Cabin pressurisation explained: cabin altitude, outflow valves, pressure controllers, relief valves, failure signs and simulator fixes.
Aircraft cabin pressurisation works by supplying conditioned compressed air to the pressure vessel and using an outflow valve to meter air overboard. The cabin pressure controller schedules that valve so cabin altitude changes gradually, stays well below aircraft altitude, and remains within the airframe’s permitted pressure-differential limit.
For our Aviation & Real-World Flying readers and flight simmers, the key point is that the cabin is not sealed once and left alone. Air enters continuously, used air leaves continuously, and the cabin pressurisation system controls the balance.
What does a pressurised cabin mean?
A pressurised cabin is an occupied pressure vessel maintained at a higher absolute pressure than the surrounding atmosphere when the aircraft is at altitude.
It does not necessarily maintain sea-level pressure, nor does pressurisation mean filling the cabin with pure oxygen. The oxygen proportion remains broadly the same as in outside air, but compressing that air raises its pressure and therefore the oxygen partial pressure available to occupants.
The pressure vessel normally includes the cockpit, passenger cabin and selected cargo spaces. Seals, windows, doors, structural joints, valves and system penetrations must all withstand repeated pressure cycles. Small controlled leakage is expected; the incoming airflow replaces it.
What supplies air for cabin pressurisation?
Most turbine aircraft obtain pressurisation air from an engine-compressor stage or an auxiliary power unit, while some designs use electrically driven compressors.
Engine bleed air is taken from the compressor before combustion, so it is not exhaust gas. It arrives hot and at high pressure; the air-conditioning packs cool, regulate and distribute it before it enters the pressure vessel. An APU can usually provide air on the ground and, on aircraft certified for it, within a defined in-flight operating envelope.
Pressurised piston aircraft may use air from a turbocharger or a dedicated compressor. Bleedless transport designs use electrically powered compressors instead, but still control cabin pressure by balancing inflow against outflow. Our explanation of how engine and APU bleed-air sources are produced and selected covers the supply side in more detail.
Filtered recirculated air can be mixed with fresh conditioned air to improve circulation and reduce the conditioning load. Recirculation does not add air mass to the pressure vessel, however, so it cannot replace the fresh-air supply needed to offset air discharged overboard and through normal leakage.
How does the outflow valve in an aircraft control cabin pressure?
The aircraft outflow valve controls cabin pressure by continuously varying how much air can escape from the pressure vessel.
Closing the valve reduces outflow, allowing continued inflow to raise cabin pressure relative to the atmosphere or slow its reduction during a climb. Opening the valve releases more air, reducing the pressure differential. Most valves modulate through a range of positions rather than operating as a simple open-or-closed device.
An outflow valve is commonly installed towards the rear of the pressure vessel, although its position and construction are type-specific. Some aircraft use more than one. Closing every outflow valve does not make the cabin perfectly airtight because seals, drains and other permitted leakage paths remain.
What does a cabin pressure controller or regulator do?
A cabin pressure controller calculates the required cabin-pressure schedule and commands the outflow valve to follow it.
Electronic systems can use cabin pressure, ambient pressure, aircraft altitude, vertical trend, weight-on-wheels status, flight phase and destination elevation. Many transport aircraft have redundant automatic controller channels, with manual outflow-valve control available after an automatic-system failure.
The term cabin pressure regulator is often used for the same general function. On older aircraft it may describe a pneumatic unit that senses cabin and ambient pressure and directly regulates an outflow valve. It should not be confused with a bleed-air pressure regulator or pack flow controller, which manages the incoming air rather than cabin pressure itself.
Valve position alone does not prove that the controller is working. A nearly closed valve with a rising cabin altitude can mean the controller is demanding maximum pressurisation but lacks enough incoming air or is unable to overcome a leak.
How does the pressure schedule change through a flight?
A normal pressure schedule changes cabin pressure gradually through each phase of flight while respecting the maximum differential-pressure limit.
- On the ground: the outflow valve is normally open enough to keep the cabin equalised with local atmospheric pressure.
