Nimrod Airworthiness
I find the whole debate on this thread over Nimrod airworthiness confused and unclear.
My understanding is that the original Comet design was extensively modified to generate a military aircraft type called Nimrod. To permit self-start, the Nimrod MR1 had an auxiliary power unit and a hot air cross-feed system installed. In the installation of the cross-feed system, the hot air ducts come close to fuel pipes, particularly in the vicinity of the starboard 7 tank (another addition to Nimrod). The airworthiness requirement for the interaction of the hot air system and the fuel system was defined in Design and Airworthiness Standards within AvP 970 (now extensively amended and published as Def Stan 00-970). The records of the clearance of the original design between the MOD and Hawker Siddley are not all available and the evidence available does not cover proximity of hot air and fuel systems. Thus, the situation is that either the proximity of the hot air system and the fuel system was not recognised or it was recognised and accepted by the MOD as an exception to AvP 970. In either case, the failing happened in the late 1960s. The aircraft then flew in a state where it was not compliant with the airworthiness standards in AvP 970 for many years.
The subsequent introduction of air-to-air refuelling in Nimrod may have increased the peak fuel pressures experienced by the aircraft refuel system.
When the Nimrod MR1 was converted into the Nimrod MR2, the situation was exacerbated when a Secondary Cooling Pack was added in the rear fuselage powered from the cross-feed ducts. This meant that whenever the Secondary Cooling Pack was in use, the Cross-Feed Ducts had to be pressurised by hot air. This extended the period when the fuel system was exposed to air ducts containing hot air. Again, the records of the clearance of the original design between the MOD and the manufacturer are not all available. The conversion of Nimrod MR1 to MR2 was an opportunity to identify the airworthiness issue and resolve it. The aircraft continued to fly in a state where it was not compliant with the airworthiness standards in AvP 970.
Much later, the Nimrod IPT had a safety case produced to document the justification of the airworthiness of Nimrod. During this exercise, a failing of the risk assessment occurred within the Nimrod IPT. This inadequate risk assessment resulted in a missed opportunity to recognise that adequate measures were not in place to cater for the close proximity of the fuel system and the hot air system. The aircraft continued to fly in a state where it was not compliant with the airworthiness standards in Def Stan 00-970.
The accident to XV230 was caused by an explosion resulting from a fuel fire most probably in the area where the fuel system and hot air systems are in close proximity near the 7 tank. There was not enough evidence to either the Board of Inquiry or the Inquest to define precisely where the fuel leaked or the ignition source. Thus, the accident sequence will never be known for certain.
After the accident a number of actions have been taken. Air-to-air refuelling is no longer carried out by Nimrods. The Secondary Cooling Pack is no longer used. The cross-feed system is only pressurised and used for engine start on the ground. The fuel system has an increased 30-day maintenance inspection regime. In addition on scheduled maintenance, there is a fuel system inspection and targeted fuel seal replacement programme under way. There is a plan to replace all hot air ducts as soon as reasonably practicable after they are available.
So where is the Nimrod force now. The immediate actions regarding the use of the hot air system where taken to remove the risks of proximity of hot air system and fuel system in flight. There is a programme to make further improvements. Does that make the Nimrod airworthy? Airworthiness is a risk management process. The aim is to manage the risk to a level that is both tolerable and as low as reasonably practicable. Should time for embodiment of safety enhancements be a factor in “reasonably practicable”? On a military aircraft, should the acceptability of risk be balanced against the potential operational benefit? These are big questions that must be answered as soon as possible. I have opinions as will many other individuals; our opinions are only that! I do not envy Charles Haddon-Cave QC his task.