Decompression

All modern airliners have pressurised cabins for the comfort of the passengers and crew. The pressure inside them is maintained by compressors, which draw in air and pump it into the cabin.

This is necessary because the atmospheric pressure at high altitudes is too low for people to inhale enough oxygen to stay conscious. By law, an oxygen supply must be provided on all flights, however short, above 14,000 feet. This threshold is lowered to 10,000 feet (about three kilometres) for flights longer than thirty minutes. In a modern airliner, the cabin pressure at cruising altitude is equivalent to that at an elevation of between 5500 and 8000 feet on earth. If pressure is lost at a very high altitude – 40,000 feet (about twelve kilometres), say – the people inside the plane are unable to function normally after only 15-20 seconds. This interval is known as the “time of useful consciousness”. It is for this reason that all aircraft with a pressurised cabin are fitted with oxygen masks, which are released automatically if the pressure drops.

The pressurisation system, properly called the Environment Control System (ECS), also provides the cabin ventilation and heating.

If a pilot reports a loss of pressure (decompression), air traffic control immediately instructs him or her to descend to a safe altitude (8000 feet or less). At this height the atmospheric pressure is greater, with enough oxygen for the passengers and crew to breathe normally. Aircraft suffering decompression usually make a precautionary landing so that the problem can be dealt with.