ΔP Faults — Residual Pressure, Negative ΔP & Profile Anomalies

This article covers the A330's differential-pressure (ΔP) abnormal conditions — residual pressure, an outflow valve not opening, a too-fast profile, an open safety valve, negative ΔP. These are not life-critical cabin-altitude failures (ata-21-24); they are anomalies of the ΔP channel itself — L2 cautions that, untreated, can escalate.

1. The four ΔP conditions

[!note]- The difference from EXCESS CAB ALT

EXCESS CAB ALT = the cabin is already "invisibly climbing" (cabin altitude > 9550 ft) → an L3 red threat to life. This article = the ΔP channel is abnormal (residual pressure / valve not opening / not closing / profile too fast) → L2 amber, not directly life-threatening but it can escalate: a ΔP anomaly → untreated → a cabin-altitude anomaly → EXCESS CAB ALT.

2. CAB PR EXCESS RESIDUAL PR — residual pressure on landing

This alert triggers when the differential pressure is still above 2.5 hPa (0.036 PSI) 12 s after the last engine shutdown. — FCOM PRO-ABN-CAB_PR

   PACK 1 ... OFF
   PACK 2 ... OFF
   CABIN CREW ... ALERT   (do not open the doors)

[!important]- 2.5 hPa = 0.036 PSI — a tiny but deliberate threshold

2.5 hPa ≈ one inch of water — a very small residual ΔP. Why so low: a cabin door has a large area; even a small ΔP across it makes a large force that can fling the door open and injure people on opening. So this is the safe-to-open threshold. The ECAM means "do not open the doors" — switch both PACKs OFF (stop the inflow so the cabin pressure stops rising), alert the cabin, and check the ΔP on the CAB PRESS SD. If the ΔP will not fall: MAN VALVE SEL BOTH + MAN V/S CTL FULL UP to force the outflow valves open. No door opens until ΔP = 0 — the same point the emergency-evacuation procedure stresses.

3. CAB PR FWD(AFT) OFV NOT OPEN — outflow valve not opening on the ground

This alert triggers on ground, when the outflow valve is not fully open (time delay 90 s). — FCOM PRO-ABN-CAB_PR

   CAB PR MODE SEL ... MAN
   MAN VALVE SEL ..... FWD (AFT)    ← the affected side, not BOTH
   MAN V/S CTL ....... FULL UP      ← force fully open (≈10 s to see a change)
   IF UNSUCCESSFUL: PACK 1 ... OFF; PACK 2 ... OFF
   STATUS: FWD (AFT) OFV CTL: MAN ONLY

[!tip]- MAN VALVE SEL FWD (AFT), not BOTH — be precise

Unlike the dual-CPC CAB PR SYS 1+2 FAULT, this affects only one outflow valve. The ECAM text says which side (FWD or AFT); select that on MAN VALVE SEL — do not select BOTH, or you lose the other side's automatic control. On the ground the valves should be fully open (releasing pressure to prevent pre-takeoff pressurisation); 90 s without full open = a real fault (a stuck stepper, a failed control circuit, a wrong feedback). If forcing it open fails → PACK 1+2 OFF (let the cabin leak to ambient). It is ground-only, so the PACK-off has no in-flight consequence — but a stuck ground outflow valve means the pressurisation system is not sound, so do not depart without maintenance.

4. CAB PR EXPECT HI CAB RATE — a predictive caution

This alert triggers when the time to reach ΔP = 0 is less than 1.5 min. — FCOM PRO-ABN-CAB_PR

   A/C V/S ... REDUCE
   (This line is not displayed during an emergency descent due to excessive cabin altitude.)

[!important]- "Time to ΔP = 0 < 1.5 min" is a prediction, not the current state

The CPC continuously computes "at the current cabin V/S + ΔP rate, how long until ΔP reaches 0". A prediction < 1.5 min means the cabin pressure is falling too fast → the cabin altitude is about to match ambient. Common causes: the descent profile out-pacing the cabin schedule (a steep ATC-cleared descent), the outflow valves wide open (manual FULL UP), or a cabin leak. Action: A/C V/S → REDUCE (slow the aircraft V/S) so the cabin can follow.

[!note]- Why it is suppressed during an emergency descent

An emergency descent is meant to be max rate — so "reduce V/S" would contradict it. The CPC intelligently suppresses this line during an EXCESS-CAB-ALT emergency descent, avoiding crew confusion. EXPECT HI CAB RATE is a "too aggressive in a normal descent" reminder, not an "emergency-descent limit".

