Τετάρτη, 20 Απριλίου 2011

Cabin pressurization [EN]

Cabin pressurization is the active pumping of compressed air into an aircraft cabin when flying at altitude to maintain a safe and comfortable environment for crew and passengers in the low outside atmospheric pressure. Pressurization is essential over 3,000 metres (9,800 ft) above sea level to protect crew and passengers from the risk of hypoxia and a number of other physiological problems that occur in the thin air above that altitude; it also serves to generally increase passenger comfort. At a cruising altitude of 39,000 feet, a Boeing 767's cabin will be pressurized to an altitude of 6,900 feet.

The need for cabin pressurization

Flights above 3,000 metres (9,800 ft) in unpressurized aircraft put crew and passengers at risk of four illnesses: hypoxia, altitude sickness, decompression sickness and barotrauma.
Hypoxia. The low 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 1,500 metres (4,900 ft) although most passengers can tolerate altitudes of 2,500 metres (8,200 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 pressurized 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.

Pressurized flight

Maintaining the cabin pressure altitude to below 3,000 metres (9,800 ft) generally avoids significant hypoxia, altitude sickness, decompression sickness and barotrauma. Emergency oxygen systems are installed, both for passengers and cockpit crew, to prevent loss of consciousness in the event that cabin pressure rapidly falls below the level equivalent to 10,000 feet with respect to mean sea level. Those systems contain more than enough oxygen for all on board, to give the pilot adequate time to descend the plane to a safe altitude, where supplemental oxygen is not needed. Federal Aviation Administration (FAA) regulations in the U.S. mandate that the cabin altitude may not exceed 8,000 feet at the maximum operating altitude of the airplane under normal operating conditions. Prior to 1996, approximately 6,000 large commercial transport airplanes were type certificated to fly up to 45,000 feet, without being required 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. For aircraft certified to operate above 25,000 feet, it "must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 feet 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 so that occupants will not be exposed to a cabin altitude exceeding 25,000 feet for more than 2 minutes, nor exceeding an altitude of 40,000 feet at any time. In practice, that new Federal Aviation Regulations amendment imposes an operational ceiling of 40,000 feet on the majority of newly designed commercial aircraft.
However, companies that build the newer designed aircraft can apply for exemption from that more restrictive rule. In 2004, Airbus successfully petitioned the FAA to allow cabin pressure of the A380 to reach 43,000 feet in the event of a decompression incident, and to exceed 40,000 feet for one minute. This special exemption allows the A380 to operate at a higher altitude than other newly designed civilian aircraft, which have not yet been granted a similar exemption. 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 average mean sea level (MSL) pressure (1013.25 hPa, or 29.921 inches of mercury) throughout the flight, because doing so would cause the designed differential pressure limits of the fuselage to be exceeded. An aircraft planning to cruise at 40,000 ft (12,000 m) is programmed to rise gradually from take-off to around 8,000 ft (2,400 m) in cabin pressure altitude, and to then reduce gently to match the ambient air pressure of the destination. 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 has been heated to around 200 °C (392 °F) and is at a very high pressure. 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 ('the packs system'). In some of the 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 other sections.
At least two engines provide compressed bleed air for all of 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.
Outflow and pressure relief valve on a Boeing 737-800
 
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. In the event that 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 pressure is as near to sea level pressure as practical, without exceeding the maximum differential limit of 8.60 psi. At 39,000 feet, the cabin pressure would be automatically maintained at about 6,900 feet (450 feet lower than Mexico City), which is about 11.5 psi of atmosphere pressure (76 kPa). Some aircraft, such as the Boeing 787, have re-introduced the use of 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, simplifies engine design, avoids the need to run high pressure pipework around the aircraft and provides greater design flexibility. Cabin altitudes are maintained at up to 2,500 metres (8,200 ft), so pressurization does not eliminate all physiological problems. Passengers with conditions such as 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 accumulated nitrogen in their bodies can form bubbles when exposed to reduced cabin pressure.

