Analysis of failed aircraft wheel assembly

A failed wheel assembly from a Hawker 125-800XP corporate passenger jet was investigated to determine the root cause of the failure. Different levels of damage sustained by the 12 tie bolt/nut couples that had held the flange to the wheel showed that failure was a cascading sequence that began at one couple and spread to adjacent couples. The suspected initiating bolt/nut couple showed stepped threads on one side of the bolt, the characteristics of which suggested that the nut had cracked, disengaged, and induced the failures of the other bolt/nut couples. Since the nut was not available for examination, our conclusion that the root cause of failure was a cracked nut could not be confirmed. Somewhat later, however, a cracked nut was observed in another wheel assembly. Examination of the bolt showed similar thread deformation, supporting the initial conclusion. The nut fracture surfaces showed an intergranular region surrounded by a ductile field – most likely the crack initiation site where the grain boundaries were weak. In our expert opinion, the root cause of the wheel assembly failure was a cracked nut that may have been embrittled by hydrogen during cadmium plating.