Pressurization is essential over 3,000 m (10,000 ft) to prevent crew and passengers from becoming unconscious through the lack of oxygen (hypoxia) in the thin air above that altitude. Pressurization also removes or alleviates a number of other adverse physiological effects of altitude (see below) and increases passenger comfort generally.
Flights above 3,000 m (10,000 ft) in unpressurised aircraft put crew and passengers at risk from four separate sources, hypoxia, altitude sickness, decompression sickness and barotrauma as follows:
Hypoxia. The low local partial pressure of oxygen at 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 1500 m (5000 ft) above sea level although most passengers can tolerate altitudes of 2500 m (8,000 ft) without ill effect. At this altitude, there is about 25% less oxygen than there is at sea level.. Hypoxia may be addressed by the administration of supplemental oxygen, usually through an oxygen mask sometimes through a nasal cannula.
Altitude sickness. The low local partial pressure of carbon dioxide (CO2) causes CO2 to out-gas from the blood raising the blood pH and inducing alkalosis. Passengers may experience fatigue, nausea, headaches, sleeplessness and on extended flights even pulmonary oedema. 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 envelopes the body in a pressurised environment, this is clearly impractical for commercial passengers.
Decompression sickness. The low local 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 for compressed air divers on ascent from depth. Symptoms may include the early symptoms of the diver's bends: tiredness, forgetfulness, headache, stroke, thrombosis subcutaneous itching but rarely the full symptoms of the bends. 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 (aerotitus) 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 and can exacerbate or precipitate pre-existing medical conditions such as pneumothorax (collapsed lung).
Pressurisation of aircraft cabins above 3000 m (10,000 ft) generally avoids significant hypoxia, altitude sickness, decompression sickness and barotrauma. An oxygen system is retained but only for emergency use and only intended to allow time to descend to a safe altitude.
The pressure maintained within the cabin is referred to as the equivalent effective cabin altitude or more normally, the ‘cabin altitude’. Cabin altitude is not normally maintained at ground level (0ft) pressure throughout the flight because doing so stresses the fuselage and uses more fuel. For an aircraft planning to cruise at 40,000ft cabin altitude is programmed to rise gradually from take-off to around 8,000ft and to then reduce gently to match the ambient air pressure of the destination. That destination may be significantly above sea level and this needs to be taken into account, for example, El Alto International Airport in La Paz, Bolivia is 4,061 metres (13,323 ft) above sea level.
Pressurisation is achieved by the design of an airtight fuselage engineered to be pressurised, a source of compressed air and 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 or turboprop propulsion engine, usually the second or third last compressor ring. By the time the cold outside air has reached this part of the compressor it has been compressively heated to around 200 °C (392 °F) and is at a very high pressure. It is then expanded and cooled to a suitable temperature by passing it through a heat exchanger and air cycle machine ('the packs system'). There is no need to further heat or refrigerate the air. Typically, compressed air is bled from at least two propulsion engines each system being fully redundant. 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 a fully redundant, duplicated electronic controller for maintaining pressurisation along with a manual back-up system.
All exhaust air is dumped to atmosphere via a valve, usually at the rear of the fuselage. This valve controls the cabin pressure and also acts as a safety relief. The pilot can alter the cabin pressure at will through this valve. Operational considerations typically require it to be set at 6,000 to 8,000ft giving a pressure differential between the cabin and the outside air of around 7.5–8 psi (52–55 kPa). If the cabin were maintained at sea level pressure while flown above 35,000 feet (10.7 km) the pressure differential would exceed 9 psi (60 kPa) limiting the structural life of the fuselage.
Bleed air extraction from the engines reduces engine efficiency only slightly but introduces a danger of oils and other chemicals from the engine being supplied to the cabin. Aircraft cabin air quality has become an occupational health and safety issue. Some aircraft, such as the Boeing 787 have re-introduced the use of electric compressors previously used on piston-engined airliners to provide pressurisation. Because the use of electric compressors increases the electrical generation load on the engines and introduces a number of stages of energy transfer 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, simplifies engine design, avoids the need to run high pressure pipework around the aircraft and provides greater design flexibility.