History

The aircraft that pioneered pressurized cabin systems include:
  • Junkers Ju 49 (1931 - a German experimental aircraft purpose built to test the concept of cabin pressurization)
  • Lockheed XC-35 (1937 - the first American pressurized aircraft also purpose built to test the concept)
  • Boeing 307 (1938 - the first pressurized piston airliner)
  • Lockheed Constellation (1943 - the first pressurized airliner in wide 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)
The first airliner with a pressurized cabin was the Boeing 307 Stratoliner, built 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 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 pressurized 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 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 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 unusually high pressure differentials, as of necessity it flew at unusually high altitude (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. The designed operating cabin altitude for new aircraft is falling and this is expected to reduce any remaining physiological problems. The lowest cabin altitude of any airliner either already flying or in current development is the A380, designed to maintain a cabin altitude of 1,520 m (5,000 ft).

Loss of pressurization

Passenger oxygen mask deployment
 
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 3,000 metres (9,800 ft) requires an emergency descent to 3,000 metres (9,800 ft) or the minimum altitude that maintains terrain clearance (MSA), whichever is higher, and the deployment of an oxygen mask for each seat. Without emergency oxygen hypoxia may lead to loss of consciousness and a subsequent loss of control of the aircraft, the Time of Useful Consciousness varying according to altitude. The air temperature will also plummet to the ambient outside temperature with a danger of hypothermia or frostbite.

Uncontrolled decompression

 Uncontrolled decompression refers to an unexpected drop in the pressure of a sealed system, such as an aircraft cabin. Where the speed of the decompression occurs faster than air can escape from the lungs, this is known as explosive decompression (ED), and is associated with explosive violence. Where decompression is still rapid, but not faster than the lungs can decompress, this is known as rapid decompression. Lastly, slow decompression or gradual decompression occurs so slowly that humans may not detect it before hypoxia sets in.
Generally uncontrolled decompression results from human error, material fatigue, engineering failure or impact, that causes a pressure vessel either not to pressurize, or to vent into lower-pressure surroundings.

The term uncontrolled decompression here refers to the unplanned depressurisation of vessels that are occupied by people, for example an aircraft cabin at high altitude, a spacecraft, or a hyperbaric chamber. For the catastrophic failure of other pressure vessels used to contain gas, liquids, or reactants under pressure, the term explosion is more commonly used, or other specialised terms such as BLEVE may apply to particular situations.
Decompression can occur due to structural failure of the pressure vessel, or failure of the compression system itself. The speed and violence of the decompression is affected by the size of the pressure vessel, the differential pressure between the inside and outside of the vessel and the size of the leak hole.
The Federal Aviation Administration recognizes three distinct types of decompression events in aircraft:
  • Explosive decompression
  • Rapid decompression
  • Gradual decompression

Explosive decompression

Explosive decompression occurs at a rate swifter than that at which air can escape from the lungs, typically in less than 0.1 to 0.5 seconds. The risk of lung trauma is very high, as is the danger from any unsecured objects that can become projectiles because of the explosive force, which may be likened to a bomb detonation.
After an explosive decompression within an aircraft, a heavy fog may immediately fill the interior as the cabin air very rapidly cools and condenses. Military pilots with oxygen masks have to pressure-breathe, whereby the lungs fill with air when relaxed, and effort has to be exerted to expel the air again.

Rapid decompression

Rapid decompression typically takes more than 0.1 to 0.5 seconds, allowing the lungs to decompress more quickly than the cabin. The risk of lung damage is still present, but significantly reduced compared with explosive decompression.

Slow decompression

Slow, or gradual, decompression occurs slowly enough to go unnoticed and might only be detected by instruments. This type of decompression may also come about from a failure to pressurize as an aircraft climbs to altitude. An example of this is the Helios Airways Flight 522 crash, in which the pilots failed to check the aircraft was pressurising automatically and then react to the warnings that the aircraft was depressurising.

