Fatigue tests being conducted on a Boeing 747- 100SR fuselage. Fatigue failure resulting from the pressurisation of the aircraft was first identified in the mid-1950s after two de Havilland Comet aircraft exploded in mid-air in seperate incidents, with the loss of all passengers. Aircraft are now tested with pressurisation cycles being imitated by pumping air into a sealed fuselage many thousands of times, simulating climbing to and descending from altitudes of around 35,000 feet. Airliner cabins are pressurised to an equivalent 8,000 ft altitude to maintain oxygen levels and ensure passenger comfort during high-altitude flights.