Pressurisation Principles — Cabin Altitude Schedule & Differential Pressure

System Overview listed the pressurisation hardware (2 CPCs, 1 RPCU, 2 outflow valves × 3 motors, 3 safety valves, 1 negative relief valve); Pack Principles covered the conditioning that feeds it. This article is the physics of pressurisation itself: what the pressure vessel is, how the cabin-altitude schedule works, what maximum differential pressure is and why it caps operating altitude, the five automatic modes across a flight, and the pilot's controls.

Failure handling is deliberately out of scope here — CPC loss, manual mode, excess cabin altitude, and ΔP exceedances are in the abnormal articles (ata-21-22 … ata-21-25).

1. The pressure vessel

[!info] Generic turbine-aircraft basis

The pressure-vessel and cabin-altitude-vs-aircraft-altitude relationships in §1–§2 are the generic turbine-aircraft basis. The A330's specific numbers (max ΔP, prepressurisation rate, cabin-V/S limits) are given with FCOM quotes in §4 onward.

Air is pumped into an aircraft's "pressure vessel" in order to reproduce the atmospheric pressures found at lower altitudes... The pressure vessel does not occupy the entire fuselage but rather uses pressure bulkheads, plus the outer skin, to contain the passenger cabin and some or all cargo areas. Control cables, wiring, and plumbing must pass through the pressure vessel, with further perforation by exits, windows, and emergency exits. To make matters worse, the aircraft fuselage changes dimensionally with every pressurization cycle. Obviously, sophisticated engineering and maintenance is required for such an aircraft. — ASA Turbine Pilot's Flight Manual, Ch. 5

Three points for the A330:

• The pressure vessel is not the whole fuselage — it is the volume enclosed by the pressure bulkheads and the outer skin, containing the cabin and the main hold (21-40, pressurised, suitable for live animals); some maintenance and equipment bays are outside it.
• Every pressurisation cycle is one fatigue cycle — the fuselage flexes slightly with each cycle, so the airframe carries a cycle-count life limit.
• Every perforation is a potential leak path — windows, doors, exits all need dedicated sealing; a breach triggers a rapid depressurisation.

On the A330, two outflow valves control the release rate, three safety valves protect against overpressure, and one negative relief valve protects against negative pressure (see System Overview §5).

2. Cabin altitude vs aircraft altitude — the core mental model

The aircraft is at FL 370, but the cabin altitude is not 37,000 ft — pressurisation holds it down to ~6,000 ft so the occupants breathe oxygen-rich air. That is the whole value of the system.

   altitude (×1000 ft)
        │
        │   aircraft altitude ──────────────────────
        │                              ╱
        │                             ╱
        │                            ╱   cabin altitude stays far
        │                           ╱    below aircraft altitude
        │                          ╱     (the system holds it down)
        │   cabin altitude ───────╱
        │
        └──────────────────────────────────► time (the climb)

   1. At FL 370, cabin altitude is typically ~6,000–8,000 ft (FAR limit 10,000 ft).
   2. As the aircraft climbs, the cabin climbs too — much more slowly: the "schedule".
   3. ΔP (cabin pressure − outside pressure) grows as the aircraft climbs.
   4. At the max-ΔP ceiling, the cabin must climb further to follow the aircraft,
      or ΔP would exceed the structural limit.

For many pressurized aircraft, certified maximum operating altitude is determined not by the airplane's service ceiling but by the ability of the pressurization to meet supplemental oxygen requirements of the FARs. Maximum operating altitudes, in such cases, are defined by the greatest altitude the aircraft can attain and still maintain legal cabin altitudes (10,000 feet under 14 CFR Parts 135 and 121, and 12,500 feet for Part 91). — ASA Turbine Pilot's Flight Manual, Ch. 5

• Maximum operating altitude ≠ service ceiling — the pressurisation capability is the binding constraint.
• Air-transport rules (e.g. 14 CFR Part 121) cap cabin altitude at 10,000 ft; in normal cruise the A330 sits at 6,000–8,000 ft.
• The A330's EXCESS CAB ALT warning is red at cabin altitude > 9,550 ft — 450 ft below the 10,000 ft regulatory cap, giving an early alert (see Excess Cabin Altitude).

