Hubbry Logo
Cabin pressurizationCabin pressurizationMain
Open search
Cabin pressurization
Community hub
Cabin pressurization
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Cabin pressurization
Cabin pressurization
Cabin pressurization
View on Wikipedia
from Wikipedia

An airliner fuselage, such as this Boeing 737, forms an almost cylindrical pressure vessel.

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.[1]

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.[2] 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.[3] 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,[3] 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 60,000 ft (18,288 m) while maintaining a cabin altitude of 6,000 ft (1,829 m). 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.[4]

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

[edit]
The pressurization controls on a Boeing 737-800

Pressurization becomes increasingly necessary at altitudes above 10,000 ft (3,048 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. For private aircraft operating in the US, crew members are required to use oxygen masks if the cabin altitude (a representation of the air pressure, see below) stays above 12,500 ft (3,810 m) for more than 30 minutes, or if the cabin altitude reaches 14,000 ft (4,267 m) at any time. At altitudes above 15,000 ft (4,572 m), passengers are required to be provided oxygen masks as well. On commercial aircraft, the cabin altitude must be maintained at 8,000 ft (2,438 m) 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
The lower partial pressure of oxygen at high altitude reduces the alveolar oxygen tension in the lungs and subsequently in the brain, leading to sluggish thinking, dimmed vision, loss of consciousness, and ultimately death. In some individuals, particularly those with heart or lung disease, symptoms may begin as low as 5,000 ft (1,524 m), although most passengers can tolerate altitudes of 8,000 ft (2,438 m) without ill effect. At this altitude, there is about 25% less oxygen than there is at sea level.[5]
Hypoxia may be addressed by the administration of supplemental oxygen, either through an oxygen mask or through a nasal cannula. Without pressurization, sufficient oxygen can be delivered up to an altitude of about 40,000 ft (12,192 m). This is because a person who is used to living at sea level needs about 0.20 bar (20 kPa; 2.9 psi) partial oxygen pressure to function normally and that pressure can be maintained up to about 40,000 ft (12,192 m) by increasing the mole fraction of oxygen in the air that is being breathed. At 40,000 ft (12,192 m), the ambient air pressure falls to about 0.2 bar, at which maintaining a minimum partial pressure of oxygen of 0.2 bar requires breathing 100% oxygen using an oxygen mask.
Emergency oxygen supply masks in the passenger compartment of airliners do not need to be pressure-demand masks because most flights stay below 40,000 ft (12,192 m). Above that altitude the partial pressure of oxygen will fall below 0.2 bar even at 100% oxygen and some degree of cabin pressurization or rapid descent will be essential to avoid the risk of hypoxia.
Altitude sickness
Hyperventilation, the body's most common response to hypoxia, does help to partially restore the partial pressure of oxygen in the blood, but it also causes carbon dioxide (CO2) to out-gas, raising the blood pH and inducing alkalosis. Passengers may experience fatigue, nausea, headaches, sleeplessness, and (on extended flights) even pulmonary edema. These are the same symptoms that mountain climbers experience, but the limited duration of powered flight makes the development of pulmonary oedema unlikely. Altitude sickness may be controlled by a full pressure suit with helmet and faceplate, which completely envelops the body in a pressurized environment; however, this is impractical for commercial passengers.
Decompression sickness
The low partial pressure of gases, principally nitrogen (N2) but including all other gases, may cause dissolved gases in the bloodstream to precipitate out, resulting in gas embolism, or bubbles in the bloodstream. The mechanism is the same as that of compressed-air divers on ascent from depth. Symptoms may include the early symptoms of "the bends"—tiredness, forgetfulness, headache, stroke, thrombosis, and subcutaneous itching—but rarely the full symptoms thereof. Decompression sickness may also be controlled by a full-pressure suit as for altitude sickness.
Barotrauma
As the aircraft climbs or descends, passengers may experience discomfort or acute pain as gases trapped within their bodies expand or contract. The most common problems occur with air trapped in the middle ear (aerotitis) or paranasal sinuses by a blocked Eustachian tube or sinuses. Pain may also be experienced in the gastrointestinal tract or even the teeth (barodontalgia). Usually these are not severe enough to cause actual trauma but can result in soreness in the ear that persists after the flight[6] and can exacerbate or precipitate pre-existing medical conditions, such as pneumothorax.

Cabin altitude

[edit]
An empty bottle, sealed at 11,000 m (37,000 ft), is crushed on descent to sea level, compared with one in its original state.

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 kPa (14.696 psi; 29.921 inHg).[7]

Aircraft

[edit]

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 (2,438 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.[citation needed]

Pilots can use a "cabin altimeter" (also known as a cabin differential pressure gauge) to measure the difference between inside and outside pressure.[8]

Keeping the cabin altitude below 8,000 ft (2,438 m) generally prevents significant hypoxia, altitude sickness, decompression sickness, and barotrauma.[9] 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.[10] 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.[citation needed] 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 7,000 ft (2,134 m) when cruising at 37,000 ft (11,278 m).[11] 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.[12] For example, the Bombardier Global Express business jet can provide a cabin altitude of 4,500 ft (1,372 m) when cruising at 41,000 ft (12,497 m).[13][14][15] The Emivest SJ30 business jet can provide a sea-level cabin altitude when cruising at 41,000 ft (12,497 m).[16][17][unreliable source?] One study of eight flights in Airbus A380 aircraft found a median cabin pressure altitude of 6,128 ft (1,868 m), and 65 flights in Boeing 747-400 aircraft found a median cabin pressure altitude of 5,159 ft (1,572 m).[18]

