Cabin Pressure in an Aircraft: Definition, Value, Difference, Mechanism

The air passengers breathe inside a jet is not the thin, frigid mixture found at high altitudes. Instead, conditioned air is pumped into the fuselage in a process called cabin pressurization. This process creates a safe and comfortable setting by maintaining an equivalent altitude - termed cabin altitude - that is far lower than the aircraft's true height. Because human physiology demands denser air than the outside atmosphere can provide, the internal pressure is expressed as an equivalent altitude above sea level, typically near 8,000 ft even when the airplane itself cruises far higher.

Expert behind this article

Jim Goodrich

Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.

What is cabin pressure in an aircraft?

Cabin pressure is the conditioned air pumped into the cabin of an aircraft. The aircraft will maintain the cabin pressure at a constant level throughout the entire flight. The pressure of the cabin falls between 6,000-8,000 feet in cruise flight at high altitudes.

Cabin pressure refers to the controlled level of air pressure maintained within the aircraft's cabin. It is regulated atmospheric pressure expressed as equivalent altitude above sea level. By the International Standard Atmosphere model, cabin pressure is defined as the equivalent altitude above mean sea level having the same atmospheric pressure, so an 8,000 ft (2,438.4 m) cabin altitude feels to occupants like standing on an 8,000 ft (2,438.4 m) mountain while the aircraft flies much higher.

Cabin pressure is created when air comes into the cabin quicker than it is released, producing a high-pressure setting inside the sealed pressure hull. This constant pressure differential is automatically limited by pressure relief valves to a predetermined value, assuring the pressure hull is placed under slightly less stress and protecting passengers and crew from rapid pressure changes.

Why is the cabin of an airplane pressurized?

The cabin of an airplane is pressurized to maintain breathable oxygen levels. High altitudes have low outside air pressure; at 40,000 feet (12,192 meters) time of useful consciousness is just a few seconds without pressurization. Cabin pressurization ensures passengers, as well as crew members, receive an adequate amount of oxygen, creating a breathable setting that protects crew and passengers from physiological problems like hypoxia, altitude sickness, decompression sickness and barotrauma. Pressurization becomes increasingly necessary at altitudes above 10,000 ft (3,048 m), and it is a legal requirement. The aircraft's cabin pressurization system helps create necessary pressure by pumping conditioned air into the cabin. The fuselage of an airplane is pressurized so that crew and passengers can breathe comfortably without supplemental oxygen.

What pressure is an airplane cabin?

The cabin pressure is kept at a lower barometric height than the aircraft's flight level. At 39,000 ft (11,887 m) the cabin pressure is about 790 hPa (11.5 psi), the automatic pressurization system providing a cabin altitude of roughly 6,900 ft (2,103 m).

Commercial aircraft must legally maintain a cabin altitude of 8,000 ft (2,438.4 m) or lower. Longevity and weight reasons set the usual working range between 6,000 ft (1,828.8 m) and 8,000 ft (2,438.4 m). Typical pressure differential values are between 540 hPa (7.8 psi) and 650 hPa (9.4 psi).

Inside this interval, The 787's internal cabin pressure is the equivalent of 6,000 ft (1,828.8 m) altitude. At this setting, cabin pressure at 6000 ft (1828.8 m) equals 81.2 kPa (11.78 psi), giving a higher pressure than the 75 kPa (10.88 psi) found at 8000 ft (2438.4 m). When cabin altitude zero pressure is 101.325 kPa, normal mean sea-level figure is 100 kPa.

Converted to psi, pressure inside the cabin at 6000 ft (1828.8 m) is about 11.8 psi (81.36 kPa), while at 8000 ft (2438.4 m) gives 10.9 psi (75.15 kPa). Barometric data at selected elevations read: 0 ft (0 m) = 100 kPa (14.5 psi), 4000 ft (1219.2 m) = 87.5 kPa (12.7 psi), 6000 ft (1828.8 m) = 81.2 kPa (11.8 psi), 8000 ft (2438.4 m) = 75 kPa (10.9 psi).

What is cabin altitude in aircraft?

Cabin altitude refers to the simulated altitude experienced inside the aircraft cabin, and it is normally maintained at 8,000 ft or less. Cabin altitude is the pressure altitude experienced inside the pressurised fuselage. The environmental control system keeps this cabin altitude lower than the aircraft's flight altitude, thereby guaranteeing passenger comfort. A cruise mode maximum of 8,000 ft (2,438.4 m) is common; at 8,000 ft (2,438.4 m) supplemental oxygen is not required, but discomfort like ear pressure and mild dyspnoea still appears.

The Boeing 787 gives a maximum cabin altitude of 6,000 ft (1,829 m), while the Gulfstream G700 offers 2,840 ft (866 m) at 41,000 ft (12,497 m), both values well below the 8,000 ft (2,438 m) threshold. If cabin altitude reaches 14,000 ft (4,267.2 m), crew members are required to use oxygen masks. Warning systems are incorporated to alert the crew of excessive cabin altitude and of any loss of pressurisation so that prompt corrective action can be taken.

What is the difference between cabin altitude and aircraft altitude?

The difference between cabin altitude and aircraft altitude is that the aircraft's true altitude - the geometric height above sea level - climbs rapidly whereas the cabin altitude changes more slowly, lagging far behind. Engineers limit this internal altitude to about 8,000 ft (2,438.4 m) so that at 8,000 ft (2,438.4 m) supplemental oxygen is not required and structural loads stay within the oval fuselage's design margins.

