Amongst the important matters investigated after the accident to a ‘Comet’ aircraft at Elba in 1954 and the recovery of the wreckage from the sea, was the severe damage which had been sustained by the turbine rotor hub-shafts. Three of the four engines were recovered with the turbine rotor disks intact, though extensive cracking was discovered in the spigot recess radii of the hub-shaft flanges. In the case of the fourth engine, the turbine rotor disk was missing, through fracture of the hub-shaft flange. A careful step-by-step examination of all the evidence enabled it to be stated quite definitely that the damage sustained by the engine hub-shaft assemblies was a consequence of abnormally severe gyroscopic bending moments imposed during disintegration of the aircraft. Nevertheless, it was considered desirable to determine by practical tests the actual magnitude of the gyroscopic bending moment which the hub-shaft assembly could sustain and the manner in which fracture would occur. The paper describes the special gyroscopic test-rig constructed for this purpose and discusses the results obtained. The results of supplementary investigations of the bending fatigue behaviour of hub-shaft assemblies and plain and notched laboratory specimens, with special reference to the high-stress low-endurance portion of the fatigue diagram, usually called the ‘high-level’ fatigue region, are also included.