Single Pack Failure — The Full Spectrum

Stage E opens with abnormals. This article covers the full spectrum of single-pack failures — every PACK 1(2) … condition in the FCOM abnormal procedures (not the dual PACK 1+2 FAULT, which is ata-21-22). This is what a pilot actually faces on the ECAM when one pack degrades.

Each pack and its controller are functionally independent: one pack's failure does not affect the other, and the remaining pack automatically goes to high flow (the PFCV HI position, ~120 % NORM) to maintain total cabin flow — see ata-21-06 and ata-21-08.

1. The six single-pack conditions

[!warning]- The core split: three "awareness only" vs three "switch the pack off"

Awareness only: OFF / Controller Fault / RAM Door. Switch off: OVHT / Bypass Mode (ground) / Valve Fault (normal). The grouping below follows this. (The dual PACK 1+2 FAULT → RAM AIR + emergency descent is ata-21-22.)

2. PACK 1(2) OFF

This alert triggers when the PACK 1(2) pb-sw is set to OFF and no failure is detected. Crew awareness. — FCOM PRO-ABN-AIR

An operator-selected OFF, not a fault — selected during an engine start (some operators), a single-pack dispatch, after a completed procedure, or an inadvertent press. No action — the cause is the selection itself. STATUS: ONE PACK ONLY IF WAI ON.

[!note]- "ONE PACK ONLY IF WAI ON"

On single-pack operation, switching wing anti-ice (WAI) ON splits the engine bleed between the WAI and the one pack — the pack's bleed may be insufficient for cabin comfort. This is a pilot-level STATUS, not a fault — be aware not to add further bleed consumption.

3. PACK 1(2) OVHT — three thresholds

This alert triggers when: The pack compressor outlet temperature rises above 260 °C, or The pack compressor outlet temperature rises above 230 °C four times during the same flight, or The pack outlet temperature rises above 95 °C. — FCOM PRO-ABN-AIR

[!warning]- The three thresholds, layered

Threshold	Point	Meaning
260 °C	compressor outlet	a single excursion triggers the ECAM OVHT
230 °C × 4	compressor outlet	cumulative fatigue (4 times = a persistent ACM/compressor problem)
95 °C	pack outlet	the pack's total output temperature too high

These differ from the 235 °C PFCV pneumatic close (an AMM hardware action) and the 180 °C ECAM-field clear. Each governs a different layer.

   PACK (AFFECTED) .......................... OFF      ← crew action
   High flow is automatically selected on the remaining pack.
   The fault light goes off when the overheat disappears.
   (If the cargo cold-air-valve auto-closure is inoperative: FWD CRG COOLING ... OFF)
   WHEN PACK OVHT OUT:
   PACK (AFFECTED) .......................... ON       ← crew restores

[!important]- "High flow automatically selected on the remaining pack"

After a single-pack loss, the remaining pack switches from NORM to HI flow automatically (no crew action) to maintain total cabin flow: the zone controller sees one PFCV closed → commands the other pack's PFCV to the HI position (120 % NORM). See ata-21-06 + ata-21-08.

[!note]- OVHT is a temporary exit, not a permanent shutdown

The procedure is OFF → wait → ON. The compressor outlet is the ACM's second-stage output; the most common cause is insufficient ram-air flow or an off-nominal ACM. It recovers when it cools. But if the 230 °C sub-threshold triggers four times in one flight, the compressor really has a problem → record it for maintenance on landing. (Distinct from RAM DOOR CLOSED, which is a stuck ram-air door — no cooling air at all; OVHT may be internal or simply insufficient ram-air flow at high speed/altitude.)

4. PACK 1(2) REGUL FAULT — two sub-conditions

The same ECAM title carries two different sub-conditions — you must read the sub-title + STATUS.

4a. PACK CONTROLLER FAULT

This alert triggers when the pack controller is failed or the air is only cooled by the heat exchanger. Crew awareness. — FCOM PRO-ABN-AIR

The corresponding anti-ice valve regulates the pack outlet temperature between approximately 9 °C and 15 °C ... The ECAM signals, associated with the corresponding pack, are lost. The flow control valve pneumatically regulates the pack flow to approximately 120 % of the NORM flow. PACK 1(2) AT FIXED TEMP. The pack delivers a fixed temperature of 12 °C ± 3 °C ... — FCOM DSC-21-10-40 + PRO-ABN-AIR

When the pack controller fails (both channels — a single channel is taken over, ata-21-08), the pack still works on its pneumatic backup: the anti-ice valve holds the outlet at a fixed 12 °C ± 3 °C, and the PFCV pneumatically regulates to ~120 % NORM. STATUS: PACK 1(2) AT FIXED TEMP.

