The Nimrod Review - Official Documents

Chapter 5 – Leak from Fuel Coupling
5.32 Following the loss of XV230, Headquarters No. 2 Group recognised in an internal document that: “recent
anecdotal evidence has highlighted a potential shortfall in the training of fitment of fuel couplings”. 33 The
Review was also informed during a trip to RAF Waddington that, when seals were removed, pipes were often
found to be misaligned and had to be remanufactured. The available evidence in this regard, however, now
goes well beyond the mere anecdotal. A detailed forensic examination of the failure of a fuel coupling and its
seals was undertaken following a significant fuel leak on Nimrod MR2 XV229 at the end of December 2008. 34
The principal objective of the investigation was to determine the cause of fuel leaks in both the port and
starboard dry bays. In my view, the report is a considered, detailed and thorough examination of a complex
problem. It concluded that the port leak was caused by “poor assembly of the FRS coupling”. The assembly
included an “oversize cast split-ring collar” which migrated into the female connector, causing an off-set in the
coupling which led to the leak. The report also noted that the condition of the fuel pipe was poor and that, in
contravention of the requirements, there was no gap between the two pipe elements. The starboard coupling
probably leaked because of stresses imposed by the method of assembly, which displaced the fuel pipe within
the coupling. The assembly method mandated required all couplings to be fixed prior to connecting the fuel
pipe to the aircraft; this procedure has now been changed to reduce the stress on the assembly. Both couplings
had been in place, with no record of disturbance, since at least July 2006; one seal was a FRS 54 K Series 1 seal,
which must have been fitted since (at the latest) 2002, when they were declared obsolete. Thus, it is clear that
misaligned couplings, i.e. those which have been assembled outside tolerance limits, may not leak immediately:
one of XV229’s couplings had been in place for at least six years before beginning to leak.
5.33
5.34
(3) Pressure
5.35
The difficulties of fitting and assembling fuel couplings within the complex, cramped environment of the Nimrod
fuselage, the manual dexterity required and the paucity of clear guidance to ground crews as to the best
methods of assembly of these coupling, make it not altogether surprising that some of the 400 fuel couplings
fitted in each Nimrod over the years may not have been assembled in accordance with the strict criteria required,
i.e. within a 1º tolerance. From a practical perspective, as QinetiQ noted in its 2007 report (see above), it is clear
that building a pipe system of the type contained in the Nimrod, and ensuring repeatability of pipe positioning
and alignment, would not be easy, especially when taking into account the ‘hand built’ nature of the aircraft
and the complexity of the pipe system. 35 The difficulties are exacerbated by the fact that the replacement of a
single seal or pipe often requires further seals and pipes to be disturbed in order to gain access to the original
item.
I have been impressed in my many meetings and discussions with the ground crews at RAF Kinloss and RAF
Waddington by their dedication and I have no doubt that they have done their best in the circumstances.
Nonetheless, some couplings have undoubtedly been assembled outside tolerance over the years, 36 either in
error, or through following procedures that induce stresses to the system. The result being that the individual
coupling components undergo greater stresses than that for which they were designed; this can cause physical
movement of components and early degradation of elastomeric seals, resulting in leaks.
Much consideration has been given to the effects of the pressures induced by AAR on the Nimrod fuel system.
The normal pressures to be expected on the fuel system during AAR of 30psi to 40psi are well within the system’s
limits. 37 Nonetheless, pressures, albeit transient, of up to 80psi have been observed during AAR. A QinetiQ report
makes reference to ‘anecdotal evidence’ of a surge up to 120psi, but the source of this information cannot
be traced. These ‘surges’ appear to occur in all aircraft types undertaking AAR and are probably caused by the
closing of the fuel system valves. With the sole exception of the single QinetiQ report, no pressures above the
Nimrod’s proof pressure have been reported. However, the slow response characteristic of the Nimrod’s probe
pressure transducer is such that pressures higher than those observed may in fact have occurred. Most pressure
surges are transient in nature with an early peak followed by a slow reduction in pressure. With a response time
33 Reference: 20071129-Fuel.
34 RAF Form 765B Ground Incident Serial No. Kin 067/08, dated 29 December 2008.
35 QinetiQ/EMEA/S&DU/CR0703772/2.0, dated November 2007: Removal and Examination of Fuel Seals from Nimrod XV236.
36 Ibid, page 18. QinetiQ calculated from witness marks on two of the seals removed from XV236 that the pipes had probably been about 6º to 7°
out of alignment.
37 75psi maximum working pressure and 112.5psi proof pressure.
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