Uncontrolled decompression

The term uncontrolled decompression here refers to the unplanned depressurisation of vessels that are occupied by people; for example, a pressurised 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 US Federal Aviation Administration recognizes three distinct types of decompression events in aircraft: explosive, rapid, and gradual decompression. Normally, the time required to release air from the lungs without restrictions, such as masks, is 0.2 seconds. Military pilots with oxygen masks must 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. Gradual 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 the cabin as an aircraft climbs to altitude. An example of this is the 2005 Helios Airways Flight 522 crash, in which the maintenance service left the pressurization system in manual mode and the pilots did not check the pressurization system. As a result, they (as well as most of the passengers and crew) suffered a loss of consciousness due to hypoxia (lack of oxygen). The plane continued to fly due to the autopilot system and eventually crashed due to fuel exhaustion after leaving its flight path. ==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 • Altitude sickness • Frostbite or hypothermia from exposure to freezing cold air at high altitude The second incident occurred on April 17, 2018, when a woman on Southwest Airlines Flight 1380 was partially blown through an airplane passenger window that had broken from a similar engine failure. Although the other passengers were able to pull her back inside, she later died from her injuries. In both incidents, the plane landed safely with the sole fatality being the person seated next to the window involved. ==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 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 prevent losing cabin pressure through them by making it nearly impossible to open them in flight, whether 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 equalized. 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. Pressurization prevented the doors of Saudia Flight 163 from being opened on the ground after the aircraft made a successful emergency landing, resulting in the deaths of all 287passengers and 14crew members from fire and smoke. Prior to 1996, approximately 6,000large commercial transport airplanes were type certified to fly up to , 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 certified to operate above 25,000 feet (FL 250; 7,600 m), it "must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of after any probable failure condition in the pressurization system." In the event of a decompression 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 for more than 2minutes, nor exceeding an altitude of at any time. In 2004, Airbus successfully petitioned the FAA to allow cabin pressure of the A380 to reach in the event of a decompression incident and to exceed 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. ==International standards==

The Depressurization Exposure Integral (DEI) is a quantitative model that is used by the FAA to enforce compliance with decompression-related design directives. The model relies on the fact that the pressure that the subject is exposed to and the duration of that exposure are the two most important variables at play in a decompression event. Other national and international standards for explosive decompression testing include: • MIL-STD-810, 202 • RTCA/DO-160 • NORSOK M710 • API 17K and 17J • NACE TM0192 and TM0297 • TOTALELFFINA SP TCS 142 Appendix H ==Notable decompression accidents and incidents==

Decompression incidents are not uncommon on military and civilian aircraft, with approximately 40–50 rapid decompression events occurring worldwide annually. However, in most cases the problem is manageable, injuries or structural damage rare and the incident not considered notable. One notable case was Southwest Airlines Flight 1380 in 2018, where an uncontained engine failure ruptured a window, causing a passenger to be partially blown out. Decompression incidents do not occur solely in aircraft; the Byford Dolphin accident is an example of violent explosive decompression of a saturation diving system on an oil rig. A decompression event is often the result of a failure caused by another problem (such as an explosion or mid-air collision), but the decompression event may worsen the initial issue. ==Myths==

A bullet through a window may cause explosive decompression In 2004, the TV show MythBusters examined whether explosive decompression occurs when a bullet is fired through the fuselage of an airplane informally by way of several tests using a decommissioned pressurised DC-9. A single shot through the side or the window did not have any effect – it took actual explosives to cause explosive decompression – suggesting that the fuselage is designed to prevent people from being blown out. Professional pilot David Lombardo states that a bullet hole would have no perceived effect on cabin pressure as the hole would be smaller than the opening of the aircraft's outflow valve. However, NASA scientist Geoffrey A. Landis points out that the impact depends on the size of the hole, which can be expanded by debris that is blown through it. Landis went on to say that it would take about 100 seconds for pressure to equalise through a roughly hole in the fuselage of a Boeing 747. He then stated that anyone sitting next to the hole would have about half a ton of force pulling them towards it. Fictional accounts of this include a scene in Goldfinger, when James Bond kills the eponymous villain by blowing him out a passenger window and Die Another Day, when an errant gunshot shatters a window on a cargo plane and rapidly expands, causing multiple enemy officials, henchmen and the main villain to be blown out to their deaths. Exposure to a vacuum causes the body to explode This persistent myth is based on a failure to distinguish between two types of decompression and their exaggerated portrayal in some fictional works. The first type of decompression deals with changing from normal atmospheric pressure (one atmosphere) to a vacuum (zero atmosphere) which is usually centered around space exploration. The second type of decompression changes from exceptionally high pressure (many atmospheres) to normal atmospheric pressure (one atmosphere) as may occur in deep-sea diving. The first type is more common as pressure reduction from normal atmospheric pressure to a vacuum can be found in both space exploration and high-altitude aviation. Research and experience have shown that while exposure to a vacuum causes swelling, human skin is tough enough to withstand the drop of one atmosphere. The most serious risk from vacuum exposure is hypoxia, in which the body is starved of oxygen, leading to unconsciousness within a few seconds. Rapid uncontrolled decompression can be much more dangerous than vacuum exposure itself. Even if the victim does not hold their breath, venting through the windpipe may be too slow to prevent the fatal rupture of the delicate alveoli of the lungs. Eardrums and sinuses may also be ruptured by rapid decompression, and soft tissues may be affected by bruises seeping blood. If the victim somehow survived, the stress and shock would accelerate oxygen consumption, leading to hypoxia at a rapid rate. At the extremely low pressures encountered at altitudes above about , the boiling point of water becomes less than normal body temperature. Dramatized fictional accounts of this include a scene from the film Licence to Kill, when a character's head explodes after his hyperbaric chamber is rapidly depressurized, and another in the film DeepStar Six, wherein rapid depressurization causes a character to hemorrhage profusely before exploding in a similar fashion. == See also ==