5. CAB PR SAFETY VALVE OPEN — two opposite branches

This alert triggers when the safety valves are not fully closed. — FCOM PRO-ABN-CAB_PR

   IF DIFF PR ABV 8.7 PSI:                 ← overpressure branch
     CAB PR MODE SEL ... MAN; MAN VALVE SEL ... BOTH; MAN V/S CTL ... AS RQRD
     IF UNSUCCESSFUL: A/C FL ... REDUCE
   IF DIFF PR BELOW 0 PSI:                 ← negative-ΔP branch
     EXPECT HI CAB RATE; A/C V/S ... REDUCE

[!warning]- The same warning, two opposite procedures — read the ΔP first

Current ΔP	Meaning	Action
> 8.7 PSI	over-pressurised (near the ~8.85 PSI safety-valve relief)	MAN + open the outflow valves to relieve
< 0 PSI (negative)	the cabin is below ambient ("reverse pressurisation")	reduce the aircraft V/S to stop the cabin climbing

One ECAM warning, opposite handling by ΔP direction — the pilot must read the CAB PRESS SD ΔP first. This is one of the easiest ATA 21 procedures to get wrong.

The overpressure branch: ΔP > 8.7 PSI is near the safety valve's mechanical relief (~8.85 PSI, ata-21-13); the 8.7 PSI caution is slightly below it, giving the pilot lead time. Relieve manually — do not rely on the safety valve (a mechanical last resort, not a routine relief). If forcing fails → A/C FL REDUCE (lower altitude → higher ambient → smaller ΔP). (Caution: below FL 190 with cabin altitude < −2060 ft the SD display is lost — keep MAN V/S CTL UP, ata-21-23.)

6. Negative ΔP — what it is and why it matters

[!important]- Negative ΔP = the cabin is below ambient

Normal pressurisation: the cabin is above ambient → ΔP > 0. Negative ΔP = the cabin below ambient. It typically happens when the aircraft climbs fast (the ambient dropping faster than the cabin can follow) → briefly the cabin altitude exceeds the aircraft altitude → ΔP < 0. Why it matters: (1) the negative-relief valve should open (mechanically, at a more negative ΔP, admitting outside air); the ECAM warns at ΔP < 0, before the valve acts — alerting the pilot first; (2) the fuselage is designed for positive ΔP (cabin above ambient) — negative ΔP loads the structure in reverse; (3) the safety valve should not leak under negative ΔP — this warning may mean the negative pressure pulled a safety valve open, letting ambient air back-flow in. Action: A/C V/S REDUCE → give the cabin time → ΔP recovers.

[!note]- Where the A330 risks negative ΔP

A fast climb (aircraft V/S 4000 ft/min+ vs cabin ~500 ft/min); a CPC failure in manual mode (the pilot over-trims the outflow valve → the cabin too low); the early phase of an emergency descent (the cabin lagging the rapid altitude change). In normal AUTO the CPC should prevent it — its appearance means CPC trouble or an over-aggressive aircraft V/S.

7. Monitoring & the decision tree

[!tip]- The four CAB PRESS SD fields to scan

Field	Normal	Abnormal
CAB V/S	±400 ft/min	pulsing > 1800 ft/min → profile anomaly
ΔP	0–8.85 PSI green	amber ≤ −0.2 PSI / ≥ +8.85 PSI
CAB ALT	follows the FL schedule	red > 9550 ft (L3 EXCESS CAB ALT)
Outflow valve	per schedule	fully closed / open / erratic

Combinations: CAB V/S 1800+ pulsing + ΔP falling → a leak / a stuck-open safety valve / wide-open outflow valves; CAB V/S 1800+ + ΔP rising → too much pack flow / a stuck-closed outflow valve; CAB ALT > 9550 ft red → L3; ΔP < 0 + cabin climbing → negative-ΔP, check the aircraft V/S; ΔP > 8.7 sustained → overpressure, prepare a manual relief.

