Overwing exits
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Overwing exits are found on passenger aircraft to provide a means of passenger evacuation onto the wing, where they either continue off the trailing edge by sliding down the extended flaps or by using an evacuation slide that deploys when the exit is opened.
Overwing exits are smaller in width and height than standard emergency exits on an aircraft, and therefore have a reduced evacuation capacity, and are typically added to aircraft where there is insufficient evacuation capacity at the main doors to obtain a 90 second evacuation, but where the addition of another set of full sized exits is not necessary to accomplish this.
Overwing exits are primarily self-help exits meaning that in an emergency evacuation the passengers seated immediately adjacent to the exit will be responsible for assessing for external hazards and opening the exit.
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[edit] Some hazards
The primary hazard involving the use of overwing exits is passengers not waiting to be given the command to evacuate, but rather initiating it themselves. While many passengers (especially frequent fliers) would believe that they are adult enough and educated enough to decide that they need to get out of an aircraft, they are not trained in the specific dangers or the sequence of events required to decide when a full-scale evacuation is needed[1]. Also, passengers will frequently open an exit as soon as evacuation begins which may result in them evacuating into danger. This occurred in the Ryanair engine fire in Stansted Airport in 2002, where passengers evacuated themselves onto a burning wing, despite airport fire services shouting at them to return inside the aircraft and evacuate via a usable exit [2].
Another hazard in the use of overwing exits is them being improperly opened (usually a result of passengers in these seats not paying attention to the verbal briefing provided pre-departure, or not observing the opening instructions on the safety card and on the exit). The majority of overwing exits involve the passenger physically removing the hatch from its frame and disposing of it outside on the wing without blocking the exit. Research conducted at the Cranfield Institute in the UK [3] showed that many passengers are prone to leaving the exit hatch on the seat next to the exit, thereby hindering other passengers egress through the exit.
[edit] Operation
There are principally two types of overwing exits (excluding the full-sized door on the likes of the Boeing 747, which is not considered an overwing exit) in use on modern aircraft. The disposable hatch type exit is the most common, where the operator must first remove the hatch from its frame before disposing of it on the wing, away from the exit. The second type is the self disposing hatch, found on more modern aircraft (such as the Boeing 737-800) and has been designed to both simplify the opening of the exit and to remove the hazard of the removed hatch blocking the exit. This is accomplished by the passengers simply pulling in and down on a handle recessed into the top of the door and initiating the exits self-opening mechanism, whereby the door rotates up and out on a hinge at the top of the exit frame.
Depending on the height of the aircraft from the ground, an overwing exit may have an automatically inflating slide that is housed within the fuselage near to the exit(s). If the exit is opened the slide will automatically deploy and inflate to provide a means of evacuation from the wing to the ground. Only the slide on the same side of the aircraft as the open hatch will inflate. However, on aircraft where there are a pair of adjacent overwing exits on both sides of the aircraft (i.e. four in total), such as on some Boeing 767-200 variants, opening one of the pair of doors will deploy the slide. Should slide inflation fail, there is a manual inflation handle within the door frame of each exit (the location of which varies but is always indicated on the safety information card) which can be pulled to inflate the slide. Should this also fail the use of another exit should be sought.
[edit] Use in a ditching
The use of overwing exits in a ditching varies from airline to airline. On aircraft fitted with overwing exits, there is typically a raised "D ring" about a third of the way from the door, on the wings upper surface and is typically painted yellow. This is accompanied by an escape rope found in the frame of the exit after opening and once this rope is attached to the D-ring it will aid passenger evacuation onto the wing to await rescue or to enter the water (depending on the airline's procedures), and on aircraft with life rafts to be launched via the overwing exit, the wing will be used to provide a boarding platform onto the life raft.
[edit] References
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