Plug Door
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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 securit ...
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 aircraft cabin, cabin of an aircraft or spacecraft in order to create a safe and comfortable environment for humans flying at high altitudes. For aircraft, this air i ...
. The higher pressure on one side forces the usually
wedge A wedge is a triangle, triangular shaped tool, a portable inclined plane, and one of the six simple machines. It can be used to separate two objects or portions of an object, lift up an object, or hold an object in place. It functions by conver ...
-shaped door into its socket to create a seal, which prevents it from being opened until the pressure is equalised on both sides. A non-plug door relies on the strength of the locking mechanism to keep the door shut.


Aircraft

Plug doors are often used on aircraft with pressurized cabins because this design of door provides a simple and reliable means of securing the door and preventing leakage of air. As the air pressure outside becomes increasingly lower than that inside the cabin, the pressure difference causes the door to seal itself closed and prevent leakage. The region of the fuselage around the opening for the door must be reinforced and this adds weight, so for large doors an alternative to the plug design is commonly used. On some aircraft, a complex hinge design allows half the door to open inward like a plug door, and the other half to open outward. Initially the door can be rotated to fit through the opening for the non-plug half so the whole door then opens outward; this is called a ''semi-plug door''. Alternatively, the door may have locking hinged panels at the top and bottom edges that can make it smaller than the opening, so it may be swung outward. Plug doors are used on most pressurized airliners, particularly for emergency exits and small passenger doors. However, since plug doors must open inward, the plug design is disadvantageous for large doors. Semi-plug doors are often used for the main passenger doors. For large cargo doors, if the door were to be swung inside the fuselage, it would prevent valuable cargo being loaded into the large space it would occupy. These doors usually open outward using a secure locking mechanism with multiple pins or hatch dogs to prevent opening while in flight with a large outward-facing pressure differential. This design of door can be lighter than a plug door of the same dimensions due to the need for reinforcing the opening around a plug door. A ''door plug'', not to be confused with a ''plug door'', is used in some of Boeing's 737-900ER and MAX 9s instead of an actual emergency exit door. It can be removed outwards for maintenance and requires stop pads on the airframe as well as stop fittings on the door panel to prevent outward movement during operation. This did not prevent the door plug from detaching on Alaska Airlines Flight 1282 on January 5, 2024, which has been attributed to missing bolts in the upper guide fittings and the lower hinge assemblies.


Passenger trains

Many passenger trains in the world use sliding plug doors: early examples of passenger trains using plug doors include the MVG Class A of the
Munich U-Bahn The Munich U-Bahn () is an Railway electrification system, electric rail Rapid transit, rapid transit network in Munich, Germany. The system began operation in 1971, and is operated by the municipally owned Münchner Verkehrsgesellschaft (MVG; Mun ...
from 1967, the first batch of trains for Line 2 of the
Milan Metro The Milan Metro () is the rapid transit system serving Milan, Italy, operated by Azienda Trasporti Milanesi. The network consists of five lines with a total network length of , and a total of List of Milan Metro stations, 125 stations (+2 in con ...
from 1970, and the DT1 of the
Nuremberg U-Bahn The Nuremberg U-Bahn is a rapid transit system in Nuremberg and Fürth, Bavaria. It is operated by ''Verkehrs-Aktiengesellschaft Nürnberg'' (VAG; Nuremberg Transport Corporation), which itself is a member of the ''Verkehrsverbund Großraum Nür ...
, also from 1970. Recent examples of trains with plug doors include the KTX-Eum high-speed train, the
Gautrain Gautrain is an Higher-speed rail, higher-speed Express train, express commuter rail system in Gauteng, South Africa, which links Johannesburg, Pretoria, Kempton Park, Gauteng, Kempton Park and O. R. Tambo International Airport. It takes 15 minu ...
commuter train, and the E-class tram in Melbourne.


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), Apollo spacecraft, used for the Apollo program, which landed astronauts on the Moon between 1969 and 1972. The CSM functi ...
, because the explosive-release hatch of
Gus Grissom Virgil Ivan "Gus" Grissom (April 3, 1926 – January 27, 1967) was an American engineer and pilot in the United States Air Force, as well as one of the original Mercury Seven selected by the NASA, National Aeronautics and Space Administration fo ...
'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 ''Liber ...
'' 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 planned 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 orbital ...
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 sp ...
, as the risk of fire in the low-pressure () in-space atmosphere was much lower. Plug hatches were also used for the inner
airlock An airlock is a room or compartment which permits passage between environments of differing atmospheric pressure or composition, while minimizing the changing of pressure or composition between the differing environments. An airlock consist ...
hatch on the
Space Shuttle The Space Shuttle is a retired, partially reusable launch system, 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. ...
. Currently, they are used on the
International Space Station The International Space Station (ISS) is a large space station that was Assembly of the International Space Station, assembled and is maintained in low Earth orbit by a collaboration of five space agencies and their contractors: NASA (United ...
, 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 spacecraf ...
.


Deep-sea vehicles

Deep-submergence vehicle A deep-submergence vehicle (DSV) is a deep-diving crewed submersible that is self-propelled. Several navies operate vehicles that can be accurately described as DSVs. DSVs are commonly divided into two types: research DSVs, which are used for ex ...
s such as the '' Alvin'' use a plug hatch which is sealed inward by the pressure of the ocean water.


References

{{Spacecraft-stub Aircraft doors Spacecraft components Transportation engineering Submarine components Pressure vessels