Power cables and ejector seats – two tales of failed risk management

File:RAF Red Arrows - Rhyl Air Show.jpgThe last week has seen findings in two inquests in England that point, I think, to failures in engineering risk management. The first concerns the tragic death of Flight Lieutenant Sean Cunningham. Flight Lieutenant Cunningham was killed by the spontaneous and faulty operation of an ejector seat on his Hawk T1 (this report from the BBC has some useful illustrations).

One particular cause of Flight Lieutenant Cunningham’s death was the failure of the ejector seat parachute to deploy. This was because a single nut and bolt being over tightened. It appears that this risk of over tightening was known to the manufacturer, it says in the news report for some 20 years.

Single-point failure modes such as this, where one thing going wrong can cause disaster, present particular hazards. Usual practice is to pay particular care to ensure that they are designed conservatively, that integrity is robust against special causes, and that manufacture and installation are controlled and predictable. It does surprise me that a manufacturer of safety equipment would permit such a hazard where danger of death could arise from human error in over tightening the nut or simple mechanical problems in the nut and bolt themselves. It is again surprising that the failure mode could not have been designed out. I suspect that we have insufficient information from the BBC. It does seem that the mechanical risk was compounded by the manufacturer’s failure even to warn the RAF of the danger.

Single point failure modes need to be addressed with care, even where institutional and economic considerations obstruct redesign. It is important to realise that human error is never the root cause of any failure. Humans make errors. Systems need to be designed so that they are robust against human frailty and bounded rationality.

File:Pylon ds.jpgThe second case, equally tragic, was that of Dr James Kew. Dr Kew was out running in a field when he was electrocuted by a “low hanging” 11kV power line. When I originally read this I had thought that it was an example of a high impedance fault. Such faults happen where, for example, a power line drops into a tree. Because of the comparatively high electrical impedance of the tree there is insufficient current to activate the circuit breaker and the cable remains dangerously live. Again there is not quite enough information to work out exactly what happened in Dr Kew’s case. However, it appears that the power cable was hanging down in some way rather than having fallen into some other structure.

Again, mechanical failure of a power line that does not activate the circuit breaker is a well anticipated failure mode. It is one that can present a serious hazard to the public but is not particularly easy to eliminate. It certainly seems here that the power company changed its procedures after Dr Kew’s death. There was more they could have done beforehand.

Both tragic deaths illustrate the importance of keeping risk assessments under review and critically re-evaluating them, even in the absence of actual failures. Engineers usually know where their arguments and rationales are thinnest. Just because we decided this was OK in the past, it’s possible that we’ve just been lucky. There is a particular opportunity when new people join the team. That is a great opportunity to challenge orthodoxy and drive risk further out of the system. I wonder whether there should not be an additional column on every FMEA headed “confidence in reasoning”.


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