The Nimrod Review - Official Documents

Chapter 5 – Leak from Fuel Coupling
12. The fact that a non-conforming part found its way into the Nimrod fleet raises serious concerns
about the MOD procurement chain for such parts and whether it is wise for specialist aviation parts
to be sourced by the non-specialist Medical and General Stores IPT.
Causation
13. I am satisfied that the Board of Inquiry was right to find that a leak from one of the fuel couplings in
starboard No. 7 Tank Dry Bay is one of the two most likely sources of fuel to have caused the XV230
fire. The refuel system within the No. 7 Tank Dry Bay would have been pressurised and full of fuel at
the relevant time as a result of Air-to-Air Refuelling. The evidence shows that significant quantities of
fuel can leak from faulty couplings within the refuel system.
14. I have concluded, on balance, that if a fuel coupling was the source of fuel, it is more likely to have
been one of the eight FRS couplings in the starboard No. 7 Tank Dry Bay, rather than the single
Avimo coupling, for three reasons: (a) recent evidence captured on video shows that FRS couplings
can, and do, suddenly spring major leaks; (b) despite the rubber manufacturing defects, no Avimo
seal has actually been found leaking (they tend to be held together by the outer metal flange); and
(c) numerically, the probabilities favour an FRS coupling being the culprit on this occasion.
Responsibility
15. The MOD bears responsibility for the fact that a leak from a fuel coupling may have been the
source of fuel causing the fire on board XV230 and is open to criticism for the shortcomings in the
maintenance system referred to above and in particular for: (a) failing to do enough to monitor
fuel leak rates over the years, and (b) failing to give better guidance for the fitting of couplings
and elimination of fuel pipe leaks. RAF maintenance personnel expended considerable efforts over
the years in curing individual leaks, but it appears that no-one in the Nimrod IPT or elsewhere was
consolidating the history of leaks, nor making sufficient efforts to analyse the underlying causes of
leaks, nor providing guidance that might have reduced the leak rate.
Post accident
16. The loss of Nimrod XV230 has now placed sharp focus on fuel coupling leaks. The Nimrod Fuel
Seal Replacement Programme undertaken in 2008-2009 has resulted in the replacement of many
centre section fuel system seals, and also addressed issues of misalignment of couplings due to
previous incorrect assembly and lack of proper guidance on how to fit couplings. The need for
such a programme should have been considered prior to the loss of XV230 as a result of: (a) the
(unobserved) increase in the fuel leak rate; and (b) the incremental slippage in the retirement of
the Nimrod MR2 with the consequent increase in its service life.
Inconvenient truth
17. The inconvenient truth is that elastomeric fuel seals do deteriorate. This can be due to: (1) age; (2)
misalignment; (3) pressure; (4) vibration or airborne stresses; (5) temperature; (6) drying out; and (7)
manufacture. Fuel coupling leaks are difficult to predict and hence any catastrophic consequences
of their failure should be mitigated by design. Elastomeric seals are widely used in military and
civilian aircraft around the world. Many legacy aircraft may contain seals which have been in place
for decades. Not enough is known about elastomeric fuel seal behaviour. More international
research is required in this area (see Chapter 23).
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