Automatic Pressurisation Failure — CPC & Manual Mode

This article covers the cabin-pressure-controller (CPC) failures — the spectrum of pressurisation degrading from AUTO mode. More frequent than dual pack failure but less severe per event.

1. The three pressurisation conditions

The other CAB PR conditions (EXCESS CAB ALT, ΔP faults) are ata-21-24 and ata-21-25. CPC architecture is ata-21-11.

2. CAB PR SYS 1(2) FAULT — one CPC failed

This alert triggers when one cabin pressure controller is failed. Crew awareness. — FCOM PRO-ABN-CAB_PR

[!note]- Why no action — the CPC redundancy

One CPC fails, the other takes over automatically. The pilot sees no change in pressurisation — cabin altitude / V/S / ΔP / outflow-valve position all normal. The ECAM only says "the system is now on the backup"; if the other CPC then fails too → it escalates to CAB PR SYS 1+2 FAULT. (The CPCs swap active/standby after each landing, so across consecutive sectors CAB PR SYS 1 FAULT need not be the same CPC.) Teaching line: a single-CPC failure is a "no-feel" fault — the ECAM is for trend judgement.

[!tip]- A hidden function — the pilot can swap the active CPC

Switching the MODE SEL pb to MAN, for at least 3 s, then returning it to AUTO will select the other system. — FCOM DSC-21-20-40

If CPC 1 is active but the pilot suspects it (e.g. the cabin V/S persistently off target), a brief MAN (≥ 3 s) then back to AUTO swaps to CPC 2 — a troubleshooting action, not an ECAM step. It works only while the system is normal; after a dual-CPC failure the system is already in MAN and pressing MODE SEL has no swap effect.

3. CAB PR SYS 1+2 FAULT — both CPCs failed

This alert triggers when both cabin pressure controllers are failed. — FCOM PRO-ABN-CAB_PR

   (If a door not locked closed on the ground: CAB PR INHIB BY DOORS)
   Due to the slow closure of the outflow valve in manual pressurization mode and
   depending on the failure, the following procedure may not avoid a depressurization.
   CAB PR MODE SEL ............... MAN     ← manual control
   MAN VALVE SEL ................. BOTH    ← control both outflow valves
   MAN V/S CTL ................... AS RQRD ← select V/S per the altitude
   Monitor cabin V/S and cabin altitude frequently and adjust as necessary.
   It may take 10 s in manual mode before the crew notices an outflow-valve change.
   The two safety valves limit ΔP to 8.85 PSI.

   TGT V/S: CLIMB 500 FT/MIN / DES 300 FT/MIN
   A/C FL   CAB ALT TGT
   410      8 000
   350      6 500
   300      5 000
   250      2 500
   < 200    0
   DURING FINAL APPR: MAN V/S CTL ... FULL UP (when ΔP = 0, open both valves)

[!warning]- The FCOM admits the procedure may not avoid a depressurisation

Due to the slow closure of the outflow valve in manual pressurization mode and depending on the failure, the following procedure may not avoid a depressurization. — FCOM PRO-ABN-CAB_PR

Most procedures say "do this and it is solved". This one says "it may not". Why: in MAN mode the outflow valve closes slowly (the pilot drives the stepper via the V/S CTL toggle, not the CPC at full speed), and the failure may involve more (a failed valve, a cabin leak). Teaching line: a dual-CPC failure does not guarantee pressurisation — be ready to descend as well.

[!warning]- Three more counter-intuitive points in MAN mode

It may take 10 s in manual mode before the crew notices a change of the outflow valve position. The pilot may notice variations (up to ± 1 000 ft) in the CAB ALT indication ... when the system switches from ... AUTO mode to MAN mode, due to reduced resolution of the backup pressure sensor. — FCOM PRO-ABN-CAB_PR + DSC-21-20-40

(1) ~10 s response delay — from the toggle to a visible SD position change. Do not repeat-press (each press = one step ~5°; repeats accumulate into a sudden large movement). (2) CAB ALT drifts ±1000 ft on AUTO→MAN — the backup sensor's reduced resolution, not a depressurisation. Tell them apart by the cabin V/S: still within range → a display issue; jumped to 1800 ft/min+ → a real depressurisation (and you can hear/feel it). (3) Below FL 190 with cabin altitude < −2060 ft, the CAB ALT / ΔP may not display in MAN mode (the backup sensor's low-end limit) → keep MAN V/S CTL UP to raise the cabin altitude and recover the display.

[!note]- The target-V/S table and the asymmetric 500/300 ft/min

The table maps the aircraft FL to a cabin-altitude target (FL 410 → 8000 ft, near the 9550 ft caution; < FL 200 → 0 ft, cabin = sea level = ground). The climb target (500 ft/min) exceeds the descent target (300 ft/min) because a cabin descent is more uncomfortable (ear/sinus) than a climb — so the descent rate is set lower. (Normal AUTO is ±400 ft/min, ata-21-11.)

[!important]- Final approach — release ΔP to 0 before opening the doors

On final approach (below airfield pressure altitude + 2500 ft): adjust ΔP = 0; when ΔP = 0, select MAN V/S CTL FULL UP to fully open both outflow valves; check ΔP is zero before opening the doors. If ΔP > 0 at door opening, the rushing air can injure people / damage a door. AUTO handles this; in MAN the pilot must. The biggest MAN-mode landing risk is opening a door before releasing ΔP — sequence: ΔP = 0 → taxi → park → doors.

INOP SYS: CAB PR 1+2.

