The sudden loss of pressure in an airliner cabin, as occurred on the Ryanair flight, is a frightening and potentially painful experience for passengers but it is does not usually endanger the aircraft. Half a dozen episodes of rapid pressure loss have been suffered by airliners over the past year.

Pilots follow a standard procedure if the cabin pressure suddenly drops while at cruising altitude. They don the oxygen masks which are constantly at their shoulder and put the aircraft into an emergency descent to about 8,000ft above sea-level, the altitude at which the air becomes comfortably breathable.

At the same time the crew would have declared an emergency to the en-route air traffic control and requested immediate clearance to the lower altitude and landing at a nearby airport. In the meantime the oxygen masks will have deployed automatically to supply passengers and other crew during the descent.

The Ryanair Boeing 737 performed exactly this procedure after it suffered pressure loss. The pilots descended from about 35,000ft to 8,000ft in five minutes, according to the authorities at Limoges airport, where it landed. The media and passengers may call such manoeuvres a "plunge" or a "free fall", but the descent rate of about 5,600ft per minute is perfectly controlled and within the normal performance of the aircraft — assuming that all its flight systems are functioning.

The cabin pressurisation is not part of the plane's flight systems. Developed first by Boeing in 1938, pressurisation came into its own in the 1950s with jet airliners, which cruise at high altitudes. Human beings begin suffering from hypoxia, or lack of oxygen, from about 12,000ft, which is the maximum altitude that pilots of unpressurised planes may fly without supplementary oxygen.

Hypoxia, if unrelieved by oxygen, quickly leads to unconsciousness and eventually death, after a dangerous initial period of light-headedness that creates symptoms akin to drunkenness. Other physiological conditions take effect at very low air pressure in a similar way to the bends suffered by divers. In 1999 a small jet crashed in South Dakota after their pilots passed out with hypoxia, leaving it on autopilot until it ran out of fuel two hours later. Payne Stewart, a golf star, died in the crash.

At 35,000ft the low atmospheric pressure is one fifth of sea-level. This requires the plane's fuselage to be pumped up like a balloon with air, usually bled from the compressor stage of the jet turbines. To limit the amount of pressure that has to be packed in, airliners do not pressurise to sea-level value, but fly at a cabin altitude of about 8,000ft — equivalent, for the human being, to being on a medium mountain.

In Ryanair's Boeing 737-800, the cabin air pressure is kept between about 6,000 and 8,000ft. The newer Boeing 787 and the Airbus A380 will maintain cabin pressures between 5,000 and 6,000ft a their highest operating altitudes.

Depressurisation can be caused by a number of factors — from a faulty valve to the rupture of the fuselage, as occurred on a Qantas airlines flight in the Far East last month. Explosive decompression that rips open the fuselage can be potentially fatal to an airliner. Fatigue from years of pressurisation and decompression can wear out the structure of an airliner's fuselage, though the condition is closely monitored throughout its lifetime.

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