-pressure switch isolates the trim line and the APU is automatically fed by the forward pump from the LH collector cell.

[!note]- Q3. What does FUEL APU AFT PUMP FAULT actually cost? The normal (aft) source. With fuel in the trim tank the APU stays available only below FL250 — an envelope limitation, not a loss.

[!note]- Q4. Why does the trim-line isolation-failure procedure switch the APU MASTER off? Locking the trim pipe removes the APU's normal supply; the procedure completes the consequence rather than leaving the APU sucking on an isolated line.

[!note]- Q5. Why is the APU LP valve wired like the engine LP valves? Same doctrine: fuel isolation is a hard-wired right (APU MASTER / APU FIRE pushbutton), independent of the FCMS and available on batteries.

Key takeaways

References

• FCOM DSC-28-10-100 (APU feed, pumps, valve); PRO-ABN-FUEL (APU AFT PUMP FAULT, APU LP VALVE FAULT, TRIM LINE FAULT).
• AMM 28-25-00 §7.A (refuel low-pressure switch logic); AMM 28-22/28-23 (pump and valve hardware).
• Some operators' MEL APU-fuel items.
• The doctrine framing and the cross-procedure table are integrative synthesis from the cited procedures.

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.