Before 1996, approximately 6,000 large commercial transport airplanes were assigned a type certificate to fly up to 45,000 ft (13,716 m) without having to meet high-altitude special conditions.[19] 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 (7,620 m) "must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 ft (4,572 m) after any probable failure condition in the pressurization system".[20] 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 25,000 ft (7,620 m) for more than 2 minutes, nor to an altitude exceeding 40,000 ft (12,192 m) at any time.[20] 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.[21][22] 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 (13,106 m) in the event of a decompression incident and to exceed 40,000 ft (12,192 m) for one minute. This allows the A380 to operate at a higher altitude than other newly designed civilian aircraft.[21]

Spacecraft

[edit]

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.[23] This requires a heavier space vehicle design, because the spacecraft cabin structure must withstand the stress of 14.7 pounds per square inch (1 atm, 1.01 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.[24]

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.[25][26] Mercury used a cabin altitude of 24,800 ft (7,600 m) (5.5 psi (0.38 bar));[27] Gemini used an altitude of 25,700 ft (7,800 m) (5.3 psi (0.37 bar));[28] and Apollo used 27,000 ft (8,200 m) (5.0 psi (0.34 bar))[29] 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 5.3 psi (0.37 bar) above ambient for Gemini, and 2 psi (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 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 5 psi (0.34 bar) in space.[30]

After the Apollo program, the United States used "a 74-percent oxygen and 26-percent nitrogen breathing mixture" at 5 psi (0.34 bar) for Skylab,[31] and a cabin atmosphere of 14.5 psi (1.00 bar) for the Space Shuttle orbiter and the International Space Station.[32]

Mechanics

[edit]
Piston-engine aircraft cabin pressurization using a dedicated compressor.[33]

An airtight fuselage is pressurized using 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 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 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. Piston-engine aircraft require an additional compressor, see diagram right.[34]

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 (Pressurization and Air Conditioning) 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.

Outflow and pressure relief valve on a Boeing 737-800

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 540 hPa (7.8 psi) and 650 hPa (9.4 psi).[35] At 39,000 ft (11,887 m), the cabin pressure would be automatically maintained at about 6,900 ft (2,100 m), (450 ft (140 m) lower than Mexico City), which is about 790 hPa (11.5 psi) of atmosphere pressure.[34]

Some aircraft, such as the Boeing 787 Dreamliner, have re-introduced electric compressors previously used on piston-engined airliners to provide pressurization.[36][37] The use of electric compressors increases the electrical generation load on the engines and introduces a number of stages of energy transfer;[38] 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.

Unplanned decompression

[edit]
Main article: Uncontrolled decompression
Typical passenger oxygen mask deployment

Unplanned loss of cabin pressure at altitude/in space 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.

Any failure of cabin pressurization above 10,000 ft (3,000 m) requires an emergency descent to 10,000 ft or the closest to that while maintaining the minimum sector 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 10,000 ft. Without emergency oxygen, hypoxia may lead to loss of consciousness and a subsequent loss of control of the aircraft. Modern airliners include a pressurized pure oxygen tank in the cockpit, giving the pilots more time to bring the aircraft to a safe altitude. 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.

For airliners that need to fly over terrain that does not allow reaching the safe altitude within a maximum of 30 minutes, pressurized oxygen bottles are mandatory since the chemical oxygen generators fitted to most planes cannot supply sufficient oxygen.

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.[39]

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.[40][41]

History

[edit]
Cessna P210 - First commercially successful pressurized single-engine aircraft

The aircraft that pioneered pressurized cabin systems include:

  • Packard-Le Père LUSAC-11, (1920, a modified French design, not actually pressurized but with an enclosed, oxygen enriched cockpit)
  • Engineering Division USD-9A, a modified Airco DH.9A (1921 – the first aircraft to fly with the addition of a pressurized cockpit module)[42]
  • Junkers Ju 49 (1931 – a German experimental aircraft purpose-built to test the concept of cabin pressurization)
  • Farman F.1000 (1932 – a French record breaking pressurized cockpit, experimental aircraft)
  • Chizhevski BOK-1 (1936 – a Russian experimental aircraft)
  • Lockheed XC-35 (1937 – an American pressurized aircraft. Rather than a pressure capsule enclosing the cockpit, the monocoque fuselage skin was the pressure vessel.)
  • Renard R.35 (1938 – the first pressurized piston airliner)
  • Boeing 307 Stratoliner (1938 – the first pressurized airliner to enter commercial service)
  • Lockheed Constellation (1943 – the first pressurized airliner in wide service)
  • Boeing 377 Stratocruiser (1947-the first pressurized double-decker plane to see long - range commercial service)
  • Avro Tudor (1946 – first British pressurized airliner)
  • de Havilland Comet (British, Comet 1 1949 – the first jetliner, Comet 4 1958 – resolving the Comet 1 problems)
  • Tupolev Tu-144 and Concorde (1968 USSR and 1969 Anglo-French respectively – first to operate at very high altitude)
  • Cessna P210 (1978) First commercially successful pressurized single-engine aircraft[43]
  • SyberJet SJ30 (2005) First civilian business jet to certify 12.0 psi pressurization system allowing for a sea level cabin at 41,000 ft (12,497 m).[44]

The first airliner to enter commercial service with a pressurized cabin was the Boeing 307 Stratoliner, built in 1938, prior to World War II, though only ten were produced before the war interrupted production. 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."[45]

World War II era flying helmet and oxygen mask

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.[46] This became impractical with the development of larger bombers where crew were required to move about the cabin. The first bomber built with a pressurised cabin for high altitude use was the Vickers Wellington Mark VI in 1941 but the RAF changed policy and instead of acting as Pathfinders the aircraft were used for other purposes. The US Boeing B-29 Superfortress long range strategic bomber was first into bomb service. 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.[47]

