Excess Cabin Altitude & Emergency Descent

This article covers the A330's most serious single ATA 21 warning — CAB PR EXCESS CAB ALT (cabin altitude > 9550 ft) — and the linked emergency-descent standard procedure. It is ATA 21's only L3 red warning — the crew has tens of seconds, directly tied to occupant survival.

1. The only L3 red warning

[!important]- EXCESS CAB ALT is one of ATA 21's four L3 red warnings

Of the 16 ATA 21 warnings, only four are L3 red (ata-21-20): EXCESS CAB ALT (here), SMOKE/FUMES, AVNCS SMOKE, CARGO SMOKE. L3 = red MASTER WARN + continuous repetitive chime (CRC) + an immediate threat to life. L1/L2 do not require immediate action; L3 = memory items executed at once.

2. The trigger thresholds

This alert triggers when: In climb or descent, the cabin altitude is above the higher of: 9 550 ft, or 1 000 ft above the airfield pressure altitude. In cruise, the cabin altitude is above 9 550 ft. — FCOM PRO-ABN-CAB_PR

[!warning]- The climb/descent vs cruise thresholds differ

Phase	Threshold
Climb / descent	max(9550 ft, airfield pressure altitude + 1000 ft)
Cruise	9550 ft fixed

Why: from a high-altitude airfield (e.g. ~12,000 ft), the cabin altitude soon exceeds 9550 ft after takeoff — that is normal (the ambient is that high) and should not warn. The "airfield + 1000 ft" is the tolerance window: airfield 12,000 ft → threshold max(9550, 13,000) = 13,000 ft; a sea-level airfield 100 ft → threshold max(9550, 1100) = 9550 ft. In cruise there is no +1000 ft tolerance — the cabin altitude is unrelated to any airfield, so 9550 ft is the absolute safety line.

3. What happens automatically

[!note]- The mask-drop threshold ≠ the EXCESS CAB ALT threshold

Passenger masks drop at 14,000 ft (an independent pressure switch, nothing to do with the CPC); the EXCESS CAB ALT warning is at 9550 ft (CPC-computed). The 4450 ft gap is the crew's reaction buffer: the 9550 ft warning sounds → the crew recognises + initiates the descent; the cabin altitude may still climb to 14,000 ft → the masks drop (protecting the passengers); the crew aims to be descending before 14,000 ft. The two are monitored independently — even with the CPC failed, the masks still drop (the independent switch).

4. The ECAM procedure & the FL 160 divide

   If above FL 100:  CREW OXY MASKS ... USE
   If below FL 160:  DESCENT ... INITIATE; CABIN CREW ... ADVISE   (normal descent)
   If above FL 160:  SIGNS ... ON; • EMER DESCENT (the memory items below)
   ENG START SEL ... IGN; ATC ... NOTIFY; EMER DESCENT ... ANNOUNCE (PA); ATC XPDR 7700 ... CONSIDER
   When descent established: check the OUTFLOW VALVES closed on the CAB PRESS SD;
     if not closed + ΔP positive → select the other CPC; if still not closing → MODE SEL MAN, adjust V/S CTL.
   MAX FL ... 100/MEA-MORA

[!important]- FL 160 is the watershed of EXCESS CAB ALT handling

If the CAB PR EXCESS CAB ALT triggers above FL 160, the flight crew should first apply the EMER DESCENT memory items, and then perform the ECAM procedure (the first steps of the ECAM procedure recall the memory items). If the CAB PR EXCESS CAB ALT triggers below FL 160, the flight crew should apply the ECAM procedure, and initiate a normal descent. — FCTM PR-AEP-CAB_PR

Above FL 160 → the EMER DESCENT memory items (speedbrake / IDLE / max speed) + SIGNS ON, because the cabin altitude could rise to 25,000+ ft → lethal hypoxia. Below FL 160 → the ECAM read-and-do + a normal descent, because even a full depressurisation below FL 160 leaves the cabin altitude ≈ 16,000 ft, physiologically survivable. The divide is the aircraft FL 160, not the cabin altitude or 9550 ft.

5. EMER DESCENT memory items

EMER DESCENT — CREW OXY MASKS ... USE; SIGNS ... ON; EMER DESCENT ... INITIATE; If A/THR not active: THR LEVERS ... IDLE; SPD BRK ... FULL; When descent established: SPEED ... MAX/APPROPRIATE; If structural damage suspected: MANEUVER WITH CARE; CONSIDER L/G EXTENSION; ENG START SEL ... IGN; ATC ... NOTIFY. — FCOM PRO-ABN-MISC

[!tip]- "Memory items" — recited from memory, in order

[MEM] is the highest priority in the ECAM/QRH design — recited without the QRH. Above FL 160, the pilot cannot reach for the QRH; they execute from memory. The order's logic: (1) CREW OXY MASKS USE — self-protection first (oxygen before operating); (2) SIGNS ON — cabin prep; (3) EMER DESCENT INITIATE — the AP/A-THR emergency-descent mode; (4) THR LEVERS IDLE (if A/THR inactive); (5) SPD BRK FULL — drag; (6) SPEED MAX/APPROPRIATE — max descent rate; (7) MANEUVER WITH CARE (if structural damage suspected); (8) CONSIDER L/G (drag / VLO/VLE limit); (9) ENG START SEL IGN — anti-flameout; (10) ATC NOTIFY. The first 6 in seconds, the last 4 within 30 s — the simulator standard rhythm.

