Cabin pressurization
Cabin pressurization is a process in which conditioned air is pumped into the cabin of an aircraft or spacecraft in order to create a safe and comfortable environment for humans flying at high altitudes. For aircraft, this air is usually bled off from the gas turbine engines at the compressor stage, and for spacecraft, it is carried in high-pressure, often cryogenic, tanks. The air is cooled, humidified, and mixed with recirculated air by one or more environmental control systems before it is distributed to the cabin.
The first experimental pressurization systems saw use during the 1920s and 1930s. In the 1940s, the first commercial aircraft with a pressurized cabin entered service. The practice would become widespread a decade later, particularly with the introduction of the British de Havilland Comet jetliner in 1949. However, two catastrophic failures in 1954 temporarily grounded the Comet worldwide. These failures were investigated and found to be caused by a combination of progressive metal fatigue and aircraft skin stresses caused from pressurization. Improved testing involved multiple full-scale pressurization cycle tests of the entire fuselage in a water tank, and the key engineering principles learned were applied to the design of subsequent jet airliners.
Certain aircraft have unusual pressurization needs. For example, the supersonic airliner Concorde had a particularly high pressure differential due to flying at unusually high altitude: up to while maintaining a cabin altitude of. This increased airframe weight and saw the use of smaller cabin windows intended to slow the decompression rate if a depressurization event occurred.
The Aloha Airlines Flight 243 incident in 1988, involving a Boeing 737-200 that suffered catastrophic cabin failure mid-flight, was primarily caused by the aircraft's continued operation despite having accumulated more than twice the number of flight cycles that the airframe was designed to endure.
For increased passenger comfort, several modern airliners, such as the Boeing 787 Dreamliner and the Airbus A350 XWB, feature reduced operating cabin altitudes as well as greater humidity levels; the use of composite airframes has aided the adoption of such comfort-maximizing practices.
Need for cabin pressurization
Pressurization becomes increasingly necessary at altitudes above above sea level to protect crew and passengers from the risk of a number of physiological problems caused by the low outside air pressure above that altitude. For private aircraft operating in the US, crew members are required to use oxygen masks if the cabin altitude stays above for more than 30 minutes, or if the cabin altitude reaches at any time. At altitudes above, passengers are required to be provided, but are not required to use, oxygen masks as well. On commercial aircraft, the cabin altitude must be maintained at or less. Pressurization of the cargo hold is also required to prevent damage to pressure-sensitive goods that might leak, expand, burst or be crushed on re-pressurization.The principal physiological problems are listed below.
; Hypoxia
; Altitude sickness
; Decompression sickness
; Barotrauma
Cabin altitude
The pressure inside the cabin is technically referred to as the equivalent effective cabin altitude or more commonly as the cabin altitude. This is defined as the equivalent altitude above mean sea level having the same atmospheric pressure according to a standard atmospheric model such as the International Standard Atmosphere. Thus a cabin altitude of zero would have the pressure found at mean sea level, which is taken to be.Aircraft
In airliners, cabin altitude during flight is kept above sea level in order to reduce stress on the pressurized part of the fuselage; this stress is proportional to the difference in pressure inside and outside the cabin. In a typical commercial passenger flight, the cabin altitude is programmed to rise gradually from the altitude of the airport of origin to a regulatory maximum of. This cabin altitude is maintained while the aircraft is cruising at its maximum altitude and then reduced gradually during descent until the cabin pressure matches the ambient air pressure at the destination.Keeping the cabin altitude below generally prevents significant hypoxia, altitude sickness, decompression sickness, and barotrauma. Federal Aviation Administration regulations in the U.S. mandate that under normal operating conditions, the cabin altitude may not exceed this limit at the maximum operating altitude of the aircraft. This mandatory maximum cabin altitude does not eliminate all physiological problems; passengers with conditions such as pneumothorax are advised not to fly until fully healed, and people suffering from a cold or other infection may still experience pain in the ears and sinuses. The rate of change of cabin altitude strongly affects comfort as humans are sensitive to pressure changes in the inner ear and sinuses and this has to be managed carefully. Scuba divers flying within the "no fly" period after a dive are at risk of decompression sickness because the accumulated nitrogen in their bodies can form bubbles when exposed to reduced cabin pressure.
The cabin altitude of the Boeing 767 is typically about when cruising at. This is typical for older jet airliners. A design goal for many, but not all, newer aircraft is to provide a lower cabin altitude than older designs. This can be beneficial for passenger comfort. For example, the Bombardier Global Express business jet can provide a cabin altitude of when cruising at. The Emivest SJ30 business jet can provide a sea-level cabin altitude when cruising at. One study of eight flights in Airbus A380 aircraft found a median cabin pressure altitude of, and 65 flights in Boeing 747-400 aircraft found a median cabin pressure altitude of.