4. CAB PR LDG ELEV FAULT — landing-elevation input lost

This alert triggers when the LDG ELEV selector is set to AUTO and the landing field elevation of the FMGS is not available. — FCOM PRO-ABN-CAB_PR

   LDG ELEV ... ADJUST    ← set the landing elevation manually with the LDG ELEV selector
                            (refer to the LDG ELEV on the CRUISE / CAB PRESS page)
   Note: for a QFE landing, set "0" ft on the LDG ELEV selector
   If the LDG ELEV selector is inoperative: LDG ELEV SET AT ZERO

[!note]- The landing-elevation chain and the failure

Normally the CPC builds the descent profile from the FMGS landing elevation (cabin altitude = 0 at landing). On the fault, the FMGS data is unavailable → the CPC does not know the landing elevation → the descent tracks imprecisely. The pilot turns the LDG ELEV selector (−2000…14,000 ft) to the destination elevation, shown on the LDG ELEV indication (CRUISE / CAB PRESS page). QFE landing: set 0.

[!warning]- "MAN LDG ELEV" memo goes amber in flight phases 1 and 2

MAN LDG ELEV: This memo appears in green, if the LDG ELEV selector is not in the AUTO position. This memo becomes amber in flight phases 1 and 2. — FCOM DSC-21-20-40

Before takeoff (phase 1/2) the pilot should check whether LDG ELEV is AUTO or manual — manual + before-takeoff → the amber prompts "is this deliberate?" (a previous crew may have left it manual). On a pre-flight check, an amber MAN LDG ELEV → check why it is manual.

5. Operations

MAN-mode manual control of cabin V/S

1. ECAM CAB PR SYS 1+2 FAULT.
2. CAB PR MODE SEL → MAN (≥ 3 s to truly switch; < 3 s back to AUTO swaps the other system).
3. MAN VALVE SEL → BOTH.
4. MAN V/S CTL → AS RQRD: read the current A/C FL → find the CAB ALT TGT in the table → if the cabin altitude differs, toggle UP (cabin climbs) / DN (cabin descends), keeping the cabin V/S within 500 ft/min climb / 300 ft/min descent.
5. Monitor (cabin V/S, cabin altitude, ΔP) about once a minute; correct disturbances (climbing too fast → DN; descending too fast → UP).

Before landing in MAN mode

Follow the A/C FL down, tracking the CAB ALT TGT (FL 350 → 6500; FL 250 → 2500; below FL 200 → 0). On final approach (below airfield pressure altitude + 2500 ft): adjust ΔP = 0 → MAN V/S CTL FULL UP. After landing: confirm ΔP = 0 on the SD → taxi → park → doors.

Combinations

[!note]- The CAB PRESS SD active-system field

SYS 1 / SYS 2 green = that CPC active; amber = that CPC faulty; MAN = MODE SEL in MAN (with VALVE SEL BOTH). Check it on every cockpit scan — which system, AUTO or MAN.

Self-test

[!note]- Q1. Does CAB PR SYS 1 FAULT require an action?

No — "Crew awareness". One CPC failed, the other takes over automatically; no change in pressurisation (cabin altitude / V/S / ΔP / outflow valve all normal). The ECAM only signals the backup is in use — monitor for escalation to a dual failure.

[!note]- Q2. After switching to MAN, CAB ALT jumps 800 ft — a depressurisation?

Probably not. The FCOM states CAB ALT may drift ±1000 ft on AUTO→MAN (the backup sensor's reduced resolution). Tell them apart: the cabin V/S still within ±300 ft/min → a display issue; jumped to 1800 ft/min+ → a real depressurisation (and you can hear/feel it). Do not start an emergency descent before judging.

[!note]- Q3. In MAN mode, you toggle V/S CTL DN, then 5 s later the SD outflow-valve position has not changed — what do you do?

Wait ~10 s. In MAN mode an outflow-valve position change takes ~10 s to show on the SD (stepper + feedback delay). Do not repeat-press — accumulated commands produce a sudden large movement. Be patient.

[!note]- Q4. In MAN mode at FL 350, the cabin altitude reads 5800 ft — normal?

A bit low. The table gives FL 350 → CAB ALT TGT 6500 ft. 5800 is 700 ft below the target — the cabin is over-pressurised (a larger ΔP). Toggle MAN V/S CTL UP briefly to let the cabin climb slowly to 6500 ft (within 500 ft/min).

[!note]- Q5. Dual-CPC failure + MAN-mode landing — can you open the doors immediately on landing?

No. In MAN mode the pilot must release ΔP: on final approach (below airfield pressure altitude + 2500 ft) adjust ΔP = 0; when ΔP = 0, MAN V/S CTL → FULL UP; check ΔP is zero before opening the doors. ΔP > 0 at door opening → rushing air can injure / damage a door. Release ΔP on final approach and re-confirm ΔP = 0 after landing before any door opens.

Key takeaways

Common misconceptions

Scope — what this article covers and defers

References

A330 specifics per FCOM PRO-ABN-CAB_PR (the CAB PR SYS 1(2) FAULT "one CPC failed" with crew awareness, the CAB PR SYS 1+2 FAULT "both CPCs failed" with the full MAN-mode procedure — the "may not avoid a depressurization" caveat, MODE SEL MAN / VALVE SEL BOTH / V/S CTL AS RQRD, the ~10 s outflow-valve delay, the safety valves limiting ΔP to 8.85 PSI, the target-V/S table with CLIMB 500 / DES 300 ft/min, and the final-approach ΔP = 0 / FULL UP with the door caution; and the CAB PR LDG ELEV FAULT with LDG ELEV ADJUST and the QFE-zero / selector-inoperative cases) and FCOM DSC-21-20-40 (the MODE SEL ≥ 3 s system-swap, the ±1000 ft CAB ALT drift on AUTO→MAN from the backup sensor's reduced resolution, the < −2060 ft display loss below FL 190, and the MAN LDG ELEV memo amber in flight phases 1/2). The "CPC like an autopilot" teaching analogy and the climb/descent asymmetry rationale 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.