6. Emergency-descent parameters

To initiate the emergency descent, the use of AP and A/THR is highly recommended. At high flight levels, the flight crew should extend the speed brakes while monitoring the VLS. This is in order to avoid the activation of the angle of attack protection, which may result in the retraction of the speed brakes and in AP disconnection. Note: When in IDLE thrust, high speed and with speed brake extended, the rate of descent is approximately 6 000 ft/min. To descend from FL 410 to FL 100, it takes approximately 5 min and 40 NM. — FCTM PR-AEP-MISC

[!warning]- At high altitude, extend the speedbrakes while watching VLS

At high altitude (FL 350+) the speed envelope is narrow — VLS (minimum manoeuvring speed) is not far below the current speed. Extending the speedbrakes → less lift → VLS rises; if VLS catches the current speed → the angle-of-attack protection activates → the speedbrakes auto-retract + the AP disconnects. So extend the speedbrakes gradually (not straight to FULL) while watching the PFD VLS; if VLS nears the current speed, stop adding speedbrake and let the IDLE thrust reduce speed first. This is the A330 high-altitude emergency-descent hidden trap.

[!tip]- ~6000 ft/min and a 5-minute descent — the pilot's mental clock

Max rate 6000 ft/min (not the 8000+ some airframes claim); FL 410 → FL 100 in ~5 min + 40 NM. Mental arithmetic: ~10 s per 1000 ft, ~7.5 s per NM. In the cabin, 5 minutes on masks, the temperature dropping fast toward the ambient (−55 °C).

7. Suspected structural damage — the anti-max-speed

If structural damage is suspected, use the flight controls with care and reduce speed as appropriate. The landing gear may be extended. In this case, speed must be reduced to VLO/VLE. If the flight crew suspects structural damage, apply both of the following: Set the SPEED/MACH pb to SPEED, to prevent an increase in the IAS, or to reduce the speed. This action minimizes the stress on aircraft structure. Carefully use the speed brakes, to avoid additional stress on aircraft structure. — FCOM PRO-ABN-CAB_PR + FCTM PR-AEP-MISC

[!warning]- Structural damage ≠ max speed

Normally an emergency descent is max-speed; with suspected structural damage it reverses — high speed + damage may worsen the break-up. Identify damage by a sudden loud bang, an abnormally high cabin-altitude rate (clearly faster than normal leakage), or abnormal airflow/vibration. Handle: SPEED/MACH pb → SPEED (not MACH — control the IAS, do not let it rise); use the speedbrakes carefully (too much adds structural stress); the landing gear may be extended for drag (at the VLO/VLE limit). Teaching line: first "keep the aircraft from breaking up", then "keep the occupants from hypoxia".

8. Cabin altitude > 14,000 ft — MASK MAN ON

If the cabin altitude goes above 14 000 ft, the flight crew must press the MASK MAN ON pb. When it is obvious that the cabin altitude will exceed 14 000 ft, the flight crew could press the MASK MAN ON pb, before the cabin altitude reaches 14 000 ft. — FCTM PR-AEP-MISC

The MASK MAN ON pb (overhead, ATA 35) releases the cabin masks manually, not relying on the automatic switch. If the cabin altitude is rising fast, press it before 14,000 ft to gain the passengers a few seconds.

[!note]- TCAS stays on TA/RA

The TCAS mode selector must remain on the TA/RA position. Avoidance of collision has the priority, even if it requires a temporary interruption of the descent maneuver. — FCTM PR-AEP-MISC

An emergency descent may pass through other aircraft's levels → TCAS must work. Do not switch it to TA-only / STBY — keep the RA auto-avoidance.

9. Operations — timeline & cleanup

Timeline (FL 350 cruise, EXCESS CAB ALT)

   T+0   ECAM CAB PR EXCESS CAB ALT (MASTER WARN + CRC); cabin altitude > 9550 ft
   T+5 s [MEMORY ITEMS] CREW OXY MASKS USE → both pilots; verify the interphone
   T+15 s SIGNS ON; EMER DESCENT INITIATE (AP mode); SPD BRK FULL (watch VLS, gradual)
   T+30 s SPEED MAX/APPROPRIATE; ENG START SEL IGN; ATC NOTIFY (PAN PAN / MAYDAY);
          EMER DESCENT (PA); XPDR 7700
   T+1 m  monitor: cabin altitude still climbing? V/S ~6000 ft/min? VLS near the speed?
   T+2 m  cabin altitude past 14,000 ft → MASK MAN ON (if not auto-released)
   T+4–5 m at FL 100 / MEA-MORA → reduce speed, retract speedbrakes, ATC + diversion

Cleanup after the descent

Subsequent to an emergency descent, once the oxygen masks are removed, the flight crew should perform all of the following: Close the oxygen stowage mask compartment. Press the PRESS TO RESET oxygen control slide, to deactivate the mask microphone, and to cut off the oxygen. Below FL 100, the flight crew should limit the rate of descent to approximately 1 000 ft/min, except during the approach phase. — FCTM PR-AEP-MISC

Close the oxygen stowage mask compartment; press PRESS TO RESET (deactivate the mask mic + cut off the oxygen); below FL 100 limit the descent rate to ~1000 ft/min (a high rate → cabin ear/sinus discomfort), except on approach.

