Failure as a Design      Criterion

   Fracture Mechanics

   Failure Analaysis


Wire Rope Failure


Undercarriage Leg Failure


Aircraft Towbar Failure
- Part 1
- Part 2
- Part 3
- Part 4
- Activity 1 - First Hypothesis
- Activity 2 - Fracture Stress


Hail Damage


Insulator Caps


Fractography Resource

Initial Failure Investigation

Damage to aircraft tends to be expensive to repair, due to stringent airworthiness regulations. The type of cover, and conditions of cover, of insurance policies varies. Additionally, there are usually various insurers for the different parties involved in the claim. In order to apportion responsibility for the accident correctly, and hence determine the liability of the various insurers in this failure, it was necessary to determine the cause and mechanism of the failure. In the present case, the insurers were interested in determining whether the failure was the result of poor maintenance (which would activate cover from a particular policy), and whether it was the result of 'sudden and unforeseen' circumstances or was brought about by 'progressive deterioration' (these factors affected cover under a different policy). Note that 'sudden and unforeseen' circumstances mean some sequence of events that could not be foreseen, or forestalled, during routine maintenance operations, e.g. a sudden and unexpected overload. Thus the consequences of poor maintenance practice would not be covered, even if their occurrence was sudden.

All available parts of the towbar were therefore supplied to a failure analysis company, including the steel shear bolt, when that was located. They noted the following points regarding the failure;
  1. 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.
  2. The failed shear bolt showed small fatigue cracks associated with the fracture surface (see the arrows in Figure 5), which was situated at the middle of the bolt length.
  3. 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, as seen in Figure 6 below.
  4. 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 (see Figure 3 repeated below).
  5. 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.
  6. The microstructure of the shear bolt was typical for a high strength steel, with a Vickers hardness measured as 364-374. The microstructure can be seen in Figure 7, and shows a quenched and tempered bainitic structure.
Fatigue_CrackSEM.JPG (115174 bytes)
Figure 5 Small fatigue cracks on shear bolt fracture surface
FatigueCrack_Towbar.jpg (66068 bytes)
Figure 6 Fatigue crack at weld in towbar body
Tractor_coupling.JPG (61675 bytes)
Figure 3 Poor maintenance practices are evinced at the bolt holes (marked with arrow)
Microstructure.jpg (153142 bytes)
Figure 7 Quenched and tempered bainitic structure of the shear bolt steel

The conclusions drawn from this investigation are given in Activity 1.

Proceed to third part of case study.


Failure Analysis  -  Fracture Mechanics  -  Failure As A Design Criterion