Before 1996, approximately 6,000 large commercial transport airplanes were assigned a type certificate to fly up to without having to meet high-altitude special conditions. In 1996, the FAA adopted Amendment 25–87, which imposed additional high-altitude cabin pressure specifications for new-type aircraft designs. Aircraft certified to operate above "must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of after any probable failure condition in the pressurization system". In the event of a decompression that results from "any failure condition not shown to be extremely improbable", the plane must be designed such that occupants will not be exposed to a cabin altitude exceeding for more than 2 minutes, nor to an altitude exceeding at any time. In practice, that new Federal Aviation Regulations amendment imposes an operational ceiling of on the majority of newly designed commercial aircraft. Aircraft manufacturers can apply for a relaxation of this rule if the circumstances warrant it. In 2004, Airbus acquired an FAA exemption to allow the cabin altitude of the A380 to reach in the event of a decompression incident and to exceed for one minute. This allows the A380 to operate at a higher altitude than other newly designed civilian aircraft.
Spacecraft
Russian engineers used an air-like nitrogen/oxygen mixture, kept at a cabin altitude near zero at all times, in their 1961 Vostok, 1964 Voskhod, and 1967 to present Soyuz spacecraft. This requires a heavier space vehicle design, because the spacecraft cabin structure must withstand the stress of 14.7 pounds per square inch against the vacuum of space, and also because an inert nitrogen mass must be carried. Care must also be taken to avoid decompression sickness when cosmonauts perform extravehicular activity, as current soft space suits are pressurized with pure oxygen at relatively low pressure in order to provide reasonable flexibility.By contrast, the United States used a pure oxygen atmosphere for its 1961 Mercury, 1965 Gemini, and 1967 Apollo spacecraft, mainly in order to avoid decompression sickness. Mercury used a cabin altitude of ; Gemini used an altitude of ; and Apollo used in space. This allowed for a lighter space vehicle design. This is possible because at 100% oxygen, enough oxygen gets to the bloodstream to allow astronauts to operate normally. Before launch, the pressure was kept at slightly higher than sea level at a constant above ambient for Gemini, and above sea level at launch for Apollo), and transitioned to the space cabin altitude during ascent. However, the high pressure pure oxygen atmosphere before launch proved to be a factor in a fatal fire hazard in Apollo, contributing to the deaths of the entire crew of Apollo 1 during a 1967 ground test. After this, NASA revised its procedure to use a nitrogen/oxygen mix at zero cabin altitude at launch, but kept the low-pressure pure oxygen atmosphere at in space.
After the Apollo program, the United States used "a 74-percent oxygen and 26-percent nitrogen breathing mixture" at for Skylab, and a cabin atmosphere of for the Space Shuttle orbiter and the International Space Station.
Mechanics
An airtight fuselage is pressurized using a source of compressed air and controlled by an environmental control system. The most common source of compressed air for pressurization is bleed air from the compressor stage of a gas turbine engine; from a low or intermediate stage or an additional high stage, the exact stage depending on engine type. By the time the cold outside air has reached the bleed air valves, it has been heated to around. The control and selection of high or low bleed sources is fully automatic and is governed by the needs of various pneumatic systems at various stages of flight. Piston-engine aircraft require an additional compressor, see diagram right.The part of the bleed air that is directed to the ECS is then expanded to bring it to cabin pressure, which cools it. A final, suitable temperature is then achieved by adding back heat from the hot compressed air via a heat exchanger and air cycle machine known as a PAC system. In some larger airliners, hot trim air can be added downstream of air-conditioned air coming from the packs if it is needed to warm a section of the cabin that is colder than others.
At least two engines provide compressed bleed air for all the plane's pneumatic systems, to provide full redundancy. Compressed air is also obtained from the auxiliary power unit, if fitted, in the event of an emergency and for cabin air supply on the ground before the main engines are started. Most modern commercial aircraft today have fully redundant, duplicated electronic controllers for maintaining pressurization along with a manual back-up control system.
All exhaust air is dumped to atmosphere via an outflow valve, usually at the rear of the fuselage. This valve controls the cabin pressure and also acts as a safety relief valve, in addition to other safety relief valves. If the automatic pressure controllers fail, the pilot can manually control the cabin pressure valve, according to the backup emergency procedure checklist. The automatic controller normally maintains the proper cabin pressure altitude by constantly adjusting the outflow valve position so that the cabin altitude is as low as practical without exceeding the maximum pressure differential limit on the fuselage. The pressure differential varies between aircraft types, typical values are between and. At, the cabin pressure would be automatically maintained at about,, which is about of atmosphere pressure.
Some aircraft, such as the Boeing 787 Dreamliner, have re-introduced electric compressors previously used on piston-engined airliners to provide pressurization. The use of electric compressors increases the electrical generation load on the engines and introduces a number of stages of energy transfer; therefore, it is unclear whether this increases the overall efficiency of the aircraft air handling system. They do, however, remove the danger of chemical contamination of the cabin, simplify engine design, avert the need to run high pressure pipework around the aircraft, and provide greater design flexibility.