The observations made during the first failure investigation are repeated here in summary.
  • The shear bolt holes were elongated near the surface, making the holes 'banana shaped' internally - this would induce an additional bend component of loading on the bolts, and indicates poor maintenance of the towbar.
  • The failed shear bolt showed small fatigue cracks associated with the fracture surface, which was situated at the middle of the bolt length.
  • The aluminium tow bar body had been fabricated from four pieces of plate welded together. The ends of the towbar contained cracks at the welds.
  • The tractor coupling bolt holes in the aluminium body of the towbar showed damage consistent with loose bolts being worked backwards and forwards during towing operations. There was also evidence of re-tightening of the bolts and squashing of the aluminium plate under their heads.
  • The shear bolt was recovered from a position on the taxiway corresponding to a later stage in the aircraft movement, than the tractor coupling bolts.
  • The microstructure of the shear bolt appeared typical for a high strength steel, with a Vickers hardness of 364-374. The microstructure showed a quenched and tempered bainitic structure.

  • This information was interpreted by the investigators to answer the question posed by the insurers, as to whether failure had been the result of a 'sudden and unforeseen' event or 'progressive deterioration'. Think about the information presented and decide what this set of investigators would have concluded. Test your deductions with the applet given below.

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