Learn how aircraft cabin pressurisation uses bleed air, packs and outflow valves, how cabin altitude is controlled, and what happens in a failure.
Aircraft cabin pressurisation works by continuously feeding conditioned air into the fuselage and controlling how quickly air escapes through an outflow valve. By restricting that outflow as the aircraft climbs, the system maintains a lower cabin altitude than the aircraft’s actual altitude while keeping pressure differential within the airframe’s structural limit.
The cabin is not sealed like a closed container. Fresh air enters continuously, used air leaves continuously, and the pressurisation controller balances the two flows.
What supplies air for cabin pressurisation?
Most turbine aircraft obtain pressurisation air from one or more engine-compressor stages, known as bleed air. This is compressed air taken before combustion, not exhaust gas. It is hot and at high pressure, so air-conditioning packs cool and regulate it before it reaches the cabin.
An auxiliary power unit can usually supply bleed air on the ground and during certain abnormal situations. Some newer aircraft instead use electrically driven compressors. Recirculated cabin air may be filtered and mixed with fresh air, but recirculation alone cannot maintain pressure without a fresh-air supply.
Our explanation of tracing engine, APU and pack airflow on the A320 BLEED page shows how these sources appear in a detailed simulator.
How does the outflow valve control cabin pressure?
The outflow valve controls cabin pressure by regulating how much air is allowed to escape from the pressure vessel. Closing it raises or maintains cabin pressure; opening it lets cabin pressure fall towards the outside ambient pressure.
- Before take-off: many systems slightly pre-pressurise the cabin to prevent an abrupt pressure change during rotation and initial climb.
- During climb: the controller progressively restricts outflow, allowing cabin altitude to climb much more slowly than aircraft altitude.
- At cruise: the valve modulates to hold the scheduled cabin altitude or the permitted pressure differential.
- During descent: the controller lowers cabin altitude towards the destination elevation, then equalises the cabin with outside pressure around landing.
Modern controllers use ambient pressure, cabin pressure, aircraft altitude, climb or descent rate and destination elevation. Older or simpler systems may require the crew to enter the landing altitude manually. Some aircraft have more than one outflow valve, but the principle is unchanged. For a cockpit example, see how A320 outflow-valve position and manual control are represented in a simulator.
What do cabin altitude and pressure differential mean?
Cabin altitude expresses the pressure inside the aircraft as an equivalent altitude in the standard atmosphere; it is not the aircraft’s physical altitude.
| Term | Meaning |
|---|---|
| Cabin altitude | The atmospheric altitude corresponding to the measured cabin pressure. |
| Cabin rate | How quickly cabin altitude is climbing or descending, independent of the aircraft’s vertical speed. |
| Pressure differential | Cabin pressure minus outside pressure, commonly shown as ΔP. |
| Maximum differential | The aircraft-specific structural or operating limit the system must not exceed. |
Holding sea-level pressure at a high cruise altitude would place excessive stress on the fuselage. Conventional airliners therefore commonly have a cabin altitude equivalent to roughly 6,000–8,000 feet near high-altitude cruise, although the target and maximum differential vary by aircraft.
What protects an aircraft from overpressure?
Dedicated relief valves protect the pressure vessel if the normal controller or outflow valve cannot keep differential pressure within limits.
- Positive-pressure relief valves open if cabin pressure becomes too high relative to outside pressure.
- Negative-pressure relief valves admit outside air if external pressure becomes greater than cabin pressure, such as during an unusually rapid descent.
- Dump or manual controls allow depressurisation when required, although their design and labels differ between aircraft.
Pressurised aircraft also use reinforced structures, pressure seals and securely locked doors. Many passenger doors are plug-type designs held against their frames by cabin pressure; other doors rely on mechanical locking systems and interlocks.
Are all aircraft cabins pressurised?
No; pressurisation is normally fitted when an aircraft’s intended operating altitude justifies the added structural weight, complexity and maintenance.
Airliners, business jets and some turboprops and piston aircraft are pressurised. Most training and utility aeroplanes are not, so occupants may need supplemental oxygen at higher altitudes. Our comparison of Cessna aircraft categories illustrates why pressurisation appears in cabin-class designs but not in many trainers and utility models.
What happens if cabin pressurisation fails?
A pressurisation failure allows cabin altitude to rise, creating a hypoxia risk even when the aircraft remains structurally sound and controllable.
A slow loss may first appear as a rising cabin altitude, an outflow valve driven towards closed, or a cabin-altitude warning. A rapid decompression can produce a loud noise, mist from condensed moisture, falling temperature and loose objects moving towards the leak. The term explosive decompression describes an extremely rapid pressure loss; it does not imply combustion.
| Condition | Typical effect |
|---|---|
| Bleed source, compressor or pack unavailable | Insufficient inflow to maintain the commanded cabin pressure. |
| Outflow valve open or dump selected | Air escapes too quickly and cabin altitude follows aircraft altitude. |
| Door seal or fuselage leak | The controller closes the outflow valve to compensate, but may eventually run out of authority. |
| Outflow valve stuck closed | Differential pressure rises until a relief valve opens. |
| Controller, sensor or landing-altitude error | Incorrect scheduling, pressure bumps or a cabin-altitude warning. |
At a type-specific cabin altitude, warnings activate and passenger oxygen masks may deploy automatically. Their oxygen supply is intended to protect occupants while the pilots use oxygen and descend to a breathable altitude; it is not a replacement pressurisation system. Flight crews follow the aircraft’s checklist because isolation steps and safe altitudes differ by type.
Why does cabin altitude keep climbing in a simulator?
In a detailed flight simulator, a climbing cabin altitude usually indicates missing airflow, an open outflow path or an incorrectly configured pressurisation controller.
- Verify the air source: confirm the required engine bleed, APU bleed or electric compressor is available for that aircraft.
- Check the packs: ensure the required air-conditioning packs are operating and no failure has removed their airflow.
- Inspect outflow controls: use automatic mode unless the aircraft procedure calls for manual control, and confirm that dump is not selected.
- Check destination elevation: enter the correct landing altitude on aircraft that require manual input; other types obtain it automatically from the flight-management system.
- Read the valve indication: a nearly closed valve with rising cabin altitude points towards insufficient inflow or a leak. An open valve despite adequate airflow suggests a mode, command or controller problem.
Control names and system depth vary between simulator aircraft. A simplified model may calculate cabin pressure in the background, while a study-level add-on may reproduce individual valves, controllers and failure states.
Why do ears pop in a pressurised aircraft?
Ears pop because cabin pressure still changes during climb and descent, even though it changes more slowly than outside pressure.
The Eustachian tubes equalise pressure across the eardrum, producing the familiar popping sensation. Swallowing or yawning often helps, and descent is commonly more noticeable because cabin pressure is increasing. Ear discomfort by itself does not indicate a pressurisation failure; warnings and abnormal cabin-altitude indications are the meaningful technical signs.