Post-war piston airliners such as the Lockheed Constellation (1943) made the technology more common in civilian service. The piston-engined airliners generally relied on electrical compressors to provide pressurized cabin air. Engine supercharging and cabin pressurization enabled aircraft like the Douglas DC-6, the Douglas DC-7, and the Constellation to have certified service ceilings from 24,000 to 28,400 ft (7,315 to 8,656 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 ft (9,144–12,497 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[48] were applied directly to the design of the Boeing 707 (1957) and all subsequent jet airliners. For example, detailed routine inspection processes were introduced, in addition to thorough visual inspections of the outer skin, mandatory structural sampling was routinely conducted by operators; the need to inspect areas not easily viewable by the naked eye led to the introduction of widespread radiography examination in aviation; this also had the advantage of detecting cracks and flaws too small to be seen otherwise.[49] Another visibly 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.[50][51] 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.[52][53]

Even following the Comet disasters, there were several subsequent catastrophic fatigue failures attributed to cabin pressurisation. Perhaps the most prominent example was Aloha Airlines Flight 243, involving a Boeing 737-200.[54] In this case, the principal cause was the continued operation of the specific aircraft despite having accumulated 35,496 flight hours prior to the accident, those hours included over 89,680 flight cycles (takeoffs and landings), owing to its use on short flights;[55] this amounted to more than twice the number of flight cycles that the airframe was designed to endure.[56] Aloha 243 was able to land despite the substantial damage inflicted by the decompression, which had resulted in the loss of one member of the cabin crew; the incident had far-reaching effects on aviation safety policies and led to changes in operating procedures.[56]

The supersonic airliner Concorde had to deal with particularly high pressure differentials because it flew at unusually high altitude (up to 60,000 ft (18,288 m)) and maintained a cabin altitude of 6,000 ft (1,829 m).[57] Despite this, its cabin altitude was intentionally maintained at 6,000 ft (1,829 m).[58] This combination, while providing for increasing comfort, necessitated making Concorde a significantly heavier aircraft, which in turn contributed to the relatively high cost of a flight. Unusually, Concorde was provisioned with smaller cabin windows than most other commercial passenger aircraft in order to slow the rate of decompression in the event of a window seal failing.[59] 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.[60] The FAA, which enforces minimum emergency descent rates for aircraft, determined that, in relation to Concorde's higher operating altitude, the best response to a pressure loss incident would be to perform a rapid descent.[61]

The designed operating cabin altitude for new aircraft is falling and this is expected to reduce any remaining physiological problems. Both the Boeing 787 Dreamliner and the Airbus A350 XWB airliners have made such modifications for increased passenger comfort. The 787's internal cabin pressure is the equivalent of 6,000 ft (1,829 m) altitude resulting in a higher pressure than for the 8,000 ft (2,438 m) altitude of older conventional aircraft;[62] according to a joint study performed by Boeing and Oklahoma State University, such a level significantly improves comfort levels.[63][64] Airbus has stated that the A350 XWB provides for a typical cabin altitude at or below 6,000 ft (1,829 m), along with a cabin atmosphere of 20% humidity and an airflow management system that adapts cabin airflow to passenger load with draught-free air circulation.[65] The adoption of composite fuselages eliminates the threat posed by metal fatigue that would have been exacerbated by the higher cabin pressures being adopted by modern airliners, it also eliminates the risk of corrosion from the use of greater humidity levels.[62]

See also

[edit]

Footnotes

[edit]

General references

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Cabin pressurization

View on Grokipedia
from Grokipedia
Cabin pressurization is the process used in and to regulate and maintain internal cabin air pressure at a level higher than the surrounding atmospheric pressure during high-altitude flights or space operations, thereby protecting occupants from the physiological effects of low oxygen levels, such as hypoxia. This system seals the passenger cabin, , and certain areas, simulating sea-level or low-altitude conditions—typically limiting the effective cabin altitude to about 8,000 feet—while the operates at cruising altitudes of 30,000 to 40,000 feet or more, where external air pressure drops to less than half of sea-level values. By preventing rapid decompression and ensuring breathable air, cabin pressurization enables efficient, comfortable, and safe long-distance travel above adverse and . The development of cabin pressurization addressed the limitations of early aviation, where high-altitude flights were restricted by the need for supplemental oxygen and the risks of thin air. Pioneered in , the first appeared in ; the , a modified Electra, became the first U.S. airplane with a pressurized cabin when it flew in May 1937, allowing sustained operations above 30,000 feet without oxygen masks for the crew. By the , pressurization became standard in commercial airliners as enabled routine high-altitude cruising, with systems evolving to include advanced materials like composites in modern designs, such as the 787, which achieves a lower cabin altitude of around 6,000 feet for enhanced passenger comfort. In operation, the system draws —known as —from the aircraft's engines or , conditions it through air cycle machines or vapor compression units for temperature and humidity control, and introduces it into the cabin while outflow valves automatically regulate exhaust to sustain the pressure differential, typically 7 to 9 pounds per (psi) depending on the aircraft's structural limits. Critical components include the cabin pressure controller, which schedules pressure changes during ascent and descent to minimize ear discomfort; safety and relief valves to prevent over-pressurization; and indicators for monitoring differential pressure, cabin altitude, and climb rate. In the event of failure, rapid decompression can occur—classified as explosive if instantaneous or insidious if gradual—triggering automatic oxygen deployment and descent protocols to restore safe pressure levels. Regulatory standards, primarily under (FAR) Part 25, mandate that pressurization systems maintain cabin altitudes below 8,000 feet during normal operations and limit exposure to higher altitudes in failures, such as no more than 2 minutes above 25,000 feet and no exposure above 40,000 feet. These requirements are verified through ground tests, flight demonstrations, and analysis of decompression scenarios, ensuring in components to handle single-point failures with high reliability. Modern advancements continue to focus on efficiency, with some aircraft using electric compressors instead of to reduce fuel consumption and improve air quality.

