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A plug door is is held in place by air pressure and utilized for various air and spacecraft applications. This door relies solely upon the difference in air pressure between interior and exterior walls for its latching and locking mechanism. A plug door opens inward and is used primarily for passenger aircraft. Plug doors began to be used by the U.S. space program for manned spacecraft in the 1960s. This door is currently employed in several different spacecraft applications.
Most doors rely on some type of latching or locking mechanism to keep them closed. A plug door utilizes the higher air pressure inside a room to keep it securely latched and locked. This wedge shaped door fits into an opening in the outer wall and is forced outward by the higher air pressure inside the structure. The door cannot be opened until the inside air pressure is equal to the outside pressure. The superior sealing ability of this door helps maintain a constant air pressure inside of a room.
A plug door is most often used in aircraft applications requiring a pressurized cabin. The door remains unlocked until the aircraft reaches a sufficient altitude to fully pressurize the cabin. During flight, the door remains locked and keeps the high pressure air inside the cabin. As the aircraft descends, the cabin pressure begins to equal the outside air pressurize and the door can be opened again. Most airlines do not allow passengers near the unlocked door until the aircraft has landed.
Plug doors open inward and occupy a significant portion of the cabin when not in use. These doors are typically used only for passenger aircraft because of the amount of interior space they require. A plug door on cargo planes would reduce the amount of interior space available and make it impractical. An outward opening door with a traditional locking mechanism is utilized for most cargo aircraft applications.
These kinds of doors are also found in certain types of spacecraft applications. A plug door was used on the capsule of U.S. spacecraft during the 1960s. This practice was later discontinued after the inward opening door prevented the escape of three astronauts during a launch pad fire. This type of door was also utilized for the Lunar Landing Module during the U.S. moon missions. Plug doors are currently being used for the outer hatches of the Space Shuttle, International Space Station, and Soyuz spacecraft.