The Nimrod Review - Official Documents

The Nimrod Review
86
Fuselage Leaks Trend
5.61
5.62
There had been an undetected but inexorable and significant increase in the leak rate in the Nimrod fleet in the
23 years before the XV230 accident. Whilst some integral wing tank leaks do not generally pose a potential
problem, leaks within the aircraft fuselage are a different matter.
The BOI’s Engineering Member, Squadron Leader Andrew Gransden, carried out a painstaking and meticulous
analysis of several thousand records61 held within the RAF’s Maintenance Data System. This exercise took many
weeks. It showed that there had been an increase in fuel leaks from couplings and seals between 1983 and 2006. 62
These had seen an increase from approximately 0.5 per thousand flying hours in the 1980s, to approximately 3.5
per thousand flying hours in the 2000s. Although, in absolute terms, this did not represent a significant number
and the rate of leakage in the 1980s was particularly low, it did represent a significant growth in reported leaks.
A subsequent report by DE&S63 suggested that the rate of increase over the period 1997 to 2006 was very low.
However, this report (compiled from BOI data) was over a much shorter timescale and utilised information for
2006 as representing the entire year, leading to a distortion of the analysis (notwithstanding that the BOI had
clearly identified that this data represented only a portion of the year in question). In my view, if one compares
like with like, the conclusions of the BOI are consistent with the DE&S report.
Comparative leak rates – other military aircraft
5.63
The DE&S study referred to above considered leak rates for a number of aircraft (Nimrod, VC10, Puma and
Tornado IDS) over the period 1997 to 2006. Although the Puma had a much lower leak rate than Nimrod,
the Tornado’s leak rate was comparable. The VC10 experienced a much higher leak rate than any of the other
aircraft, peaking at 16 faults per 1000 flying hours in 1998. This was attributed to a change of maintenance
procedures for the VC10 fuel system, which had required significant disruption of the fuel system and provoked
additional leaks. The rate of leaks for VC 10s had, however, fallen and in 2006 was only slightly higher than that
of the Nimrod. Nonetheless, it would appear from the limited data available that the Nimrod has a comparable
leak record with some other MOD aircraft.
Warning signs
5.64
There were a number of incidents over the years which indicated the potential for fuel leaks to cause significant
problems. These incidents can be seen as lost opportunities. They were potentially serious events which should
have highlighted the risks inherent in fuel coupling leaks interacting with patent ignition sources. They are
discussed further at Chapter 8.
Post accident leak history
5.65
Following the loss of XV230, there was an increase in reported fuel leaks. This can probably be explained by an
increased inspection schedule and the regular checks for leaks under pressure.
Shortcomings in fuel seal maintenance regime
5.66
It is important to place the issue of Nimrod fuel leaks in context. No Nimrod crew would knowingly accept an
aircraft with a fuel leak (with the exception of minor leaks from the wing fuel tanks, which had been assessed
as not presenting a threat to the aircraft and recorded as such in the aircraft documentation). The majority of
detected leaks were, and still are, relatively small leaks, found by the ground crew during maintenance and
immediately rectified. Nonetheless, in my view, there appears to have been an inadequate appreciation of
the risks from fuel coupling leaks in the fuselage, unsatisfactory maintenance and no overarching direction to
61 Approximately 46,000 in total.
62 XV230 BOI Report Annex I, and Addendum A1-2. Where possible integral wing tank leaks were excluded from the analysis as, from the Board’s
knowledge of the Nimrod’s systems and the reported location of the fire on XV230 it felt able to disregard fuel tank leaks as a potential source of
fuel.
63 “Review of Lifing and Maintenance Policy for Aircraft System Seals”, DE&S(WYT)/366/8/2/CASD, dated 22 November 2007.