Modern commercial aircraft operate at an altitude which is above 35000 feet, where human life cannot sustain. To provide a comfortable environment in the cabin of the aircraft, it is sealed and the flow of air in and out of this metal tube is carefully controlled. The flow of air in the cabin is provided by compressed air taken from the engines. This mechanical system maintains near sea level (5000-7000 feet) pressure. This is termed pressurization or compression, that means squeezed or compressed air within a confined space.
If for any reason, the mechanical system of pressurization fails, a breakage or leakage or explosion occurs, the result will be decompression or depressurization; then the pressure in the cabin would suddenly drop to match the outside air pressure and the aircraft temperature will also fall until it reaches outside temperature of -45C to -60C.
There are two types of Decompression: - (a) Slow and (b) Rapid Decompression.
A slow decompression may occur where, for example, a door seal fails, and resulting in a gradual rise in cabin pressure and a slow drop in cabin temperature, a hissing sound will be heard and loose items will flutter.
If a rapid decompression occurs, the sudden equalization of air pressure causes the cabin air to condense, which results in a temporary misting effect. Any loose objects in the cabin will fly around and there will be sharp drop of cabin temperature.This is one of the reasons why cabin crew suggests passengers to fasten their seatbelt, whenever they are in seat, during the safety demonstration.
Aircraft flying at an altitude of 35,000 feet somehow loosing its pressurization system completely, then the occupants of the a/c will get maximum 25-30 seconds to establish an alternative oxygen supply. If they were unable to do so they will die within two minutes.
To combat this threat, aircraft have a warning system which alerts the crew if the cabin altitude is approaching dangerous levels. If this alert is received, the pilots should put on masks which will provide them with oxygen while they rapidly descend the aircraft to an altitude where the occupants can breathe without any assistance or 02 masks. In this time the passengers will be provided with oxygen from drop-down masks which will give them oxygen for 12 to 15 minutes, by which time the aircraft should be at a lower level.
When decompression occurs, cabin crew’s first duty is to hold a thing & take one 02 mask. When the aircraft comes in normal position that is in lower altitude cabin crew should go around the cabin with a portable oxygen bottle and find out any unconscious is there.
If the cabin altitude reaches 14,000ft., O2 masks automatically drop, usually from compartments above your head.
Now there is a question comes into your mind that what is TUC OR EPT?
TUC is time of useful consciousness, or EPT is effective performance time, which indicates the time difference between decompression occurs & a person’s useful or effective performance (i.e. ability to think clearly) deteriorates.
In a rapid decompression, you may only have twenty seconds of useful consciousness 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.
During decompression a person’s TUC or EPT decreases with time and after a certain period he or she will collapse.
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