FROM THE TIMES  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 ft above sealevel the altitude at which the air becomes comfortably breathable  At the same time the crew would have declared an emergency to the enroute 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  performed exactly this procedure after it suffered pressure loss The pilots descended from about ft to ft 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 ft per minute is perfectly controlled and within the normal performance of the aircraft  assuming that all its flight systems are functioning