Physiological Need

Human Response to Low Pressure

As atmospheric pressure decreases with increasing altitude, the partial pressure of oxygen (PO₂) in inspired air diminishes, impairing oxygen uptake in the lungs and subsequent delivery to tissues. At sea level, PO₂ is approximately 160 mmHg, but it halves to about 80 mmHg at 18,000 feet (5,500 m), effectively reducing the oxygen fraction to an equivalent of 10.5% at sea-level pressure despite the constant 21% oxygen composition in air. This hypobaric hypoxia triggers physiological responses such as hyperventilation and increased heart rate, but above 10,000 feet (3,000 m), symptoms emerge including euphoria, impaired judgment, headache, cyanosis, visual impairment, and drowsiness, which can progress to unconsciousness without intervention. The severity of hypoxia is quantified by the (TUC), defined as the maximum period after sudden oxygen deprivation during which an individual can perform rational, life-saving actions. TUC shortens nonlinearly with altitude due to rapid arterial oxygen desaturation; for instance, it ranges from 3–5 minutes at 25,000 feet to mere 9–15 seconds at 45,000 feet. Representative TUC durations, based on unacclimatized individuals at rest, are summarized below:
Altitude (feet MSL)TUC Duration
25,0003–5 minutes
30,0001–2 minutes
35,00030–60 seconds
40,00015–20 seconds
45,0009–15 seconds
These values decrease further with physical exertion or preexisting conditions. Prolonged exposure to hypobaric conditions induces , encompassing acute mountain sickness with symptoms like , , and from , typically onset above 8,000 feet but worsening at altitudes. (DCS), triggered by rapid pressure reduction, causes inert gas bubbles—primarily —to form in tissues and blood, manifesting as joint pain (the "bends"), neurological deficits, or skin mottling, with incidence rising above 18,000 feet and peaking over 25,000 feet in nonpressurized environments. In extreme hypobaric vacuum above 63,000 feet, develops as tissue vapor pressure exceeds ambient pressure, causing fluid ebullition, massive swelling, and swift loss of consciousness within 10–15 seconds, though brief exposures may allow survival with prompt repressurization. Pressure changes during aircraft ascent and descent exacerbate hypobaric risks through , where unequal pressures across tissue barriers cause injury. Ear barotrauma occurs when the fails to ventilate the , leading to tympanic membrane distortion, pain, or perforation, affecting up to 25% of air travelers. Sinus barotrauma, more common on descent due to compressing external pressure trapping air in obstructed sinuses, results in facial pain, hemorrhage, or mucosal , with frontal sinuses most vulnerable and prevalence reaching 55% in pilots with preexisting .

Pressurization Requirements

Cabin pressurization requirements for commercial are primarily governed by regulations from aviation authorities such as the (FAA) and the (EASA), which mandate that the cabin must not exceed 8,000 feet (2,438 meters) during normal operations at the aircraft's maximum certified operating altitude. This limit ensures occupant safety by maintaining a breathable environment equivalent to moderate altitude conditions. In the event of a pressurization system failure, the aircraft must be capable of an descent to a cabin of 10,000 feet (3,048 meters) or below within two minutes from a cruising altitude of up to 41,000 feet (12,497 meters), with exposure to higher altitudes minimized thereafter. These standards align with the framework established by the (ICAO) in Annex 8, which outlines airworthiness certification principles but delegates detailed performance requirements to national authorities like the FAA and EASA. A key aspect includes limits on maximum differential, typically around 8 to 9 psi (55 to 62 kPa) for commercial jet airliners, to prevent structural overload while achieving the required cabin altitude; for instance, many wide-body jets are certified for a differential of 8.9 psi. The physiological basis for the 8,000-foot limit is to preserve a of oxygen (PO₂) of approximately 118 mmHg in the cabin air, comparable to that at 8,000 feet above , which supports adequate oxygenation for healthy individuals without supplemental oxygen. Above a cabin pressure altitude of 15,000 feet (4,572 meters), regulations require supplemental oxygen for all occupants to counteract hypoxia risks, with crew members needing it continuously above 10,000 feet for extended periods and pilots above 12,500 feet. Military aircraft standards, governed by documents such as MIL-STD-1472 for human engineering, permit higher cabin altitudes—often up to 10,000 feet or more in fighters—due to pilot training, oxygen mask usage, and mission-specific tolerances that exceed commercial limits for brevity and performance. This contrast reflects the differing priorities: passenger comfort and safety in commercial operations versus operational agility in military contexts.

System Design

Aircraft Pressurization

Aircraft cabin pressurization systems maintain a safe internal environment by supplying conditioned air to the and regulating its relative to the external atmosphere. The basic architecture typically involves sourcing pressurized air from extracted from the stages of engines, which is then conditioned and distributed into the sealed . Outflow valves, positioned at the rear of the , control the rate of air egress to achieve the desired differential, ensuring a continuous flow of while preventing over- or under-pressurization. In some modern designs, dedicated cabin air compressors provide an alternative to , reducing penalties and enabling all-electric systems. The key metric in these systems is the differential pressure, defined as ΔP = P_cabin - P_ambient, where P_cabin is pressure and P_ambient is the external . This differential allows to simulate lower altitudes despite cruising at high altitudes, with typical maximum values around 8-9 psi in commercial aircraft like the to balance passenger comfort and structural limits. Federal Aviation Administration regulations mandate that pressurized cabins maintain a of no more than 8,000 feet under normal operations to mitigate hypoxia risks. Core components include cabin pressure controllers (CPCs), which automatically adjust outflow valve positions based on aircraft altitude, climb/descent rates, and scheduled profiles to maintain target cabin altitudes. Safety valves, or positive relief valves, activate if the differential exceeds design limits—such as 9.1 psi in the —to vent excess and protect the structure. Negative relief valves counter potential vacuum conditions during rapid descents by allowing ambient air to enter the cabin, opening at approximately -0.5 to -1.0 psi differential. Pressurization air is integrated with the (ECS) via air cycle machines (ACMs), which cool the hot through expansion and heat exchange processes before mixing it with recirculated cabin air for distribution. This conditioning removes moisture and contaminants, ensuring breathable air at comfortable temperatures while supporting pressurization. In variations across types, high-altitude business jets like the Gulfstream G650 achieve lower cabin altitudes (around 4,000-5,000 feet at 51,000 feet cruise) with higher differentials up to 10.6 psi, prioritizing passenger comfort on long flights, whereas short-haul commercial jets like the A320 target 6,000-8,000 feet at 39,000 feet with 7.8-8.6 psi for efficiency on frequent cycles. Emerging electric vertical takeoff and landing () designs incorporate electric compressors, such as 10-15 kW units, to provide bleed-air-independent pressurization for , enabling lighter, more efficient systems without turbine dependency.