3. Inputs and outputs — pressurisation is "metered release", not "pumping up"

   PFCV → pack → mixing unit
        (steady inflow)        ┌─────────────────────────────┐
            ───────────────────►│   PRESSURE VESSEL           │
                                │   (cabin + holds)           │
                                │                             │
                                │   steady inflow in ↑        │
                                │   metered release out ↓     │
                                │     via 2 outflow valves    │
                                │     (6 motors, CPC sets rate)│
                                │                             │
                                │   mechanical backups:       │
                                │     3 × safety valve (over) │
                                │     1 × negative relief     │
                                └──────────────┬──────────────┘
                                               ▲
            pilot controls: LDG ELEV selector · MODE SEL (AUTO/MAN)
                          · MAN V/S CTL (manual only) · DITCHING pb

The core relationship:

Cabin pressure = f(inflow, release rate). The system does not "pump the cabin up" — it modulates the release.

The inflow (PFCV → pack → mixing unit) is steady (set by the PACK FLOW detent); the release rate (outflow valves) is the variable the CPC controls. This is the physical root of the "outflow valve is a release valve, not a switch" point in System Overview §7.

4. The cabin-altitude schedule — how the A330 computes cabin altitude

The controller normally uses the landing elevation and the QNH from the FMGEC, and the pressure altitude from the ADIRS. If FMGEC data are unavailable, the controller uses the Captain BARO Reference from the ADIRS and the LDG ELEV selection. — FCOM DSC-21-20-30

For the pilot:

• FMGEC data all valid → fully automatic schedule — nothing to set.
• FMGEC lost → the CPC falls back to the LDG ELEV selector + ADIRS baro — so the landing-elevation knob must be set correctly.
• No cruise FL → the CPC has no target cabin altitude — which is why the cruise FL must be entered in the FMS for the schedule to be correct.

5. Maximum differential pressure + the limiter

[!info] Generic basis + A330 numbers

Max ΔP is a generic concept; the A330's figures (8.70 PSI limiter / 8.85 PSI amber) come from the FCOM.

The main measure of a pressurization system's efficiency is known as its maximum differential (or max diff). This is simply the maximum ratio of cabin pressure to outside air pressure that the pressurization system and vessel can sustain. Max diff varies significantly by aircraft type. This is due to many factors, including pressure vessel design, engine bleed air capacity, and aircraft weight and power considerations. — ASA Turbine Pilot's Flight Manual, Ch. 5

The A330's limiter:

This function is only available in automatic mode. If the differential pressure is above 8.70 PSI, the CPC maintains the Δp constant, to avoid over-pressurization. As a result, the outflow valves open and the CAB V/S increases. Once the differential pressure has decreased below the threshold, normal automatic control of the valves resumes; this generally causes the valves to go towards the closed position. — FCOM DSC-21-20-30

[!warning]- The limiter activates silently — almost no cockpit indication

Except for the outflow valve position indication and the Δp value on the ECAM PRESS page, there is no indication in the cockpit that the limiter function is activated. — FCOM DSC-21-20-30

Operationally: if the cabin starts climbing on its own (CAB V/S rising with no LDG ELEV input) and the outflow valves are noticeably open and ΔP is pinned near 8.7 PSI → the limiter is probably working (the airframe is near max ΔP).

[!warning]- CAUTION — never counteract the limiter in manual mode

Once the Δp limiter function has opened the valves, do not counteract the automatic operation by trying to close the valves in manual mode. Due to the slow movement of the outflow valves in manual mode, the valves cannot be closed fast enough and the cabin altitude quickly increases above 20 000 ft (even if an emergency descent is initiated simultaneously). The automatic mode provides the safest and the quickest way to reduce differential pressure, and recover normal pressure control. — FCOM DSC-21-20-30

This is one of the easiest wrong decisions to make: seeing the automatic system raise the cabin V/S and wanting to "take over" — which makes it worse. The fix for high ΔP is to descend so ΔP falls, not to close the valves.

6. The five pressurisation modes — one full flight

FCOM DSC-21-20-30 describes five automatic modes.

6.1 Ground (GND) — before takeoff and 80 s after landing

Before takeoff, and 80 s after landing, the system keeps the outflow valves fully open to ensure there is no residual pressure differential inside the aircraft. At touchdown, to release the remaining cabin overpressure, a depressurization sequence controls the cabin V/S at +500 ft/min. — FCOM DSC-21-20-30

• Outflow valves full open before takeoff — on the ground ΔP = 0, and it must stay there (a residual ΔP could jam a door or exit).
• Full open 80 s after landing — the 80 s allows the residual overpressure to bleed off.
• Touchdown cabin V/S = +500 ft/min — the residual overpressure is released gently (not dumped, to spare ears).

