The detail in this report is amazing. The micrographs of the fracture surfaces absolutely stunning. I have never before seen cracks propagating through the grains of a material rather than follow around the grain boundaries. Perhaps of even more significance is that cracks were forming and propagating extensively through the interior material without leaving any significant trace at the surface, apparently due to the compressive carburization hardening process. This certainly has provoked a degree of controversy among the various groups responsible for the investigation as to what initiated the cracking and the speed and method of propagation. The lack of surface damage limited the possibility of spalling metallic chips being generated and detected by devices intended to identify early signs of damage and potential failure of components.
My interpretation of this mode of crack propagation is that the materials used were operating at the absolute limits of their ability to cope with the stresses involved. Once damage started to occur, progressively greater stress was put on the remaining intact structure. A common enough failure mode, but in this case, the cracks were passing through grain structures that were designed to resist cracks.
I can only conclude that the engineering design of the gearbox exceeded the ability of current materials and manufacturing techniques to cope. Simply beefing up the components a little bit will not be enough. Without even more robust materials, cracks will still form and propagate through the metal. It may take longer for a failure to occur, but it will still happen. We appear to have hit the ultimate limits of material science for the time being.
This catastrophic failure mode is a rare event with only three remarkably similar accidents and a couple of near misses recorded. The problem is that there is no reliable current method of detecting impending failure before it occurs. That suggests that the MGB needs to be totally reengineered or the aircraft operated at a much reduced capacity to provide an extra safety margin.
Either way, I can't see it ever being allowed back into North Sea operations again.
Last edited by G0ULI; 29th Apr 2017 at 01:52.