Fallacies

Exposure to a vacuum causes the body to explode

This persistent myth is based on a misunderstanding of explosive decompression possibly fuelled by many misrepresentations in popular media such as the film Licence to Kill; in this film one character's head explodes after his hyperbaric chamber is rapidly depressurized. Research and experience in space exploration and high-altitude aviation have shown that while exposure to vacuum causes swelling, human skin is tough enough to withstand the drop of one atmosphere although the resulting hypoxia will cause unconsciousness after a few seconds. Misconceptions may also arise from confusion with diving accidents where the pressure differentials are much higher than those in space. In the Byford Dolphin accident a catastrophic pressure drop of eight atmospheres caused massive, lethal, barotrauma but this does not apply to a loss of only one atmosphere although some pulmonary barotrauma is possible if the breath is forcibly held.

Bullets cause explosive decompression

Aircraft fuselages are designed with ribs to prevent tearing; the size of the hole is one of the factors that determines the speed of decompression, and a bullet hole is too small to cause rapid or explosive decompression.

A small hole will blow people out of a fuselage

The television program Mythbusters examined this belief informally using a pressurised aircraft and several scale tests. The Mythbusters approximations suggested that fuselage design does not allow this to happen. There has been no scientific verification of their results: conclusions obtained by Mythbusters should not be taken as scientifically conclusive. An air hostess was blown from Aloha Airlines Flight 243 when a large section of cabin roof (about 18' x 25') detached; the report states she was swept overboard rather than sucked through the hole.

Decompression injuries

The following physical injuries may be associated with decompression incidents:
  • Hypoxia is the most serious risk associated with decompression, especially as it may go undetected or incapacitate the aircrew.
  • Barotrauma: an inability to equalize pressure in internal air spaces such as the middle ear or gastrointestinal tract, or more serious injury such as a burst lung.
  • Decompression sickness.
  • Physical trauma caused by the violence of explosive decompression, which can turn people and loose objects into projectiles.
  • Altitude sickness
  • Frostbite or hypothermia from exposure to freezing cold air at high altitude.

Implications for aircraft design

Modern aircraft are specifically designed with longitudinal and circumferential reinforcing ribs in order to prevent localised damage from tearing the whole fuselage open during a decompression incident. However, decompression events have nevertheless proved fatal for aircraft in other ways. In 1974, explosive decompression onboard Turkish Airlines Flight 981 caused the floor to collapse, severing vital flight control cables in the process. The FAA issued an Airworthiness Directive the following year requiring manufacturers of wide-body aircraft to strengthen floors so that they could withstand the effects of in-flight decompression caused by an opening of up to 20 square feet (1.9 m2) in the lower deck cargo compartment. Manufacturers were able to comply with the Directive either by strengthening the floors and/or installing relief vents called "dado panels" between the passenger cabin and the cargo compartment.
Cabin doors are designed to make it almost impossible to lose pressurization through opening a cabin door in flight, either accidentally or intentionally. The plug door design ensures that when 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.
Prior to 1996, approximately 6,000 large commercial transport airplanes were type certificated to fly up to 45,000 feet, without being required to meet special conditions related to flight at high altitude. In 1996, the FAA adopted Amendment 25-87, which imposed additional high-altitude cabin-pressure specifications, for new designs of aircraft types. For aircraft certificated to operate above 25,000 feet (FL 250), it "must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 feet 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 aircraft must be designed so that occupants will not be exposed to a cabin altitude exceeding 25,000 feet for more than 2 minutes, nor exceeding an altitude of 40,000 feet at any time. In practice, that new FAR amendment imposes an operational ceiling of 40,000 feet on the majority of newly designed commercial aircraft.
In 2004, Airbus successfully petitioned the FAA to allow cabin pressure of the A380 to reach 43,000 feet in the event of a decompression incident, and to exceed 40,000 feet for one minute. This special exemption allows that new aircraft to operate at a higher altitude than other newly-designed civilian aircraft, which have not yet been granted a similar exemption.

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