Pobjoy
Your question was asked and debated here when the SI report became available. As Martin-Baker chose not to defend themselves (in order to protect MoD) there was no opportunity to ask. I’m sure it would have come up in court during what was to have been a six-week trial.
So, all that can be said (unless someone involved decades ago wants to come forward) is that the shackle bolt and nut arrangement was deemed to be the engineering design solution, and this was accepted by MoD. The type of bolt changed in the Mk9 seat, but the fitting instruction to MoD remained the same. ‘Tighten the nut until contact is made with the Drogue Shackle. Do not torque’.
This is the information Martin-Baker were fined £1.1M for not providing. MoD’s APs and Martin-Baker manuals from the 50s - 90s containing it were provided to the court and HSE. So too the RAF training films clearly showing the final test that was omitted. So too witness evidence from the head of Tech Pubs in MoD. The HSE simply told the Judge the films were ‘irrelevant’, and flatly denied the existence of the APs and manuals that were provided to both it and the Judge.
Following these instructions would prevent over-tightening and deformity of the shackle. Instead, the nut was over-tightened to such a degree the bolt was bent and new thread cut.
The question then is why, after the accident, did MoD re-issue its instruction to torque the nut to 50 lbf-in; yet again completely ignoring the instructions from Martin-Baker.
Of course, the main question is why in situ servicing was being carried out when the manuals and APs unambiguously said ‘Remove seat from aircraft’. This meant a false declaration was made that the seat was serviceable, because its serviceability could not be verified. Someone gambled when issuing the order not to follow Martin-Baker’s instructions. He lost, and Sean Cunningham died. As dctyke said earlier, we'd all love to see the risk assessment and justification.