Spacecraft Pressurization

Spacecraft pressurization systems maintain a controlled internal environment to support human physiology in the vacuum of , typically using pure oxygen or nitrogen-oxygen mixtures at pressures ranging from 5 to 14.7 pounds per (psi), in contrast to that rely on ambient air compression. These closed-loop systems prioritize minimal and , supplying breathable gas from stored or generated sources while managing contaminants and to prevent physiological issues like hypoxia or . In the Apollo command module, the atmosphere consisted of 100% oxygen at 5 psi during orbital operations, achieved after venting used for launch protection; this low-pressure pure-oxygen environment reduced structural requirements but necessitated rigorous fire safety measures following the incident. The (ISS), by comparison, employs a sea-level equivalent of 14.7 psi with a 21% oxygen and 79% mixture, including humidity control at 50-65% relative humidity to enhance crew comfort and equipment longevity. During launch and reentry, face significant structural challenges from differential pressures, where the internal cabin pressure contrasts sharply with external or atmospheric forces, imposing tensile loads up to several times the vehicle's weight; ablative heat shields, which char and erode to dissipate reentry exceeding 2,000°C, must integrate with the to avoid compromising hull integrity. These dynamics require robust pressure hulls designed to withstand combined acoustic, vibrational, and thermal stresses without leakage. Pressurization is integral to the Environmental Control and Life Support System (ECLSS), which sustains cabin pressure by regulating oxygen partial pressure between 2.83 and 3.35 psia while scrubbing via regenerable beds in the Carbon Dioxide Removal Assembly (CDRA), capturing the crew's metabolic CO2 production, approximately 1 kg per day per crew member for subsequent water recovery. This closed-loop integration recycles air and water, achieving over 90% efficiency on the ISS to minimize resupply needs. Post-2020 developments in SpaceX's include pressurized modules offering over 600 cubic meters of habitable , integrated with advanced ECLSS technologies for maintenance and environmental control, leveraging cryogenic systems to manage cooling and prevent boil-off in support of long-duration missions.

Operational Control

Cabin Altitude Profiles

Cabin altitude profiles describe the programmed variations in internal pressure during flight phases to balance passenger comfort, physiological safety, and structural limits. These profiles are managed by automated systems that adjust outflow valves to control the rate of pressure change, ensuring the cabin environment simulates lower altitudes than the external . The goal is to minimize rapid pressure shifts that could cause discomfort, such as in the ears or sinuses, while adhering to certification standards. During the climb phase, the cabin altitude increases gradually from to the scheduled cruise level, typically at a rate of 300 to 750 feet per minute (fpm), depending on aircraft type and operator preferences. For instance, the uses rates around 600 fpm to allow time for pressure equalization across the Eustachian tubes, reducing ear discomfort. This schedule builds the pressure differential slowly until the maximum design differential is achieved, preventing excessive stress on the . In cruise, the system maintains a constant cabin altitude, commonly set between 6,000 and 8,000 feet, which corresponds to the oxygen of a terrestrial environment at moderate . This is achieved despite external altitudes of 30,000 to 43,000 feet, where would otherwise be untenable. Modern long-haul aircraft like the 787 optimize this further by sustaining a 6,000-foot cabin altitude at up to 43,000 feet external, using advanced composite structures to handle higher differentials of about 9 psi. Regulatory limits ensure this normal operating cabin altitude does not exceed 8,000 feet under standards. The descent profile involves a controlled reduction in cabin altitude to match ground-level , typically at rates of 300 to 500 fpm to further ease equalization and avoid discomfort from faster changes. Pressurization controllers include aural and visual warnings if rates exceed safe thresholds, such as 750 fpm, prompting crew intervention. Long-haul flights, which sustain higher external cruise altitudes for efficiency, allow for consistent low cabin altitudes like 6,000 feet over extended periods, enhancing comfort on routes exceeding several hours. In contrast, short-haul operations often cruise at lower external altitudes (e.g., 20,000-30,000 feet), resulting in higher cabin pressures equivalent to 5,000-6,000 feet, as the aircraft spends less time building full differential. Flight path variations, such as step climbs or encounters with , require profile adjustments; pilots may select manual modes or predefined schedules to temporarily alter rates, ensuring the cabin follows a safe without abrupt shifts. For example, in , reduced climb rates prevent excessive differential buildup during irregular ascents.

