Prior to 1996, approximately 6, 000 large commercial transport airplanes were certified to operate above 25, 000 feet ( FL 250; 7, 600 m ), it " must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of after any probable failure condition in the pressurization system . " In the event of a decompression which results from " any failure condition not shown to be extremely improbable, " the aircraft must be designed so that occupants will not be exposed to a cabin altitude exceeding for more than 2 minutes, nor exceeding an altitude of at any time.