6.2 Takeoff (TO) — prepressurise to avoid a surge at rotation

To avoid a pressure surge at rotation, the controller prepressurizes the aircraft at a rate of -328 ft/min until the pressure differential reaches 0.1 PSI. At lift off, the controller initiates the climb phase. — FCOM DSC-21-20-30

At rotation the airflow around the fuselage changes abruptly and the local skin pressure dips; if the cabin were still open to ambient (ΔP ≈ 0), occupants would feel a brief surge. Prepressurising the cabin to ΔP = 0.1 PSI (≈ 200 ft equivalent cabin descent) before rotation removes that. Key figures: prepressurisation rate −328 ft/min (cabin descending), target ΔP 0.1 PSI.

6.3 Climb internal mode (CI) — the cabin follows, slowly

CAB V/S varies, according to a preprogrammed law, in order to reach the scheduled CAB ALT at the top of climb defined by the FMGES cruise FL. The CAB V/S is limited to 1 000 ft/min. — FCOM DSC-21-20-30

• The aircraft climbs at ~3,000 ft/min, but cabin V/S ≤ 1,000 ft/min — the cabin climbs more slowly.
• The 1,000 ft/min limit is a comfort limit — faster is uncomfortable for ears (children, the elderly, anyone with a cold are more sensitive).
• The scheduled cabin altitude is computed from the FMGES cruise FL — without a cruise FL in the FMS, the CPC has no target.
• At FL 370 the cabin sits ~6,000–7,000 ft; at FL 410 it sits ~8,000–9,000 ft (approaching the 9,550 ft red threshold).

6.4 Descent — internal (DI) and external (DE) modes

In descent the cabin V/S is limited to 750 ft/min, bringing cabin pressure to landing-field pressure + 0.1 PSI before touchdown. The mode splits into:

• Internal descent (DI) — optimises the rate so the cabin reaches landing-field pressure + 0.1 PSI before landing.
• External descent (DE) — the cabin altitude varies on the FMGES estimated time, reaching landing-field pressure + 0.1 PSI before landing.

Both leave the cabin at a slight positive pressure (landing-field + 0.1 PSI) at touchdown — symmetric with the takeoff 0.1 PSI prepressurisation, preventing a ΔP surge at touchdown and avoiding "catching the cabin" (§12). Detailed V/S laws are in CPC.

7. The three gauges

Pilots control pressurization by setting one or more variables in the pressurization controller. Keep in mind that there are two types of aircraft performance involved in pressurization: the airplane's climb/descent performance and that of the cabin. Pressurization systems of all types are monitored in the flight deck via cabin altitude, cabin rate of climb, and pressure differential indicators. — ASA Turbine Pilot's Flight Manual, Ch. 5

A330 ECAM CAB PRESS fields and colour coding:

On the CRUISE page: ΔP shows pulsing green at 1.5–8.85 PSI in final approach (flight phase 7); CAB V/S between −25 and +25 ft/min is treated as zero and not displayed. (Per FCOM DSC-21-20-40.)

8. Pilot controls — LDG ELEV / MODE SEL / V/S CTL

8.1 LDG ELEV selector

AUTO: The pressurization system uses the FMGEC data to construct an optimized pressure schedule. To leave the AUTO position, pull out and turn the selector. Other positions: The pressurization schedule does not use the landing elevation from the FMGEC, but instead uses the landing elevation selected with this knob (from -2 000 to 14 000 ft) as its reference. — FCOM DSC-21-20-40

• Almost all flights leave it in AUTO — the FMS destination elevation feeds the CPC automatically.
• Manual selection is for FMGEC failure or a missing/incorrect FMS destination.
• Range −2,000 to 14,000 ft covers nearly all airfields (only the very highest commercial airfields, around 14,000 ft, sit at the top of the range).
• LDG ELEV field AUTO green = FMGES data valid; AUTO amber = FMGES did not send landing elevation → the pilot must select manually.
• Leaving AUTO requires pull out + turn (a guarded action).

8.2 MODE SEL pb (AUTO / MAN)

AUTO: Automatic mode is operating. One of the two systems controls the outflow valves. MAN: This legend appears in white, and FAULT does not come on. The flight crew then uses the MAN V/S CTL selector to control the system. Note: Switching the MODE SEL pb to MAN, for at least 3 s, then returning it to AUTO will select the other system. FAULT lt: This legend appears in amber and the ECAM caution only comes on when both automatic systems are faulty. — FCOM DSC-21-20-40

[!warning]- Hidden function: force a swap of the active automatic system

MODE SEL → MAN for ≥ 3 s → back to AUTO = force a swap to the other CPC.

Use: the active CPC is behaving oddly (unsteady cabin V/S, ΔP wander, suspect landing-elevation computation) but ECAM has not flagged FAULT → manually trigger a swap to see if the other system does better. A tool that is documented but not prompted by ECAM (see Automatic Pressurisation Failure).

