The Nimrod Review - Official Documents

5.67
5.68
Chapter 5 – Leak from Fuel Coupling
monitor the frequency or extent of fuel leaks generally: they were seen as essentially a routine maintenance
issue preventing aircraft flying, but not necessarily as a threat to the aircraft. Each incident appears to have
been treated in isolation, with no attempt to gather evidence of patterns or trends. As fuel represents the most
unpredictable ‘third’ of the fire equation, 64 there should always be careful and unremitting focus on managing,
tracking, monitoring and reducing fuel coupling leaks to ALARP.
In my view, the maintenance regime operated for many elements of the Nimrod fuel system prior to the loss of
XV230 in September 2006, was unsatisfactory because:
5.67.1 Although the ‘Corrective Maintenance’ regime reflected common practice in the aviation field, it failed
to detect an increase in fuel leaks, despite the conduct of the data reviews mandated to ensure the
efficacy of the system. Perhaps the policy’s greatest failing, though, is that it appears to have imbued
a sense that fuel leaks were not an occurrence that required careful monitoring. Fuel coupling leakage
was not perceived as a significant risk in itself, as the majority of leaks were considered to be extremely
small and no one had realised the ignition threat posed by the Cross-Feed/SCP ducting.
5.67.2 Fuel leak mapping and zonal surveys of the fuel system, conducted prior to and during periodic
maintenance, were of limited value in detecting fuel leaks as they were undertaken either with the fuel
system empty, or full but not under pressure. The fuel system was not tested or inspected under pressure,
unless a component was replaced under the Corrective Maintenance policy, or following scheduled
maintenance to confirm correct reassembly of the fuel system. (The AAIB 65 report, produced as part of
the investigation into the loss of XV230, highlighted that the maintenance regimes for some civilian
aircraft included checking the fuel system for leaks while under pressure.) 66 Further, visual inspections
were confined to external signs of damage, deterioration, corrosion, or leaks. Couplings were not
dismantled to check the condition of their rubber seals; however, such a process would have involved
the wholesale replacement of the seals as once disturbed they are automatically replaced. It is difficult
to see, therefore, how incipient fuel leaks might be detected with any level of confidence.
5.67.3 There was no system of formal random ‘sampling’ of components such as seals to check their in-service
condition, or any pro-active method to anticipate leaks.
5.67.4 There was a lack of guidance in the Aircraft Maintenance Manual (AMM) regarding the fitting of
fuel couplings which meant that ground crew had no immediately available reference to ensure that
the couplings were fitted correctly. Couplings have been found incorrectly assembled allowing some
rotation of the coupling body. The Illustrated Spares Catalogue did not identify the part numbers of the
couplings illustrated, such that maintainers had to rely on using the part number of the removed part
to identify the necessary replacement – something which could perpetuate previous misidentifications.
The MOD was at fault in particular because:
5.68.1 Although Reliability Centred Maintenance was supposed to be used to determine fault trends, the MOD
failed to detect the increase in fuel leaks, as it was not focused on individual fuel system components,
nor was it directed to investigate fuel leak rates. The Maintenance Data System (MDS) managed through
the Logistics Analysis and Research Organisation (LARO) captured maintenance work orders (MWO)
but was not used to extract trend analysis for fuel leaks. The platform ‘fault trend analysis’ produced
annually did not go down to component level. There was thus no means of investigating easily fuel leak
rates, or indeed noticing increases over time; a fact illustrated by the BOI’s need to interrogate several
thousand records to produce their own analysis.
5.68.2 The system was reliant solely on IRs or SFRs, raised by air or ground crew, to reveal problems. However,
unless a fuel leak was determined to have posed a threat to an aircraft, it would generally simply be
recorded as a fault and rectified in the normal manner (although all leaks discovered whilst airborne
64 A fire requires fuel, heat and oxygen to ignite.
65 Air Accident Investigation Branch.
66 BOI Report, Exhibit 12, page 48.
87