- Before take-off: many systems close the valve slightly and pre-pressurise the cabin, reducing the pressure change felt during rotation and initial climb.
- During climb: the controller restricts outflow so cabin altitude rises much more slowly than aircraft altitude.
- At cruise: the valve modulates to maintain the scheduled cabin altitude without exceeding the permitted pressure differential.
- During descent: cabin altitude decreases towards the destination elevation as cabin pressure rises. The controller must also prevent outside pressure from becoming greater than cabin pressure.
- After landing: the system equalises the cabin with ambient pressure and opens the outflow path as required for ground operation and door opening.
Some aircraft require the crew to enter landing elevation manually. Others obtain it from the flight-management system or destination data. An incorrect value can cause an early or late cabin descent, pressure bumps near landing or failure to equalise correctly.
What does cabin altitude mean?
Cabin altitude is the standard-atmosphere altitude corresponding to the pressure measured inside the cabin, not the aircraft’s physical height above the ground.
| Term | Meaning |
|---|---|
| Cabin altitude | The equivalent atmospheric altitude represented by the cabin’s absolute pressure. A higher cabin altitude means lower pressure. |
| Cabin rate | The speed at which cabin altitude is increasing or decreasing, usually shown in feet per minute. |
| Ambient pressure | The atmospheric pressure outside the aircraft at its actual altitude. |
| Pressure differential | Cabin absolute pressure minus outside absolute pressure, commonly displayed as ΔP and often measured in psi. |
| Maximum differential | The aircraft-specific structural or operating limit that the controller and relief valves must protect. |
An aircraft at 37,000 feet might have a cabin altitude of roughly 6,000–8,000 feet, depending on its design, loading and pressure schedule. That does not mean the aeroplane has descended; it means the pressure inside resembles the standard atmospheric pressure found at that lower altitude. Our worked explanation of how aircraft altitude, cabin altitude and differential pressure relate at FL370 provides the cruise context.
Maintaining sea-level pressure at high cruise altitude would demand a much greater pressure differential and a heavier pressure vessel. Designers instead choose a cabin-altitude schedule that balances occupant comfort, structural loading, aircraft weight and air-system capability.
What does a positive-pressure relief valve protect?
A positive-pressure relief valve protects the airframe by opening independently when cabin pressure becomes too high relative to outside pressure.
- Positive-pressure relief valves discharge cabin air if differential pressure reaches the valve’s aircraft-specific limit.
- Negative-pressure relief valves admit outside air when ambient pressure becomes greater than cabin pressure, preventing the fuselage from being loaded in the opposite direction.
- Dump or depressurisation controls deliberately open an outflow path when commanded by the crew or another aircraft system.
The positive-pressure relief valve is a safety device, not the normal cabin pressure regulator. Under routine conditions, the controller and outflow valve keep differential pressure below the relief threshold. A stuck-closed outflow valve or faulty controller can cause the relief valve to open and close repeatedly as pressure reaches its limit.
Valve architecture varies. An aircraft may have separate positive and negative relief valves, combined safety valves, or negative-relief functions incorporated into an outflow-valve assembly.
What happens if cabin pressurisation fails?
A cabin pressurisation failure allows cabin altitude to rise and can expose occupants to hypoxia even while the aircraft remains structurally intact and controllable.
A slow loss may first appear as an abnormal cabin rate, increasing cabin altitude, an outflow valve driven towards closed or a cabin-altitude warning. Rapid decompression can produce a loud report, mist from condensing moisture, a sudden temperature drop and airflow towards the opening. Explosive decompression means an extremely rapid pressure loss; it does not imply fire or combustion.