[!warning]- AUTO → MAN switching jumps the cabin altitude by up to ±1,000 ft

The pilot may notice variations (up to ± 1 000 ft) in the CAB ALT indication on the ECAM PRESS page, when the system switches from the cabin pressure control AUTO mode to MAN mode, due to reduced resolution of the backup pressure sensor. — FCOM DSC-21-20-40

Manual mode uses the backup pressure sensor, whose resolution is coarse — the displayed cabin altitude drops from AUTO's tens-of-feet precision to a ±1,000 ft order. This is why manual mode is not "precise manual control" — it is coarse control with the pilot watching cabin V/S and adjusting.

8.3 MAN V/S CTL switch

If both automatic systems fail, the flight crew may use the CABIN PRESS control panel to take over manual control of cabin pressurization: Press the MODE SEL pushbutton to select MAN, and Push the MAN V/S CTL toggle switch to the UP or DN position to increase or decrease cabin altitude. Depending on the VALVE SEL position, the flight crew manually controls both, or only one, outflow valve(s). If only one outflow valve is selected, the other one remains under automatic control. — FCOM DSC-21-20-30

• MAN V/S CTL is a spring-loaded toggle — UP/DN commands a change in cabin V/S.
• Slow response — outflow valve movement can lag up to 5 s on the SD page.
• Don't jab it — give the valve time (one nudge → wait 5–10 s → assess → again).

9. Outflow valves after landing

In AUTO: at touchdown the depressurisation sequence releases residual overpressure at cabin V/S +500 ft/min; 80 s after landing the valves go fully open (ground mode) and the active CPC swaps.

[!warning]- Counter-intuitive: in manual mode the outflow valves do not auto-open at touchdown (unless an RPCU is fitted)

As the pressurization system is manually controlled, in the case no RPCU is installed, the outflow valves do not automatically open at touchdown. If the RPCU is installed, the outflow valves open automatically. — FCOM DSC-21-20-30

The A330 is fitted with an RPCU (Residual Pressure Control Unit — see System Overview §5), so the valves open automatically even after a manual-mode landing (preventing a residual overpressure from jamming a door). But the pilot should still remember: manual mode + an (extremely unlikely) RPCU failure → the valves must be opened by hand after landing (MAN V/S CTL UP to raise cabin altitude and release pressure). See Outflow Valve.

The manual-mode display range: ΔP and CAB ALT show "XX" for cabin altitude below −2,060 ft or above 26,250 ft — the backup sensor's working range (per FCOM DSC-21-20-30).

10. DITCHING pb

To prepare for ditching, the flight crew must press the DITCHING pushbutton on the CABIN PRESS control panel to close the outflow valves, the emergency ram air inlet, the avionics ventilation overboard valve, and the pack flow control valves. — FCOM DSC-21-20-30

All four closed openings are below the aircraft's waterline — closing them keeps water out and keeps the aircraft afloat longer for rescue. Full detail in Ditching.

11. The pressurisation scan across a flight

12. Catching the cabin

[!warning]- Generic counter-intuitive point: "catching the cabin"

Negative pressure relief valves... come into play when the pilots forget, upon initiating descent, to set the controller for landing. Say that the airplane is cruising at 25,000 feet, with a cabin altitude of 4,000 feet... When properly set for descent, the controller gradually descends the cabin at a few hundred feet per minute, while the plane itself may be descending at several thousand feet per minute. However, if the controller is not reset for landing, the cabin will stay at 4,000 feet until the airplane reaches the matching 4,000-foot pressure altitude. From that point on, the negative pressure relief valves will vent the cabin... This situation, known as "catching the cabin", is not particularly serious, except that the cabin's descent rate now matches that of the airplane... (Descent rates of 3,000 fpm are common in turbine aircraft.) Catching the cabin, therefore, makes it difficult to keep both ATC and the passengers' ears happy. — ASA Turbine Pilot's Flight Manual, Ch. 5

How the A330 avoids it: FMGES + LDG ELEV in AUTO means the controller automatically builds the descent schedule from the FMS destination — there is no manual "set the controller for landing" step. The only risk paths are: (1) LDG ELEV in a manual position with the wrong value; (2) the FMS destination missing or wrong; (3) the destination changed but the manual LDG ELEV knob not updated. The A330's negative relief valve is the mechanical backstop — even if catching the cabin occurs, it opens to prevent a negative ΔP (see Safety / Negative-Relief Valves).