How does airplane cabin pressure compare to sea level?

Cabin air pressure at cruising altitude is lower than air pressure at sea level. Mean sea level pressure is taken to be 101.325 kPa (14.696 psi, 29.921 inHg), while a typical airliner maintains an 8,000 ft (2 438.4 m) cabin altitude and at that level the partial pressure of oxygen is about 74 percent of what it is at sea level. At 30,000 ft (9144 m) atmospheric pressure drops to 3.8 psi (26.2 kPa) - 26 percent of sea-level pressure - so the quantity of oxygen in the air is 26 percent of that found at sea level.

Airliners don't try to duplicate the air pressure at sea level. Although bleed air generates sea-level pressure even at high altitude, full sea-level pressurization puts too much strain on the fuselage and accelerates its fatigue. The fuselage is already reinforced to manage stress over thousands of pressurization cycles. Crews maintain cabin pressure near sea level only during lower altitudes or during descent, while ambulance flights pressurize the cabin almost to sea level for medical reasons.

What is aircraft cabin differential pressure?

Aircraft cabin differential pressure is the difference between the air pressure inside of an airplane's cabin and the air pressure outside of the airplane. A differential pressure gauge is often combined with a cabin altimeter indicating cabin altitude. Pressure differential is controlled by differential control so it does not exceed the maximum for which the aircraft is designed. Typical values are between 540 hPa and 650 hPa, but the number varies between aircraft types.

A low aircraft cabin differential pressure places the pressure hull under less stress, so the fuselage of pressurized aircraft is placed under slightly less stress when the pressure differential is lower. A high aircraft cabin differential pressure means the larger the difference, the more severe the decompression and the longer it takes to lose pressure completely. Concorde had a high pressure differential because it flew at unusually high altitude.

How is an aircraft cabin pressurized?

Airplanes pressurize their cabins by pumping conditioned air into an airtight fuselage. Modern jetliners use bleed air tapped from the engines' compressor sections. The engines do not need all this air for combustion, so some is cooled in air-conditioning packs and then ducted into the sealed section called the pressure hull. The pressurization system works in combination with the air-conditioning system that continuously cycles air through the cabin: most airplanes completely exchange the air inside the cabin in three to five minutes, recirculating some while the rest is vented.

The outflow valve balances this inflow. Positioned at the rear fuselage, the valve opens and closes on a regular basis to release air at the same rate it enters, keeping the pressure differential within safe limits. If the valve closes fully, the cabin retains pressure. If internal pressure exceeds safe limits, a positive pressure release valve pops open to release excess air.

Cabin pressure is measured by pressure sensors installed in the cabin ducting. These sensors monitor differential pressure between the inside of the pressure hull and the ambient atmosphere, sending data to the environmental control system so that the outflow valve can be modulated automatically.

How is the cabin pressure of a pressurized aircraft usually controlled?

The cabin pressure of a pressurized aircraft is maintained by regulating the amount of air that flows out of the cabin. High-pressure air is bled off from the engines, then cooled by PACKs before it enters the cabin. While the fresh air from the engine must always enter, an equal amount of air must always leave the cabin to exhaust away harmful contaminants. The outflow valve controls the cabin pressure by releasing this air to the atmosphere. It opens, closes, or modulates to establish the amount of air pressure maintained in the cabin.

Although many aircraft are equipped with outflow valves operated by computers installed onboard the aircraft, computers control the cabin pressure regulator. The pressure regulator adjusts the outflow valves and sets the desired cabin altitudes, rate of cabin altitude change, and barometric pressure settings. The valve, generally a hole located around the tail area with a pair of sealable doors, constantly adjusts its position. If the engine-bleed air were allowed to flow unchecked, it would generate sea-level pressure. The automatic controller maintains the proper cabin pressure altitude by constantly adjusting the outflow valve position without exceeding the maximum pressure differential limit on the fuselage.

During flight, the automatic controller keeps the cabin pressure constant. Modern aircraft are designed to keep the cabin at an equivalent low-altitude pressure even when the airplane is flying at 30,000 feet (9,144 meters). Pilots have access to the mode controls of the cabin pressure control system, and if the automatic pressure controllers fail, the pilot can manually control the cabin pressure valve according to the backup emergency procedure checklist. The outflow valve acts as a safety relief valve to preclude overpressure situations.

What is the height of airplane cabins?

Cabin height ranges from about four feet in the Vision Jet to more than eight feet in the Challenger 604/605. Midsize jets like the Legacy 450 and Challenger 3500 give 6.0 feet (1.83 m) of standing room, while the Gulfstream 450/IV/SP and Global Express/XRS reach 7 ft 4 in (2.24 m) and 8 ft 2 in (2.49 m) respectively. Very-light jets stay near 4 ft 6 in (1.37 m): the Citation Mustang is 4 ft 6 in (1.37 m), the Phenom 100 is 4 ft 9 in (1.45 m), and the CJ3 is 4 ft 8 in (1.42 m). Large-cabin Falcons climb higher: the Falcon 900 offers 6 ft 3 in (1.91 m), the Falcon 2000 gives 6 ft 2 in (1.88 m), and the Falcon 7X tops out at 7 ft 8 in (2.34 m).