Because cabin altitudes are maintained at up to 2,500m (8,000ft) pressurisation does not eliminate all physiological problems. Passengers with conditions such as a pneumothorax are advised not to fly until fully healed; pain may still be experienced in the ears and sinuses by people suffering from a cold or other infection; SCUBA divers flying within the 'no fly' period after a dive may risk decompression sickness because their dive tables are calibrated to sea level. The aircraft Captain may elect to maintain cabin altitude at sea level on request to address compelling pressure-sensitive medical needs of a passenger but at an operational cost to the airline arising from fuselage fatigue, see section below.
The airliners that pioneered pressurised cabin systems include:
The first airliner with a pressurized cabin was the Boeing 307 Stratoliner, built 1938, prior to World War II, though only ten were produced. 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 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 air and operated below 20,000 ft where the piston engine is more efficient. Designing a pressurised fuselage to cope with this altitude was within the engineering and metallurgical knowledge of the time. The introduction of jet airliners required a large increase in cruise altitude to 30,000 ft where the jet engine is more efficient. This increase in altitude required far more rigorous engineering of the fuselage and in the beginning not all the engineering problems were understood.
The world’s first commercial jet airliner was the British de Havilland Comet (1949) designed with a service ceiling of 36,000' (10973 m). It was the first time that a large diameter, pressurised 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 pressurised 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 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 you see 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 unusually high pressure differentials, as of necessity it flew at unusually high alitude (up to 60,000 ft) while the cabin altitude was maintained at 6000 ft. This made the vehicle significantly heavier and contributed to the high cost of a flight. Concorde also had to have smaller than normal cabin windows to limit decompression speed in the event of window failure.
Nowadays, nearly all commercial airliners can maintain their cabin altitude at sea level throughout the flight if the captain sees a compelling reason to do so. In practice, cabin altitude is usually maintained well above sea level to reduce fuel consumption and the costs of fuselage fatigue inspections, which are driven by the number and depth of pressurisation cycles.
The designed operating cabin altitude for proposed aircraft now in development is falling and this is expected to reduce any remaining physiological problems. The Boeing 787 will feature a standard cabin altitude of 1,800m (6,000ft); the Airbus A350 is considering a cabin altitude as low as 1,500m (5,000ft).
Gradual or slow decompression, sometimes caused by a failure to pressurize the cabin with an increase in altitude, is dangerous because it may not be detected. The Helios Airways 2005 accident is a good example . Warning systems may be ignored, misinterpreted or fail and self-recognition of the subtle effects of hypoxia really depends upon previous experience and hypoxia familiarization training. Unfortunately, in most countries this has been largely restricted to military hypobaric chamber training with its risk of decompression sickness and barotrauma. Newer reduced oxygen breathing systems are more accessible, safer and provide valuable practical experience. Adding such practical training to knowledge required by regulatory authorities is likely to increase hypoxia awareness and aviation safety.
Hypoxia may result in loss of consciousness without emergency oxygen. The Time of Useful Consciousness varies depending on the altitude. Additionally, the air temperature will plummet to the ambient outside temperature with a danger of hypothermia or frostbite.
Failure of cabin pressurisation above 3000 m (10,000 ft) for whatever reason requires an emergency descent to below 10,000ft and the deployment of an oxygen mask above each seat. In almost all pressurised jet airliners passenger oxygen masks are automatically deployed when the cabin altitude exceeds 14,000 feet. The Boeing 737 emergency equipment is typical.
It is generally impossible to lose pressurisation through opening a cabin door in flight, either accidentally or intentionally. If the pressure inside the cabin exceeds the pressure outside the doors are forced shut and will not open until the pressure is equalised. Cabin doors, including the emergency exits, but not all cargo doors, open inwards, or must first be pulled inwards and then rotated before they can be pushed out through the door frame because at least one dimension of the door is larger than the door frame. Even if the pressure was first equalised the doors are locked from the cockpit in flight anyway.