Pressure Regulation Methods

Cabin pressurization systems in primarily rely on feedback control loops to maintain safe and comfortable internal environments. These loops utilize sensors, such as absolute and differential transducers, to continuously monitor cabin altitude and the differential between the cabin and ambient atmosphere. The cabin controller (CPC), a digital electronic unit, processes this sensor data and commands adjustments to the outflow valve's position through servo motors, modulating the rate of air exhaust to balance incoming from the engines or . This closed-loop mechanism ensures the cabin altitude remains the target metric for occupant safety, typically aiming for levels equivalent to 6,000–8,000 feet above during cruise. In isobaric mode, the predominant operational setting for commercial , the CPC maintains a constant cabin altitude by dynamically adjusting the outflow to counteract changes in aircraft altitude and external pressure. This mode employs proportional-integral-derivative (PID) algorithms, where the proportional term responds to the current pressure error, the integral term corrects accumulated discrepancies, and the derivative term anticipates rate changes to minimize overshoot and stabilize pressure. Widely adopted in modern fleets like the and Airbus A320, PID-based control achieves precise regulation, with cabin pressure rates limited to 300–500 feet per minute to avoid discomfort. For emergencies or system faults, pilots can engage manual override through controls, directly positioning the or activating dump valves to rapidly equalize cabin pressure with ambient conditions. This mode bypasses the automatic CPC, allowing selective pressurization or depressurization as per emergency checklists, such as during suspected or rapid descent requirements. In certified systems, manual controls include rate selectors and position indicators to facilitate precise intervention without . Redundancy is integral to certified pressurization systems, featuring dual CPCs—one designated as primary and the other as standby—that automatically switch upon failure detection via built-in diagnostics. Each CPC interfaces with independent outflow valve motors and sensors, ensuring continued operation if one channel fails. Pneumatic backups, including positive and negative pressure relief valves, provide passive protection against overpressurization or vacuum conditions, while hydraulic or electric redundancies support valve actuation in high-reliability architectures like those in the Airbus A320 family. Advanced methods in aircraft integrate predictive algorithms within the to anticipate pressure adjustments based on factors like weather-induced altitude changes or passenger load variations. These algorithms, often enhancements to PID with elements, optimize outflow valve scheduling for smoother transitions and energy efficiency, as implemented in newer platforms like the Boeing 787. Such predictive approaches reduce pressure oscillations by forecasting climb/descent profiles from navigation data.

Safety and Risks

Decompression Events

Decompression events in aircraft cabins occur when there is a sudden loss of pressure, categorized primarily as rapid or explosive decompression, each presenting distinct dynamics and risks. Rapid decompression typically results from a breach such as a small hole or crack in the , allowing pressurized air to escape at a rate governed by the time constant τ ≈ V / (A * C_d * c_s), where V is the cabin volume, A is the outflow area, C_d is the (often around 0.75 for orifices like windows), and c_s is the local (approximately 340 m/s). For small holes, this time constant yields decompression times of 2-10 seconds, during which cabin pressure drops significantly but not instantaneously, enabling some time for responses. Explosive decompression arises from major structural failures, such as a window blowout or large tear, where the gas expansion velocity approaches the (approximately 343 m/s at , varying with temperature), leading to near-instantaneous equalization across the breach. This violent outflow creates extreme forces, including high-velocity winds, often hurricane-force (over 33 m/s) near the breach, capable of propelling objects and occupants. The physiological impacts of these events are severe and multifaceted. Immediate hypoxia occurs as oxygen plummets, with (TUC) decreasing with altitude from 2–3 minutes at 25,000 feet to 15–20 seconds at 40,000 feet, potentially causing unconsciousness without supplemental oxygen. Wind blast injuries from the rushing air can result in , lacerations, or ejection of unsecured individuals, while adiabatic expansion causes rapid cooling, exposing occupants to temperatures dropping below -50°C and risking or . A notable case study is on April 28, 1988, where a 737-200 experienced explosive decompression due to a 20-foot fuselage tear at 24,000 feet, caused by metal fatigue; one was fatally ejected by the decompression forces, while 65 others sustained injuries primarily from wind blast and debris. More recently, on June 24, 2024, Flight 189, a 737-800, suffered a pressurization failure shortly after takeoff from , causing a rapid descent of 26,900 feet and injuring 29 passengers due to hypoxia and ; the aircraft landed safely. Detection of decompression events relies on differential pressure alarms that activate when cabin altitude exceeds safe limits, typically triggering at 10,000-14,000 feet, alongside automatic deployment of oxygen masks at approximately 14,000 feet cabin altitude to mitigate hypoxia.

Mitigation Strategies

Aircraft fuselages incorporate fail-safe designs featuring multiple load paths to mitigate the risks associated with pressurization failures, allowing loads to redistribute to redundant structural elements such as frames, skin panels, and stiffeners in the event of a crack or partial failure. These designs prioritize pressurized structures, ensuring that the remaining fuselage can withstand limit loads until damage is detected through scheduled inspections. To protect occupants during decompression events, whether explosive or gradual, aircraft are equipped with emergency oxygen systems including drop-down masks connected to chemical oxygen generators that provide a 12-22 minute supply of oxygen, sufficient for descent to breathable altitudes. These generators produce oxygen via a chemical reaction initiated upon mask deployment, delivering it through continuous-flow masks at rates maintaining safe partial pressures up to 40,000 feet. In response to pressurization loss, standard descent protocols require pilots to immediately initiate an emergency descent to 10,000 feet at the maximum safe rate, typically 6,000 feet per minute in commercial jets, using full engine thrust reverse, speed brakes, and flaps as needed to maximize drag while preserving control. This procedure prioritizes rapid altitude reduction to restore adequate ambient oxygen levels without exceeding structural limits. Preventive includes pre-flight pressurization tests to verify system integrity, such as leak checks by pressurizing the cabin to operational differentials and monitoring for decay over a specified period. Additionally, cyclic fatigue inspections assess components for pressurization-induced cracks, conducted at intervals based on flight cycles and full-scale data to ensure compliance with damage tolerance requirements. Crew training emphasizes proficiency in decompression response through simulator scenarios that replicate rapid cabin altitude increases, requiring pilots to don quick-donning oxygen masks in under 5 seconds while initiating the emergency descent checklist. These sessions also cover passenger mask deployment verification and communication protocols to minimize physiological risks during the event.

