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 passengers and crew 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 if necessary, before it is distributed to the cabin by one or more environmental control systems. The cabin pressure is regulated by the outflow valve.
Need for cabin pressurization
Pressurization becomes increasingly necessary at altitudes above 12,500 feet (3,800 m) to 14,000 feet (4,300 m) 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. It also serves to generally increase passenger comfort and is a regulatory requirement above 15,000 feet (4,600 m) in the U.S.A. The principal physiological problems are listed below. 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 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 101,325 pascals (14.696 psi).
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 8,000 ft (2,400 m). 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 8,000 ft (2,400 m) generally prevents significant hypoxia, altitude sickness, decompression sickness, and barotrauma. Federal Aviation Administration (FAA) 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 6,900 feet (2,100 m) when cruising at 39,000 feet (12,000 m). 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 4,500 ft (1,400 m) when cruising at 41,000 feet (12,000 m). The Emivest SJ30 business jet can provide a sea-level cabin altitude when cruising at 41,000 feet (12,000 m). One study of 8 flights in Airbus A380 aircraft found a median cabin pressure altitude of 6,128 feet (1,868 m), and 65 flights in Boeing 747-400 aircraft found a median cabin pressure altitude of 5,159 feet (1,572 m).
Before 1996, approximately 6,000 large commercial transport airplanes were type-certificated to fly up to 45,000 ft (14,000 m) 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 25,000 ft (7,600 m) "must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 ft (4,600 m) after any probable failure condition in the pressurization system". In the event of a decompression which 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 25,000 ft (7,600 m) for more than 2 minutes, nor to an altitude exceeding 40,000 ft (12,000 m) at any time. In practice, that new Federal Aviation Regulations amendment imposes an operational ceiling of 40,000 ft (12,000 m) 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 43,000 ft (13,000 m) in the event of a decompression incident and to exceed 40,000 ft (12,000 m) for one minute. This allows the A380 to operate at a higher altitude than other newly designed civilian aircraft.
Russian engineers have chosen to use 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 (1 bar) 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 chose to use 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 24,800 feet (7,600 m) (5.5 pounds per square inch (0.38 bar)); Gemini used an altitude of 25,700 feet (7,800 m) (5.3 pounds per square inch (0.37 bar)); and Apollo used 27,000 feet (8,200 m) (5.0 pounds per square inch (0.34 bar)) in space. This allowed for a lighter space vehicle design. Before launch, the pressure was kept at slightly higher than sea level (a constant 5.3 pounds per square inch (0.37 bar) above ambient for Gemini, and 2 pounds per square inch (0.14 bar) above sea level at launch for Apollo), and transitioned to the space cabin altitude during ascent. However, the high pressure pure oxygen atmosphere proved to be 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 40% nitrogen/60% oxygen mix at zero cabin altitude at launch, but kept the low-pressure pure oxygen in space.
After Apollo, the United States chose to use air-like cabin atmospheres for its Skylab, Space Shuttle, and the International Space Station.
Pressurization is achieved by the design of an airtight fuselage engineered to be pressurized with a source of compressed air and controlled by an environmental control system (ECS). The most common source of compressed air for pressurization is bleed air extracted from the compressor stage of a gas turbine engine, from a low or intermediate stage and also from an additional high stage; the exact stage can vary depending on engine type. By the time the cold outside air has reached the bleed air valves, it is at a very high pressure and has been heated to around 200 °C (392 °F). 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.
The part of the bleed air that is directed to the ECS is then expanded and cooled to a suitable temperature by passing it through a heat exchanger and air cycle machine known as the packs 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 (APU), 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 7.8 psi (54 kPa) and 9.4 psi (65 kPa). At 39,000 feet (12,000 m), the cabin pressure would be automatically maintained at about 6,900 feet (2,100 m) (450 feet (140 m) lower than Mexico City), which is about 11.5 psi (79 kPa) 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. It does, 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.
Unplanned loss of cabin pressure at altitude is rare but has resulted in a number of fatal accidents. Failures range from sudden, catastrophic loss of airframe integrity (explosive decompression) to slow leaks or equipment malfunctions that allow cabin pressure to drop undetected to levels that can lead to unconsciousness or severe performance degradation of the aircrew.
Any failure of cabin pressurization above 10,000 feet (3,000 m) requires an emergency descent to 8,000 feet (2,400 m) or the closest to that while maintaining the Minimum Safe Altitude (MSA), and the deployment of an oxygen mask for each seat. The oxygen systems have sufficient oxygen for all on board and give the pilots adequate time to descend to below 8,000 ft (2,400 m). Without emergency oxygen, hypoxia may lead to loss of consciousness and a subsequent loss of control of the aircraft. The time of useful consciousness varies according to altitude. As the pressure falls the cabin air temperature may also plummet to the ambient outside temperature with a danger of hypothermia or frostbite.
In jet fighter aircraft, the small size of the cockpit means that any decompression will be very rapid and would not allow the pilot time to put on an oxygen mask. Therefore, fighter jet pilots and aircrew are required to wear oxygen masks at all times.
On June 30, 1971, the crew of Soyuz 11, Soviet cosmonauts Georgy Dobrovolsky, Vladislav Volkov, and Viktor Patsayev were killed after the cabin vent valve accidentally opened before atmospheric re-entry. There had been no indication of trouble until the recovery team opened the capsule and found the dead crew.