Why an A330 pilot still needs this: it is the reason to re-check LDG ELEV before every descent — AUTO green with a sensible value (near the destination elevation). It is the descent scan item in Typical Day Operations.

Self-test

[!note]- Q1. How many pressurisation modes does the A330 have, and what are the key numbers?

Five: ground (GND), takeoff (TO, prepressurise), climb internal (CI), descent internal (DI), descent external (DE). Key numbers (FCOM DSC-21-20-30/40): outflow full open 80 s before takeoff and after landing; touchdown release at +500 ft/min; TO prepressurise at −328 ft/min to ΔP 0.1 PSI; CI cabin V/S ≤ 1,000 ft/min; descent cabin V/S ≤ 750 ft/min to landing-field pressure + 0.1 PSI; limiter at ΔP > 8.70 PSI; ECAM amber at ΔP ≤ −0.2 or ≥ 8.85 PSI; CAB ALT red > 9,550 ft; CAB V/S pulsing > 1,800 ft/min; manual-mode "XX" below −2,060 or above 26,250 ft; AUTO→MAN cabin-altitude jump up to ±1,000 ft.

[!note]- Q2. Why must the pilot NOT counteract the max-ΔP limiter in manual mode?

Per FCOM DSC-21-20-30: in manual mode the outflow valves move slowly (display lag up to 5 s); trying to close them to stop ΔP rising cannot close them fast enough, and the cabin altitude rapidly climbs above 20,000 ft (even with a simultaneous emergency descent). The automatic mode is the safest and fastest way to reduce ΔP. Operationally: cabin V/S rising on its own + outflow valves open + ΔP pinned near 8.7 PSI = the limiter working = the airframe is near max ΔP = descend to reduce ΔP; do not close the valves. This is why the FCOM uses a CAUTION here, not a note.

[!note]- Q3. What does the LDG ELEV selector use in AUTO vs manual, what is the range, and when must it be set manually?

AUTO uses FMGEC data (the FMS destination elevation) to build the optimised schedule. Manual uses the knob value (−2,000 to 14,000 ft). Set manually when FMGEC fails, or the FMS destination is missing/wrong, or the destination was changed and the FMS not yet updated. Leaving AUTO is a pull-out-and-turn action (guarded). The range covers nearly all commercial airfields. Source: FCOM DSC-21-20-40.

[!note]- Q4. The active CPC seems off but ECAM has not flagged FAULT. Can you force a swap to the other CPC?

Yes — a documented but un-prompted function. Per FCOM DSC-21-20-40: MODE SEL → MAN held ≥ 3 s → back to AUTO forces a swap to the other CPC. Use it when the active CPC behaves abnormally (unsteady cabin V/S, suspect landing-elevation computation, unsmooth ΔP control) without an ECAM FAULT, to see whether the other system is steadier. Detail in Automatic Pressurisation Failure.

[!note]- Q5. What is "catching the cabin", how does the A330 avoid it, and why must an A330 pilot still know it?

Catching the cabin (generic): the controller is not reset for landing before descent, so the cabin stays at its cruise altitude until the aircraft descends to the matching pressure altitude; from there the negative relief valve vents the cabin and the cabin descends at the aircraft's rate (3,000 fpm typical) — uncomfortable for ears. The A330 avoids it because FMGES + LDG ELEV AUTO build the descent schedule automatically (no manual "set for landing" step). An A330 pilot still needs it: it is the reason to re-check LDG ELEV before every descent (AUTO green, sensible value), and it explains the negative relief valve's role. Source: Turbine text Ch. 5 + FCOM.

Key takeaways

Common misconceptions

Scope — what this primer covers and defers

References

A330 specifics per FCOM DSC-21-20-10 (pressurisation general — hardware and the "any one of three motors" redundancy, used in System Overview §5), DSC-21-20-30 (automatic control modes — the five modes with their rates, the 8.70 PSI maximum-differential limiter and its caution against manual counteraction, manual control, the manual-mode display range, and the DITCHING pb), and DSC-21-20-40 (LDG ELEV selector, MODE SEL with the active-system-swap note and the ±1,000 ft AUTO→MAN jump, ECAM CAB PRESS and CRUISE field colours). All A330 figures verified verbatim against the English knowledge base. Generic pressurisation physics — the pressure vessel, max-diff-defines-ceiling, the three gauges, and the catching-the-cabin behaviour of the negative relief valve — per ASA Turbine Pilot's Flight Manual, Ch. 5. The prepressurisation aerodynamic rationale, the comfort basis for the 1,000 ft/min limit, and the descent-symmetry reasoning are integrative syntheses.

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.