Historical Development

Early Innovations

The development of cabin pressurization was driven by the physiological need to mitigate hypoxia risks for crews and passengers during high-altitude flights, where oxygen levels drop significantly above 10,000 feet. The Lockheed XC-35, introduced in 1937, marked the first American aircraft specifically designed and built with a pressurized cabin for high-altitude research. This experimental twin-engine aircraft, a modified version of the Lockheed Electra, utilized engine-driven turbo-superchargers to bleed into the cabin, maintaining an equivalent altitude of 12,000 feet even when flying at 30,000 feet. The system allowed sustained operations above weather and enemy defenses, demonstrating the feasibility of pressurized flight for military applications. During , the , entering service in 1944, represented a major advancement in pressurized bomber design. This long-range featured interconnected pressurized compartments for the crew, achieving a maximum pressure differential of 6.6 psi to sustain a cabin altitude of 8,000 feet up to a flight altitude of 30,000 feet, and partially up to 40,000 feet. The innovation significantly reduced hypoxia-related impairments for crews on extended high-altitude bombing missions over , enabling safer and more effective operations despite the aircraft's vulnerability to flak and fighters. Transitioning to , the in the early 1940s became the first pressurized airliner to enter service, revolutionizing passenger travel. Funded by company founder , who had established the firm in and prioritized innovative transport designs, the Stratoliner accommodated 33 passengers in a spacious, climate-controlled cabin. It cruised at altitudes up to 20,000 feet, maintaining a cabin pressure equivalent to 8,000 feet up to approximately 15,000 feet aircraft altitude, with higher cabin altitudes at maximum cruise, using engine-driven compressors for pressurization and allowing flights above turbulent weather layers. Early pressurization efforts faced significant engineering hurdles, particularly in sealing the fuselage to prevent air leaks and managing the added weight from reinforced structures and heavy compressors. Fuselage sealing required precise fabrication of airtight doors, windows, and joints using rubber gaskets and sealants, as even minor breaches could lead to rapid pressure loss; meanwhile, the structural reinforcements to withstand pressure differentials imposed significant weight penalties, reducing and range. These challenges were iteratively addressed through testing on prototypes like the XC-35, paving the way for reliable post-war implementations.

Modern Advancements

The introduction of the Boeing 707 in 1958 marked a pivotal advancement in cabin pressurization for the , utilizing efficient systems driven by engine turbocompressors to supply compressed air for high-altitude transatlantic flights. This system drew high-temperature bleed air from the engines to power turbo compressors, which then pumped fresh ram air into the cabin while an outflow valve regulated pressure to maintain a comfortable environment equivalent to 6,000–8,000 feet altitude during cruises up to 43,000 feet. Unlike earlier propeller aircraft limited by lower altitudes, this enabled reliable, fuel-efficient long-haul operations over oceans and mountains, fundamentally transforming . The (1952) was the first with cabin pressurization, though early metal fatigue issues from pressure cycles led to design improvements in subsequent models. In the , cabin pressure control systems evolved with the integration of microprocessors into digital controllers, minimizing manual pilot intervention and enhancing precision. These automated systems, building on early pneumatic designs from the , used electronic logic to monitor sensors and adjust outflow valves in real-time, reducing errors and improving reliability across commercial fleets. By the 2000s, the pioneered all-electric pressurization, replacing traditional with electrically driven compressors powered by high-capacity generators, which eliminated energy losses from engine and improved overall efficiency by up to 20%. This "bleedless" architecture also allowed for higher cabin humidity and better air quality, reducing passenger fatigue on long flights. Recent innovations from 2020 to 2025 include the seamless integration of sustainable aviation fuels (SAF) into existing pressurization systems without altering performance, as SAF's drop-in compatibility ensures no modifications to or electric compression mechanisms. Additionally, supersonic vehicles like the feature pressurized cabins designed for Mach 1.7 cruises, with engines positioned aft of the passenger section to isolate heat and noise while maintaining structural integrity under repeated cycles. Efficiency gains have been realized through reduced maximum differential pressures in modern airframes; for instance, the operates at a differential of up to 9.5 psi to achieve cabin altitudes around 6,000 feet at 41,000 feet cruise, enabling higher flight levels that save fuel compared to older 8.5–9 psi designs.