   SD CAB PRESS warning (amber/red) → what is ΔP?
     ΔP > 8.7 PSI     → SAFETY VALVE OPEN branch 1: MAN + BOTH + V/S CTL; if unsuccessful A/C FL REDUCE
     ΔP < 0 PSI       → SAFETY VALVE OPEN branch 2: A/C V/S REDUCE (negative ΔP)
     ΔP normal + CAB V/S 1800+ → EXPECT HI CAB RATE: A/C V/S REDUCE
     ΔP residual (landing)     → EXCESS RESIDUAL PR: PACK 1+2 OFF + alert the cabin
     outflow valve not open (ground 90 s+) → FWD/AFT OFV NOT OPEN: MAN + FWD/AFT + FULL UP

[!note]- ΔP amber vs CAB ALT red — the colour levels

ΔP amber (the −0.2 / +8.85 PSI boundaries) and CAB V/S 1800 ft/min pulsing are cautions — attend to them, but there is time. CAB ALT > 9550 ft red is L3 — an immediate emergency descent. That is why ΔP faults are not on the life-threat list — they are L2 "handle by procedure"; only escalation to a red CAB ALT raises the level.

Self-test

[!note]- Q1. 30 s after landing, CAB PR EXCESS RESIDUAL PR — let the cabin open the doors?

No. The procedure: PACK 1+2 OFF + alert the cabin. The 2.5 hPa (0.036 PSI) threshold looks tiny, but a large door under even a small ΔP makes a large force — opening can fling it and injure people. Wait for ΔP = 0; if it will not fall, MAN VALVE SEL BOTH + MAN V/S CTL FULL UP to force a release.

[!note]- Q2. In cruise, CAB PR SAFETY VALVE OPEN with the SD showing ΔP = −0.3 PSI — how do you handle it?

Branch 2 (ΔP < 0, negative): EXPECT HI CAB RATE prompt + A/C V/S REDUCE; check the aircraft V/S is not too high (an over-aggressive climb); reduce it so the cabin can follow. Do **not** go MAN VALVE SEL BOTH (that is the ΔP > 8.7 PSI branch). The two branches are opposite — read the ΔP first.

[!note]- Q3. Climbing to FL 350, CAB PR EXPECT HI CAB RATE — what does it mean?

The CPC predicts that, at the current rate, ΔP will reach 0 in < 1.5 min — commonly the aircraft climbing too fast (V/S 4000 ft/min+) for the cabin to follow, or the outflow valves wide open. Action: A/C V/S → REDUCE. It is a predictive caution — a trend anomaly, not a current ΔP anomaly.

[!note]- Q4. Before takeoff, CAB PR FWD OFV NOT OPEN — what does it mean, and do you depart?

On the ground the outflow valves should be fully open (releasing pressure to prevent pre-takeoff pressurisation); 90 s without full open = a physical fault. Handle: MODE SEL MAN → MAN VALVE SEL FWD (per the ECAM text) → MAN V/S CTL FULL UP → wait ~10 s. If unsuccessful: PACK 1+2 OFF + maintenance → do not depart (a stuck ground outflow valve means the pressurisation is not sound, a risk in cruise).

[!note]- Q5. CAB PR SAFETY VALVE OPEN + ΔP > 8.7 PSI; after a minute of manual control ΔP is still > 8.7 PSI — next step?

A/C FL → REDUCE (descend). With the outflow valve stuck closed (manual cannot move it), the only way to cut ΔP is to raise the ambient pressure — i.e. descend, so the external pressure rises and the relative ΔP shrinks. Caution: below FL 190 with cabin altitude < −2060 ft the SD display is lost → keep MAN V/S CTL UP to maintain the display.

Key takeaways

Common misconceptions

Scope — what this article covers and defers

References

A330 specifics per FCOM PRO-ABN-CAB_PR (the EXCESS RESIDUAL PR 2.5 hPa / 0.036 PSI 12 s trigger with PACK 1+2 OFF + cabin alert, the FWD(AFT) OFV NOT OPEN 90 s ground trigger with MAN VALVE SEL the affected side + FULL UP, the EXPECT HI CAB RATE < 1.5 min predictive trigger with A/C V/S REDUCE and its emergency-descent suppression, and the SAFETY VALVE OPEN not-fully-closed trigger with its ΔP > 8.7 PSI and ΔP < 0 PSI branches) and FCOM DSC-21-20 (the safety-valve relief near 8.85 PSI, the negative-relief valve admitting at a more negative ΔP, and the ECAM ΔP amber at −0.2 / +8.85 PSI / CAB ALT red > 9550 ft). The safe-to-open force reasoning, the negative-ΔP structural and back-flow analysis, and the decision tree 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.