| Indication | Likely cause | What it means |
|---|---|---|
| Cabin altitude rising too quickly with the outflow valve nearly closed | Insufficient pack or compressor airflow, or a pressure-vessel leak | The controller is trying to retain air but has run out of authority. |
| Cabin altitude rising with the outflow valve commanded open | Manual mode, dump selection, incorrect schedule or controller fault | Air is being discharged faster than required. |
| Differential pressure at its upper limit | Outflow valve stuck closed or incorrect controller command | The positive-pressure relief system may operate to protect the fuselage. |
| Pressure bumps during descent or landing | Wrong landing elevation, sensor error or late equalisation | The cabin schedule is not matching the destination pressure correctly. |
| Outflow valve fully open after a door or seal problem | This is not the expected compensating response | Check the displayed convention and controller mode; a controller normally closes the valve to offset a leak. |
A single pack or bleed-source failure does not automatically mean immediate depressurisation; many aircraft can remain pressurised with reduced capacity under type-specific restrictions. Loss of all usable inflow, a major leak or an outflow valve stuck open is more serious.
At aircraft-specific thresholds, the crew receives warnings and passenger oxygen masks may deploy. Oxygen protects occupants temporarily while pilots use their own oxygen equipment, run the approved checklist and descend when necessary; it does not restore pressurisation. We cover the operational sequence separately in our guide to what crews and passengers face during cabin decompression.
Why does cabin altitude keep climbing in a flight simulator?
In a detailed flight simulator, an unexpectedly climbing cabin altitude normally points to missing airflow, an open outflow path, a controller configuration error or an activated failure.
- Confirm that the behaviour is abnormal: cabin altitude should normally rise during an aircraft climb, just at a lower rate. Diagnose a problem when it rises too quickly, exceeds the normal schedule or triggers a warning.
- Verify the air source: check that the required engine bleeds, APU bleed or electric compressors are available. Engine start alone does not guarantee that pressurisation air has been selected.
- Check the packs: make sure enough air-conditioning packs are operating for the aircraft and flight phase. One pack may be sufficient on some types, while another aircraft or operating condition may require more.
- Review the APU-to-engine transition: a mistake we see constantly is switching off the APU and its bleed source before establishing the normal engine-bleed and pack configuration.
- Select the correct controller mode: use automatic control unless the aircraft procedure calls for manual operation. Verify that a dump, depressurise or manual-open command has not been left selected.
- Set landing elevation where required: enter the destination’s airfield elevation on aircraft with manual landing-altitude selection. Do not add cruise altitude or pressure altitude in that field.
- Compare all indications: read aircraft altitude, cabin altitude, cabin rate, differential pressure and outflow-valve position together. A closed valve with worsening cabin altitude points towards weak inflow or a leak; an unnecessarily open valve points towards its command or control mode.
- Check simulated failures and aircraft state: an unlatched door, saved failure, maintenance state or incomplete turnaround configuration may persist between flights in a complex add-on.
Do not assume every valve indication uses the same convention. One cockpit may display percentage open, another may show actuator travel, and a simplified aircraft may provide only a schematic. Read the indication label before treating zero as open or closed.
Large time acceleration, slewing or teleporting can also leave an add-on’s pressure schedule out of step with aircraft altitude. Return to normal simulation rate and allow the system to stabilise before deciding that the model has failed. Some simplified aircraft do not model individual packs, controllers or relief valves at all, so their switches may be cosmetic or their cabin pressure may be calculated in the background.
Are all aircraft cabins pressurised?
No; pressurisation is fitted when the intended altitude and mission justify the stronger structure, seals, compressors, controls and maintenance it requires.
Airliners, business jets and some turboprop and piston aircraft are pressurised. Most light trainers and utility aeroplanes are not. Occupants of an unpressurised aircraft experience essentially the same atmospheric pressure as outside and may require supplemental oxygen at altitude; the applicable considerations are explained in our guide to oxygen use in unpressurised aircraft.
Why do ears pop in a pressurised aircraft?
Ears pop because cabin pressure still changes during climb and descent, even though the pressurisation system makes that change slower than the pressure change outside.
The Eustachian tubes equalise pressure across the eardrum, causing the familiar popping sensation. Swallowing or yawning often helps, and descent is commonly more noticeable because cabin pressure is increasing. Ear popping by itself is not evidence of a pressurisation failure; abnormal cabin indications, warnings and oxygen-mask deployment are the meaningful system signs.