Fatigue of aircraft parts due to small but numerous vibrations has been known about for some time, but in 1944–5 cases of structural fatigue were found which were thought to have been caused by loads greater, but less frequent, than the usual vibration loads. Such loads could be ground loads due to taxi-ing, take-off and landing or they could be air loads caused by gusts and manœuvres.

A general study of the fatigue properties of typical structures was made in the laboratory and it became apparent that the loads likely to do the most damage were those occurring about a million times in the life of the aircraft.