A plug door
is a door designed to seal itself by taking advantage of pressure difference on its two sides and is typically used on pressurised aircraft. The higher pressure on one side forces the wedge-shaped door into its socket, making a good seal and preventing it from being opened until the pressure is released.
Non-plug doors rely on the strength of the locking mechanism
A lock is a mechanical or electronic fastening device that is released by a physical object or secret information , or combination of more than one of these....
to keep the door shut whereas a plug door relies on the pressure differential to keep it shut.
The term "plug door" may also refer to a sliding bus door that is not actually held in place by pressure but is mechanically wedged into place when closed.
The plug door is often seen on aircraft with pressurized cabins
Cabin pressurization is the pumping of compressed air into an aircraft cabin to maintain a safe and comfortable environment for crew and passengers when flying at altitude.-Need for cabin pressurization:...
. 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.
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 can not 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 to prevent opening while in flight.
Inward opening plug hatches were used during the early US space program on the "Block I" Apollo Command Module
The Command/Service Module was one of two spacecraft, along with the Lunar Module, used for the United States Apollo program which landed astronauts on the Moon. It was built for NASA by North American Aviation...
because this ensured a reliable seal to the pressurised module. NASA decided to return to outward opening hatches on the Command Module after the plug hatch prevented rapid egress of the crew from the Apollo 1
Apollo 1 was scheduled to be the first manned mission of the Apollo manned lunar landing program, with a target launch date of February 21, 1967. A cabin fire during a launch pad test on January 27 at Launch Pad 34 at Cape Canaveral killed all three crew members: Command Pilot Virgil "Gus"...
fire, trapping and killing the three astronauts. Plug doors were retained for the CM docking hatch and the two hatches on the Apollo Lunar Module
The Apollo Lunar Module was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman to carry a crew of two from lunar orbit to the surface and back...
as a fire in the low-pressure air atmosphere was more manageable; the Apollo 1 fire occurred in a high-pressure pure oxygen atmosphere. Currently, plug doors are used only on the outer hatch doors on the Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
and International Space Station
The International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...
as well as on the hatch between the Orbital Module and Descent Module on the Russian Soyuz spacecraft
Soyuz , Union) is a series of spacecraft initially designed for the Soviet space programme by the Korolyov Design Bureau in the 1960s, and still in service today...
A plug door on a bus has a pantographic hinge that moves the door panel outwards from its plug socket and then parallel to the side of the bus to clear the opening. On closing, the door is wedged and locked into the opening. This arrangement makes a very good airtight and sound proof seal but is usually limited to buses such as coaches that make infrequent stops as the door operation is slow and mechanically complex.