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A plug door is a
door A door is a hinged or otherwise movable barrier that allows ingress (entry) into and egress (exit) from an enclosure. The created opening in the wall is a ''doorway'' or ''portal''. A door's essential and primary purpose is to provide security b ...
designed to seal itself by taking advantage of pressure difference on its two sides and is typically used on aircraft with
cabin pressurization 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 passengers and crew flying at high altitudes. For aircraft, this air is u ...
. The higher pressure on one side forces the usually wedge-shaped door into its socket, making a good seal and preventing it from being opened until the pressure is released. Conversely, a non-plug door relies on the strength of the locking mechanism to keep the door shut.


Aircraft

The plug door is often seen on aircraft with pressurized cabins. Due to the air pressure within the aircraft cabin being higher than that of the surrounding atmosphere, the door seals itself closed as the aircraft climbs and the pressure differential increases. This prevents accidental opening of the door. In the event of a decompression, with there no longer being a pressure differential, the doors may be opened, and as such most airlines' operating procedures require cabin crew to keep passengers away from the doors until the aircraft has safely landed. On some aircraft the plug door opens partially inward, and through a complex hinge design can be tilted to fit through the fuselage opening, or the door may have locking hinged panels at the top and bottom edges that can make it smaller than the opening, so that it may be swung outward. Plug doors are used on most modern airliners, particularly for the small passenger doors. However, since plug doors must open inward, the design is disadvantageous for cargo doors. Due to its large area, the cargo door on an airliner cannot be swung inside the fuselage without taking up a considerable amount of valuable cargo space. For this reason, these doors often open outward and use a locking mechanism with multiple pins or hatch dogs to prevent opening while in flight.


Spacecraft

An inward opening plug hatch design was used on the Block I
Apollo Command Module The Apollo command and service module (CSM) was one of two principal components of the United States Apollo spacecraft, used for the Apollo program, which landed astronauts on the Moon between 1969 and 1972. The CSM functioned as a mother shi ...
, because the explosive-release hatch of
Gus Grissom Virgil Ivan "Gus" Grissom (April 3, 1926 – January 27, 1967) was an American engineer, pilot in the United States Air Force, and member of the Mercury Seven selected by National Aeronautics and Space Administration's (NASA) as Project Mercur ...
's Mercury capsule ''
Liberty Bell 7 Mercury-Redstone 4 was the second United States human spaceflight, on July 21, 1961. The suborbital Project Mercury flight was launched with a Mercury-Redstone Launch Vehicle, MRLV-8. The spacecraft, Mercury capsule #11, was nicknamed the ''Lib ...
'' prematurely blew at the end of the flight, causing the capsule to sink in the Atlantic Ocean and nearly resulting in Grissom's drowning. The Apollo cabin was pressurized at launch to above standard sea level pressure, which sealed the hatch. A cabin fire during a 1967
Apollo 1 Apollo 1, initially designated AS-204, was intended to be the first crewed mission of the Apollo program, the American undertaking to land the first man on the Moon. It was planned to launch on February 21, 1967, as the first low Earth orbit ...
ground test raised the pressure even higher (), and made the hatch impossible to remove for the crew to escape. This killed Grissom along with his entire crew, Edward H. White and Roger Chaffee. Because of this, NASA decided to change to a quick-release, outward opening hatch on the Command Module. Plug hatches were retained for the CM docking hatch and the two hatches on the
Apollo Lunar Module The Apollo Lunar Module (LM ), originally designated the Lunar Excursion Module (LEM), was the lunar lander spacecraft that was flown between lunar orbit and the Moon's surface during the United States' Apollo program. It was the first crewed ...
, as the risk of fire in the low-pressure () in-space atmosphere was much lower. Plug hatches were also used for the inner airlock hatch on the
Space Shuttle The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program n ...
. Currently, they are used on the
International Space Station The International Space Station (ISS) is the largest Modular design, modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos ( ...
, as well as on the hatch between the Orbital Module and Descent Module of the Russian
Soyuz spacecraft Soyuz () is a series of spacecraft which has been in service since the 1960s, having made more than 140 flights. It was designed for the Soviet space program by the Korolev Design Bureau (now Energia). The Soyuz succeeded the Voskhod spacecr ...
.


Deep-sea vehicles

Deep-submergence vehicles such as the ''
Alvin Alvin may refer to: Places Canada *Alvin, British Columbia United States *Alvin, Colorado *Alvin, Georgia *Alvin, Illinois * Alvin, Michigan *Alvin, Texas * Alvin, Wisconsin, a town *Alvin (community), Wisconsin, an unincorporated community Other ...
'' use a plug hatch which is sealed inward by the pressure of the ocean water.


References

{{reflist Vehicle doors Aircraft doors