Oxygen diluter

Note: To save oxygen, set the oxygen diluter selector to the N position. With the oxygen diluter selector set to 100 %, oxygen quantity may not be sufficient for the entire descent profile. — FCOM PRO-ABN-CAB_PR

[!important]- The diluter — N, not 100 %

N (NORMAL): a mix of cabin air + pure O₂, diluted on demand (saving O₂). 100 %: pure O₂ (high consumption). EMER: high-pressure O₂ (severe hypoxia). In an emergency descent, N is the default — preventing hypoxia while saving O₂ so it lasts the whole descent to FL 100. Not 100 %, because the portable O₂ supply at 100 % output is limited (~25 min) vs ~60–90 min at N (enough for the whole descent + reserve).

Checking the outflow valves — the root cause

When descent is established, and if time permits, check that the OUTFLOW VALVES are closed on the CAB PRESS SD page. If they are not closed and ΔP is positive, select the other CPC. If the OUTFLOW VALVE is still not closing, set the cabin pressure MODE SEL pb to MAN and adjust the V/S CTL sw. — FCOM PRO-ABN-CAB_PR

If the valves are closed but the cabin altitude still climbs → there is a leak (an unseen hole) and the descent is the only remedy. If the valves are not closed + ΔP positive → a CPC fault (commanded closed but unresponsive) → select the other CPC or go manual.

Self-test

[!note]- Q1. At a ~12,000 ft airfield, climbing through a 12,500 ft cabin altitude — does EXCESS CAB ALT trigger?

No. In climb/descent the threshold is max(9550 ft, airfield pressure altitude + 1000 ft). Airfield 12,000 ft → threshold max(9550, 13,000) = 13,000 ft; 12,500 ft has not reached it. In cruise the threshold is a fixed 9550 ft (airfield-independent).

[!note]- Q2. EXCESS CAB ALT triggers at FL 200 cruise — do you do the EMER DESCENT memory items?

No. The FCTM states: below FL 160 → the ECAM read-and-do + a normal descent, no memory items. Below FL 160 even a full depressurisation is not lethal (~16,000 ft worst case). The divide is the aircraft FL 160, not the cabin altitude or 9550 ft.

[!note]- Q3. EXCESS CAB ALT at FL 380 + suspected structural damage — descend at max speed?

No. With suspected structural damage: SPEED/MACH pb → SPEED (not MACH — prevent the IAS rising); use the speedbrakes carefully (too much adds stress); reduce the descent rate (consider the landing gear for drag at VLO/VLE). Counter-intuitive — usually it is max-speed, but structural damage reverses it. First keep the aircraft from breaking up, then the occupants from hypoxia.

[!note]- Q4. During the descent the cabin altitude reaches 13,800 ft — press MASK MAN ON?

Press it early. The FCTM: above 14,000 ft you must press MASK MAN ON; if it will obviously exceed 14,000 ft, you may press it earlier. 13,800 ft and still climbing → press MASK MAN ON now, gaining the passengers a few seconds rather than waiting for the 14,000 ft auto-release (also a few seconds delayed).

[!note]- Q5. Stable at FL 100 after the descent — what after removing the masks?

Three things + one ongoing: (1) close the oxygen stowage mask compartment; (2) press PRESS TO RESET (deactivate the mask mic + cut off the O₂); (3) check the outflow valves closed + ΔP = ambient; ongoing: below FL 100 limit the descent rate to ~1000 ft/min (except on approach).

Key takeaways

Common misconceptions

Scope — what this article covers and defers

References

A330 specifics per FCOM PRO-ABN-CAB_PR (the EXCESS CAB ALT trigger — climb/descent max(9550, airfield + 1000 ft), cruise 9550 ft — the full ECAM procedure, the structural-damage SPEED reduction, the oxygen-diluter N, and the outflow-valve root-cause check), FCOM PRO-ABN-MISC (the EMER DESCENT [MEM] memory items), and FCTM PR-AEP-CAB_PR / PR-AEP-MISC (the FL 160 memory-items-vs-read-and-do divide, the ~6000 ft/min and FL 410→100 in ~5 min/40 NM figures, the speedbrake/VLS angle-of-attack-protection caution, the SPEED-not-MACH structural-damage handling, the 14,000 ft MASK MAN ON, the TA/RA TCAS, the post-descent cleanup, and the below-FL-100 1000 ft/min limit). The mask-drop 9550-vs-14000 buffer reasoning and the "first the aircraft, then the occupants" teaching line are integrative syntheses. All engineering detail is from the A330 knowledge base; the procedures and ATC calls are presented generically without operator-specific SOP or callsign, 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.