Uncontrolled decompression is an unplanned drop in the pressure of a
sealed system, such as an aircraft cabin, and typically results from human
error, material fatigue, engineering failure, or impact, causing a pressure
vessel to vent into its lower-pressure surroundings or fail to pressurize at
all. Such decompression may be classed as
Explosive, Rapid, or Slow: Explosive decompression is violent, the
decompression being too fast for air to safely escape from the lungs.
Rapid decompression, while still fast, is slow enough to allow the lungs to
vent. Slow or gradual decompression occurs so
slowly that it may not be sensed before hypoxia sets in.
Description 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 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 relative humidity of cabin air rapidly changes as
the air 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.
= 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 as an
aircraft climbs to altitude. An example of this is the 2005 Helios Airways
Flight 522 crash, in which the pilots failed to check the aircraft was
pressurising automatically and then to react to the warnings that the aircraft
was depressurising, eventually losing consciousness from hypoxia.
Pressure vessel seals and testing Seals in high-pressure vessels are also
susceptible to explosive decompression; the O-rings or rubber gaskets used to
seal pressurised pipelines tend to become saturated with high-pressure
gases. If the pressure inside the vessel is suddenly released, then the gases
within the rubber gasket may expand violently, causing blistering or
explosion of the material. For this reason, it is common for military and
industrial equipment to be subjected to an explosive decompression test before
it is certified as safe for use. Myths
= Exposure to a vacuum causes the body to explode=
This persistent myth is based on a failure to distinguish between two types
of decompression: the first, from normal atmospheric pressure to a vacuum; the
second, from an exceptionally high pressure to normal atmospheric pressure.
The first type, a sudden change from normal atmospheric pressure to a vacuum,
is the more common. 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. It is also possible that pulmonary barotrauma will occur if the
breath is forcibly held. The second type is rare, since the only
normal situation in which it can occur is during decompression after deep-sea
diving. In fact, there is only a single well-documented occurrence: the 1983
Byford Dolphin incident in the North Sea, in which a catastrophic pressure
drop of eight atmospheres, from nine atmospheres to one atmosphere
instantaneously, caused massive and lethal barotrauma, including the actual
explosion of one diver. A similar but fictional death was shown in the James
Bond film Licence to Kill, when a character's head explodes after his
hyperbaric chamber is rapidly depressurized. Neither of these
incidents would have been possible if the pressure drop had been only from
normal atmosphere to a vacuum. = Bullets cause explosive decompression=
Aircraft fuselages are designed with ribs to prevent tearing; the size of the
hole is one of the factors that determine 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.
Flight Attendant C.B. Lansing was blown from Aloha Airlines Flight 243 in 1988
when a large section of cabin roof) detached; the report states she was
swept overboard rather than blown through the resulting hole. The Air
Crash Investigation documentary report on Flight 243 notes that the 'tear line'
construction of the aircraft was supposed to prevent such a large slab
failure. Working from passenger accounts, forensic evidence including
NTSB photographs, and stress calculations, experts speculated that
the air hostess was blown against the foot-square hole initially permitted by
the tear strips, blocking it: this would have caused a 10 atmosphere pressure
spike, hence the much greater material failure. One corrosion engineer takes
the view that the tear straps could also have been defeated by the airstream
impact through Lansing's body. 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 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
nearly 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. Pressurization apparently 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 287 passengers and 14 crew members from
fire and smoke. Prior to 1996, approximately 6,000 large
commercial transport airplanes were type certified 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 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 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.
International standards The Depressurization Exposure Integral
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. In the majority of cases the problem is
relatively manageable for aircrew. Consequently where passengers and the
aircraft do not suffer any ill-effects, the incidents tend not to be considered
notable. Injuries resulting from decompression incidents are rare.
Decompression incidents do not occur solely in aircraft—the Byford Dolphin
incident is an example of violent explosive decompression on an oil rig. A
decompression event is an effect of a failure caused by another problem, but
the decompression event may worsen the initial issue.
See also Cabin pressurization
Time of useful consciousness Space exposure
Decompression Decompression
Notes ^ Notable exceptions include the Airbus
A380, Boeing 787, and Concorde References
External links Human Exposure to Vacuum
Will an astronaut explode if he takes off his helmet?