[!note]- What 12 °C ± 3 °C means for the cabin

Normal pack outlet is a cool positive temperature, set by the controller to the lowest zone demand; a controller failure fixes it at 12 °C ± 3 °C. The pilot loses pack-level outlet-temperature control; all further trimming is then only via the trim-air system, which has limited heat-up authority. So a warm cabin demand is limited — the cabin temperature is constrained.

4b. PACK IN BYPASS MODE

This alert triggers when the air cycle machine is failed. If the Air Cycle Machine (ACM) fails (compressor/turbine seizure), the affected pack may be operated in the heat-exchanger cooling mode. Warm pre-conditioned bleed air enters the cooling path, via the pack flow control valve, and goes to the primary heat exchanger. Then, the compressor check valve opens, and air is cooled only by the heat exchanger. — FCOM PRO-ABN-AIR + DSC-21-10-40

[!important]- Bypass mode is the engineer's reserve cooling path

Normally an ACM failure stops the pack. The A330 has a compressor check valve that opens on an ACM failure (pushed by the compressor-outlet back-pressure), routing the air around the compressor straight to the main heat exchanger (cooled by ram air). The pack still works in flight, at much reduced efficiency. See ata-21-07.

   PACK 1(2) IN BYPASS MODE
   On ground:  PACK (AFFECTED) ... OFF   ← to avoid overheating on the ground
   In flight:  PACK (AFFECTED) AVAIL IN FLT

[!warning]- Why ground OFF but in-flight available — the ram-air flow

On the ground (stationary) the ram-air flow is tiny → the heat exchanger cannot cool enough → the pack overheats → switch it off. In flight (with airspeed) the ram air is plentiful → the heat exchanger still gives an acceptable (if degraded) temperature → the pack is usable. The FCOM states it: a pack with a seized ACM must be switched off on the ground due to the unavailability of ram-air cooling. STATUS: PACK PERF AFFECTED.

5. PACK 1(2) RAM DOOR CLOSED

This alert triggers when the pack ram air inlet or outlet door is failed closed. — FCOM PRO-ABN-AIR

Crew awareness — no action. STATUS: PACK PERF AFFECTED.

[!warning]- Counter-intuitive: RAM DOOR CLOSED does not require switching the pack off

A closed ram-air door seems to mean no cooling → overheat → switch off. But the procedure is awareness only, because: (1) PACK OVHT is monitored independently — if the compressor outlet actually exceeds 260 °C or the outlet 95 °C, the ECAM additionally triggers PACK 1(2) OVHT, and then you run the OVHT procedure; (2) the pack still works at reduced efficiency (a stuck door is not zero cooling — some leakage + pack-bay fan effect); (3) it avoids needlessly losing half the air-conditioning. The FCOM trusts the pilot to watch the BLEED page: RAM DOOR CLOSED + a rising compressor-outlet trend → switch the pack off proactively; if the temperature is stable, keep using it.

6. PACK VALVE 1(2) FAULT

This alert triggers when the position of the pack flow control valve disagrees with the selected position. — FCOM PRO-ABN-AIR

The PFCV has two microswitches (open / closed); the pack controller compares the commanded position with the feedback. A disagreement (> ~2 s) triggers the fault — commonly a stuck valve, insufficient bleed pressure, or a failed switch.

   If only one PACK VALVE closed (on ground at engine start):
     PACK (affected) INHIB BY DOORS    ← engine-start exception, no action
   In all other cases:
     PACK (AFFECTED) ... OFF

[!important]- Why no action at engine start — "door not closed" prevents pressurisation

At engine start, if the pack controller detects a door not closed, it deliberately closes the same-side pack valve to prevent inadvertent pressurisation. The ECAM reads the "should-be-open-but-closed" valve as a position disagreement, but it is a deliberate design — so the procedure shows INHIB BY DOORS and the pilot must not switch off the other pack. ("The other pack valve stays abnormally in the OPEN position" — "abnormally" is the ECAM wording, not a fault.) In all other cases the disagreement is a real fault → PACK OFF, the remaining pack auto-HI. INOP SYS: PACK 1(2), LD FWD TEMP, LD AFT TEMP (cargo trim air also comes from that pack — ata-21-05).