The aircraft that pioneered pressurized cabin systems include:
In the late 1910s, attempts were being made to achieve higher and higher altitudes. In 1920, flights well over 37,000 ft (11,000 m) were first achieved by test pilot Lt. John A. Macready in a Packard-Le Père LUSAC-11 biplane at McCook Field in Dayton, Ohio. The flight was possible by releasing stored oxygen into the cockpit, which was released directly into an enclosed cabin and not to an oxygen mask, which was developed later. With this system flights nearing 40,000 ft (12,000 m) were possible, but the lack of atmospheric pressure at that altitude caused the pilot's heart to enlarge visibly, and many pilots reported health problems from such high altitude flights. Some early airliners had oxygen masks for the passengers for routine flights.
In 1921, a Wright-Dayton USD-9A reconnaissance biplane was modified with the addition of a completely enclosed air-tight chamber that could be pressurized with air forced into it by small external turbines. The chamber had a hatch only 22 in (0.56 m) in diameter that would be sealed by the pilot at 3,000 ft (910 m). The chamber contained only one instrument, an altimeter, while the conventional cockpit instruments were all mounted outside the chamber, visible through five small portholes. The first attempt to operate the aircraft was again made by Lt. John A. McCready, who discovered that the turbine was forcing air into the chamber faster than the small release valve provided could release it. As a result, the chamber quickly over pressurized, and the flight was abandoned. A second attempt had to be abandoned when the pilot discovered at 3,000 ft (910 m) that he was too short to close the chamber hatch. The first successful flight was finally made by test pilot Lt. Harrold Harris, making it the world's first flight by a pressurized aircraft.
The first airliner with a pressurized cabin was the Boeing 307 Stratoliner, built in 1938, prior to World War II, though only ten were produced. The 307's "pressure compartment was from the nose of the aircraft to a pressure bulkhead in the aft just forward of the horizontal stabilizer."
World War II was a catalyst for aircraft development. Initially, the piston aircraft of World War II, though they often flew at very high altitudes, were not pressurized and relied on oxygen masks. This became impractical with the development of larger bombers where crew were required to move about the cabin and this led to the first bomber with cabin pressurization (though restricted to crew areas), the Boeing B-29 Superfortress. The control system for this was designed by Garrett AiResearch Manufacturing Company, drawing in part on licensing of patents held by Boeing for the Stratoliner.
Post-war piston airliners such as the Lockheed Constellation (1943) extended the technology to civilian service. The piston engined airliners generally relied on electrical compressors to provide pressurized cabin air. Engine supercharging and cabin pressurization enabled planes like the Douglas DC-6, the Douglas DC-7, and the Constellation to have certified service ceilings from 24,000 ft (7,300 m) to 28,400 ft (8,700 m). Designing a pressurized fuselage to cope with that altitude range was within the engineering and metallurgical knowledge of that time. The introduction of jet airliners required a significant increase in cruise altitudes to the 30,000–41,000 feet (9,100–12,500 m) range, where jet engines are more fuel efficient. That increase in cruise altitudes required far more rigorous engineering of the fuselage, and in the beginning not all the engineering problems were fully understood.
The world’s first commercial jet airliner was the British de Havilland Comet (1949) designed with a service ceiling of 36,000 ft (11,000 m). It was the first time that a large diameter, pressurized fuselage with windows had been built and flown at this altitude. Initially, the design was very successful but two catastrophic airframe failures in 1954 resulting in the total loss of the aircraft, passengers and crew grounded what was then the entire world jet airliner fleet. Extensive investigation and groundbreaking engineering analysis of the wreckage led to a number of very significant engineering advances that solved the basic problems of pressurized fuselage design at altitude. The critical problem proved to be a combination of an inadequate understanding of the effect of progressive metal fatigue as the fuselage undergoes repeated stress cycles coupled with a misunderstanding of how aircraft skin stresses are redistributed around openings in the fuselage such as windows and rivet holes.
The critical engineering principles concerning metal fatigue learned from the Comet 1 program were applied directly to the design of the Boeing 707 (1957) and all subsequent jet airliners. One immediately noticeable legacy of the Comet disasters is the oval windows on every jet airliner; the metal fatigue cracks that destroyed the Comets were initiated by the small radius corners on the Comet 1’s almost square windows. The Comet fuselage was redesigned and the Comet 4 (1958) went on to become a successful airliner, pioneering the first transatlantic jet service, but the program never really recovered from these disasters and was overtaken by the Boeing 707.
Concorde had to deal with particularly high pressure differentials because it flew at unusually high altitude (up to 60,000 feet (18,000 m)) and maintained a cabin altitude of 6,000 ft (1,800 m). This made the aircraft significantly heavier and contributed to the high cost of a flight. The Concorde also had smaller cabin windows than most other commercial passenger aircraft in order to slow the rate of decompression if a window failed. The high cruising altitude also required the use of high pressure oxygen and demand valves at the emergency masks unlike the continuous-flow masks used in conventional airliners.
The designed operating cabin altitude for new aircraft is falling and this is expected to reduce any remaining physiological problems.