The next item on the safety demonstration agenda is the subject of decompression or depressurisation. If for any reason the pressurisation system fails, or a break occurs in the aircraft structure, the result will be a decompression. A slow decompression may occur where, for example, a door seal fails, resulting in a gradual rise in cabin altitude and a decrease in cabin temperature (the external temperature at cruising altitude is over minus fifty degrees Celsius).

If a rapid decompression occurs, the sudden equalisation of air pressure causes the cabin air to condense, which results in a temporary misting effect. Gases in the body will vent, although a blocked nose from a cold or hay fever may prevent the ears from clearing, causing ear-ache and in severe cases, may result in perforated ear-drums. Any loose objects in the cabin will fly around and if there is a hole in the fuselage (for example a broken window), objects will initially be sucked towards it. This is one of the reasons why the crew, during the safety demonstration, suggest that you should keep your seatbelt fastened whenever in your seat (the other being an unexpected encounter with clear air turbulence, where aircraft have been known to drop hundreds of feet in seconds). Amongst those decompressions caused by failures in the fuselage, there are two catalogued in which passenger's lives were saved due to the 'Fasten Seatbelt' signs having been illuminated at the time.

This United Airways 747 had just taken off from Honolulu and was at 23,000 ft over the Pacific when the forward cargo door flew open, taking a 15 X 10 ft section of the fuselage with it. Nine passengers were lost during the initial decompression. The remaining 343 people on board evacuated safely when the aircraft successfully landed back at Honolulu.

If the cabin altitude reaches 14,000ft., masks automatically drop, usually from compartments above your head. Often, the masks just dangle above the head initially, and it is the pulling action to make it reach your face which sets off the oxygen supply. In a rapid decompression, you may only have twenty seconds of useful consciousness (i.e. when you are still able to think clearly); you may remain conscious for some time after, but will be unable to think or co-ordinate properly. This is usually accompanied by a feeling of euphoria - a typical sign of oxygen starvation to the brain. If you are standing, sit down immediately, strap in and grab a mask.

If the masks above you don't drop, there are spare masks at most other seat rows - grab one from an adjacent row, they will reach. There have been many cases of passengers failing to take a decompression seriously and not put on a mask, especially when it's a slow one, and have become quite severely hypoxic.

The next thing that will become obvious in a decompression is the steep angle of the emergency descent that the flight crew will have instituted. They will be taking the aircraft down to a level at which it is possible to breathe without the need for emergency oxygen (usually 10,000ft or the minimum safe altitude, which is around 4,000ft higher than the terrain). The descent will be a little rough, as the flight crew will make full use of the speed brakes - this allows them to point the nose towards the ground without the aircraft speed accelerating unduly. Only when the aircraft has levelled out will the cabin crew begin to move around the cabin, checking on the passengers. They may wear portable oxygen bottles, in case the aircraft is still above 10,000ft due to high terrain below.

Perhaps the most publicised example of a cabin decompression due to a failure of the fuselage was theAloha Airways 737 which lost a third of its roof in flight, due to metal fatigue in the aircraft’s structure, which resulted in the loss of only one life, a stewardess who was walking through the cabin at the time.
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