7. Procedures & operations

Switching the PACK off

1. Confirm the ECAM text (which pack? which fault?).
2. PACK 1 or PACK 2 pb-sw → OFF (one press).
3. Watch the SD BLEED page: the PFCV shows Crossline amber (closed), the pack-outlet temperature box clears, the other pack's flow jumps to HI.
4. Check STATUS/INOP: ONE PACK ONLY IF WAI ON.
5. If the procedure shows WHEN PACK OVHT OUT → PACK ON, restore only after the STATUS prompt appears.

[!tip]- Pilot detail on switching a PACK off

One press toggles on/off — do not press twice. The OFF light = a selected-off; the FAULT light = a fault-off (regardless of pb position). After switching off, the cabin airflow drops (only drops, not cut — the other pack's HI flow compensates, but ~30–40 % flow is still lost).

Single-pack altitude & dispatch

The FCOM gives no explicit single-pack altitude limit, but: a single-pack dispatch is covered by the MEL; with WAI ON the single pack's bleed may be insufficient for normal cabin flow (an effective limit); and a long sector (> 4 h) is marginal for cabin comfort. A single pack does not require descending, but watch the BLEED-page cabin flow, CO₂ (if fitted), and passenger feedback.

ECAM cancel/recall

Single-pack warnings are mostly L2 (caution, amber + single chime). CLR clears the current item; STS recalls the STATUS. When the procedure completes (PACK OFF + cool + PACK ON) and the overheat clears, the ECAM auto-clears; if the condition persists, it re-triggers.

Self-test

[!note]- Q1. Does PACK OVHT trigger on the compressor-outlet or the pack-outlet temperature?

Either — three thresholds: compressor outlet 260 °C once, or 230 °C four times in one flight, or pack outlet 95 °C. On the SD BLEED page the pilot mainly watches the compressor-outlet field (amber at 260 °C); the pack outlet is not usually shown as a number.

[!note]- Q2. PACK REGUL FAULT has two sub-conditions — how does the pilot tell them apart?

By the sub-title + STATUS. PACK CONTROLLER FAULT → STATUS PACK 1(2) AT FIXED TEMP, no action. PACK IN BYPASS MODE → STATUS PACK PERF AFFECTED + PACK OFF on the ground, available in flight. The main ECAM title is the same for both — read the sub-title and STATUS.

[!note]- Q3. Why does the FCOM not require switching the pack off for RAM DOOR CLOSED? What if it overheats?

PACK OVHT is monitored independently — if the door-closed condition drives the temperature above 260/95 °C, the ECAM additionally triggers PACK 1(2) OVHT, and then you switch off per the OVHT procedure. This layered design lets the pilot judge the temperature trend on the BLEED page and avoids losing half the air-conditioning for a mere door fault. In practice: RAM DOOR CLOSED + a rising compressor-outlet trend → switch off proactively without waiting.

[!note]- Q4. A PACK VALVE FAULT appears during an engine start — switch the pack off?

No. The procedure shows INHIB BY DOORS — at engine start, with a door not closed, one pack valve is closed deliberately (to prevent inadvertent pressurisation) and the other stays OPEN; this is by design, not a fault. Do not use the "all other cases" branch; the system releases automatically once the doors are closed.

[!note]- Q5. On single-pack operation, WAI ON gives ONE PACK ONLY IF WAI ON — a fault?

Not a fault — a STATUS (not an L1/L2 warning). It means with one pack + WAI the bleed is split and may be insufficient for cabin comfort. Be aware the bleed margin has dropped — add no further consumption (e.g. engine anti-ice), watch the cabin flow / bleed pressure, and brief the passengers on a long sector.

Key takeaways

Common misconceptions

Scope — what this article covers and defers

References

A330 specifics per FCOM PRO-ABN-AIR (the conditions PACK 1(2) OFF, OVHT with its 260 / 230×4 / 95 °C thresholds, REGUL FAULT in its two sub-conditions — PACK CONTROLLER FAULT with the fixed 12 °C ± 3 °C and PACK IN BYPASS MODE with the ground-OFF/in-flight-available split, RAM DOOR CLOSED, and PACK VALVE FAULT with the engine-start INHIB BY DOORS exception — each with its procedure, STATUS and INOP) and FCOM DSC-21-10-30 / DSC-21-10-40 (the functional independence and auto-HI of the remaining pack, the anti-ice-valve 9–15 °C backup regulation, and the ACM-failure heat-exchanger bypass via the compressor check valve). The three-thresholds-layered reading (against the 235 °C PFCV close and 180 °C field clear), the ground-vs-in-flight ram-air rationale, and the awareness-vs-action grouping are integrative syntheses. All engineering detail is from the A330 knowledge base; the procedures are presented generically without operator-specific SOP, no cross-type comparison is made, and no fleet tail numbers appear.

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.