DV:
Whether we like it or not, hazard H66 has a hazard severity rating of CATASTROPHIC. It is currently set on the data base has having a probabilty of IMPROBABLE. Is was recommended by QinetiQ in 2008 that because there had been two duct failures this should be reset to OCCASIONAL. This would give it a HRI of "A".
Yes, I fully understand what has been written by BAeS and Qinetiq. I think we need to look at how the HRI is arrived at:
1. The published frequency of the hazard is based on judgement of both historical and technical analysis of the system. I have absolutely no argument with the upgrade to OCCASIONAL by QQ in the case of a duct leak. It is debatable what the degree of any leak or failure is likely to be, but OCCASIONAL is an acceptable failure rate.
2. The effect of the hazard is in 2 areas: local and overall. In the very worst case, a blown duct might burn out a hole in the upper skin, burn out a few wires, and/or overheat a fuel/hydraulic coupling seal. This is the local effect. However, the overall effect to the aircraft, which is where the HRI comes in, is minimal due to the fact there is a crew on board to deal with the problem(s). The aircraft will not be lost, it will not even have to make an emergency landing. The published procedures in the aircrew manuals will deal all the local failures in a structured and calm manner. The weight might need to be reduced, which could take up to 30 minutes, and then the aircraft will make a safe and controlled approach to land. The crew would not even need to make it a MAYDAY unless they received a fire warning, which, due to nature of the event, will either be spurious due to burnt wiring or a temporary hyd or fuel fire (which would require a sustained and very hot air leak), but easily extinguished. That is the very, very, worst case, and the aircraft is still flying and the rear crew are still eating their doughnuts. Regardless of the degree of the failure, the effect to the aircraft, by book definition, is not CRITICAL or CATASTROPHIC by any means. Therefore, there is no legal requirement to reduce the risk to ALARP.
I don't know how, or why, BAeS and QQ declared the hazard as CATASTROPHIC. I do know that they did not consult the aircrew when they erroneously calculated the effect to the aircraft when a HP air duct bursts on a Nimrod in flight.
I don't think I can bring anything more to this discussion.
Regards
Ed