References

  1. https://www.faa.gov/sites/faa.gov/files/09_phak_ch7.pdf
  2. https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_25-20.pdf
  3. https://www.smithsonianmag.com/air-space-magazine/how-things-work-cabin-pressure-2870604/
  4. https://www.smithsonianmag.com/air-space-magazine/flying-in-comfort-32503022/
  5. https://www.boeing.com/commercial/787/by-design
  6. https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C/part-25/subpart-D/section-25.841
  7. https://www.ncbi.nlm.nih.gov/books/NBK232874/
  8. https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/phak/19_phak_ch17.pdf
  9. https://pmc.ncbi.nlm.nih.gov/articles/PMC2151873/
  10. https://www.faa.gov/pilots/safety/pilotsafetybrochures/media/dcs.pdf
  11. https://ntrs.nasa.gov/citations/19920013110
  12. https://www.ncbi.nlm.nih.gov/books/NBK470207/
  13. https://www.law.cornell.edu/cfr/text/14/25.841
  14. https://applications.icao.int/postalhistory/annex_8_airworthiness_of_aircraft.htm
  15. https://www.ncbi.nlm.nih.gov/books/NBK207472/
  16. https://www.ncbi.nlm.nih.gov/books/NBK234096/
  17. https://www.ecfr.gov/current/title-14/chapter-I/subchapter-F/part-91/subpart-C/section-91.211
  18. https://skybrary.aero/articles/outflow-valve-ofv
  19. https://aerosavvy.com/aircraft-pressurization/
  20. https://www.clean-aviation.eu/research-and-innovation/clean-sky-2/results-stories/electra-leads-the-charge-in-cabin-air-systems
  21. http://www.b737.org.uk/pressurisation.htm
  22. https://www.tasecs.com.tr/ecs-solutions/cabin-pressurization-control-systems-cpcs/cabin-pressure-control-system-componentss
  23. https://www.transglobaltraining.com/air-conditioning-pack/
  24. https://www.tasecs.com.tr/ecs-solutions/air-conditioning-systems/cooling-systems/air-cycle-cooling-systems
  25. https://bjtonline.com/business-jet-news/flying/the-quest-for-lower-cabin-altitude
  26. https://aeronamic.com/product/10kw-e-compressor-cabin-pressurization/
  27. https://www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-003-habitable-atmosphere.pdf
  28. https://www.nasa.gov/reference/environmental-control-and-life-support-systems-eclss/
  29. https://www.nasa.gov/history/50th-anniversary-of-nasa-deciding-on-a-mixed-gas-atmosphere-for-apollo-a-direct-result-of-the-apollo-fire/
  30. https://ntrs.nasa.gov/api/citations/20150010425/downloads/20150010425.pdf
  31. https://www.nasa.gov/news-release/nasas-orion-spacecraft-proves-sound-under-pressure/
  32. https://ntrs.nasa.gov/api/citations/20240003625/downloads/JSC-65829%2520Loads%2520-Dyn_Req_for_SF_Hardware-cover.pdf
  33. https://www.nasa.gov/wp-content/uploads/2023/07/eclss-technical-brief-ochmo.pdf
  34. https://www.nasa.gov/ochmo-tb-004-carbon-dioxide-2/
  35. https://www.spacex.com/updates
  36. https://www.humanmars.net/2025/11/lunar-starship-hls-interiors-by-spacex.html
  37. https://airwaysmag.com/new-post/how-aircraft-pressurization-systems
  38. https://pmc.ncbi.nlm.nih.gov/articles/PMC1790988/
  39. https://www.caa.co.uk/passengers-and-public/passenger-guidance/health-guidance/guidance-for-health-professionals/physiology-of-flight/
  40. https://aviation.stackexchange.com/questions/1728/does-the-b787-dreamliner-operate-with-higher-cabin-pressure
  41. https://docs.flybywiresim.com/pilots-corner/a32nx/a32nx-briefing/flight-deck/ovhd/cab-press/
  42. https://pmc.ncbi.nlm.nih.gov/articles/PMC7152029/
  43. https://aviation.stackexchange.com/questions/69730/why-would-short-haul-flights-be-pressurised-at-a-higher-cabin-pressure
  44. https://skybrary.aero/articles/aircraft-pressurisation-systems
  45. https://aerospace.honeywell.com/us/en/about-us/blogs/why-do-aircraft-use-cabin-pressurization
  46. https://www.boldmethod.com/learn-to-fly/systems/aircraft-cabin-pressurization-how-it-works/
  47. https://www.aircraftsystemstech.com/2017/05/control-of-aircraft-cabin-pressure.html
  48. http://ieeexplore.ieee.org/document/6492958/
  49. https://patents.google.com/patent/US20070049188A1/en
  50. https://aviation.stackexchange.com/questions/23997/why-does-the-cabin-pressurisation-switch-have-a-manual-mode
  51. https://contrail.in/a320-pressurization-system-2/
  52. https://www.pprune.org/tech-log/129039-pressurisation-systems.html
  53. https://patents.google.com/patent/WO2007040680A1/en
  54. https://www.researchgate.net/publication/335080941_Dynamic_modeling_of_a_cabin_pressure_control_system
  55. https://apps.dtic.mil/sti/tr/pdf/AD0020374.pdf
  56. https://ntrs.nasa.gov/api/citations/19690004637/downloads/19690004637.pdf
  57. https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR8903.pdf
  58. https://www.faa.gov/sites/faa.gov/files/data_research/research/med_humanfacs/oamtechreports/AM99-04.pdf
  59. https://skybrary.aero/sites/default/files/bookshelf/1269.pdf
  60. https://skybrary.aero/articles/korean-air-boeing-737-800-ke189-loss-cabin-pressurisation-and-rapid-descent
  61. https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_25.571-1B.pdf
  62. https://www.federalregister.gov/documents/2013/01/09/2013-00238/requirements-for-chemical-oxygen-generators-installed-on-transport-category-airplanes
  63. https://www.faa.gov/pilots/safety/pilotsafetybrochures/media/oxygen_equipment.pdf
  64. http://aviationstudys.blogspot.com/2015/06/testing-of-pressurisation-systems.html
  65. https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_25_571-1D_.pdf
  66. https://sm4.global-aero.com/articles/the-need-to-ensure-skill-proficiency-in-oxygen-mask-use/
  67. https://hartzellprop.com/aircraft-spotlight-lockheed-xc-35/
  68. https://airwaysmag.com/new-post/first-successful-pressurized-airplane-cabin
  69. https://www.thisdayinaviation.com/tag/lockheed-xc-35/
  70. https://www.aopa.org/news-and-media/all-news/2011/september/pilot/the-lady-has-a-history
  71. https://airandspace.si.edu/stories/editorial/defending-superbomber-b-29s-central-fire-control-system
  72. https://www.historylink.org/file/3598
  73. https://time.com/archive/6834309/aviation-boeing-at-50/
  74. https://ig.space/commslink/airborne-ecs-pioneer-boeing-307-the-first-pressuri/
  75. https://asma.org/wp-content/uploads/2008/01/cruising-altitudes.pdf
  76. https://www.707jet.com/systems/pneumatic-system/
  77. https://www.sae.org/papers/cabin-pressure-control-systems-general-aviation-aircraft-800606
  78. https://airwaysmag.com/new-post/the-boeing-787-bleedless-air-system
  79. https://www.sciencedirect.com/science/article/pii/S2772656824000757
  80. https://simpleflying.com/boom-supersonic-overture-quadjet/
  81. https://simpleflying.com/airbus-a350-low-cabin-altitude/
Add your contribution
Related Hubs
User Avatar
No comments yet.