To combat this threat, aircraft have a warning system which alerts the crew if the cabin altitude is approaching dangerous levels.

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The reason is simple: the typical business jet or airliner cruising at 45, 000 feet will have a cabin altitude of about 8, 000 feet.

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Although jets are pressurized so that the cabin environment resembles an altitude of around 8, 000 feet, some people report that flying (possibly from the changes in pressure and resulting decrease in oxygen concentration) causes symptoms such as headache, fatigue and nausea.

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Like the paper-clip, the airframe and its alloy skin are stressed first in one direction as the cabin is pressurised while climbing to its cruising altitude, and then in the opposite direction when depressurised during descent for landing.

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