The Nimrod Review - Official Documents

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Chapter 23 – Age Matters
A wide range of recommendations are made relating to: the review of skills, training and awareness; Forward
and Depth policies, procedures and practice; the importance of fault recording; the frequency and depth of
zonal inspections; and the need for the safety management system to reflect the ‘as-maintained’ rather than
the ‘assumed as-maintained’ condition of the aircraft.
Out-of-Service Date continually extended
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23.30 Attention to a number of these matters was bedevilled by the fact that the OSD of the MR2 fleet was
continually put back because the In-Service Date of the MRA4 slipped on no less than six occasions in the period
1999-2008 (see Chapter 14).
Elastomeric seals – lessons to be learned
Inconvenient truth
23.31 As explained in Chapter 5, 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 fuel seal behaviour. More international research is required in this area.
‘To life or not to life?’
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A major issue remains: ‘To life or not to life?’. The question is whether or not to place a hard life on elastomeric
seals used in aircraft fuel and hydraulic systems, such that they must be replaced after a defined period.
There are two schools of thought, one in favour and the other against. Respectable arguments can be made
either way. Experienced opinions within the RAF itself diverge. The BOI recommendation that a life be determined
for FRS coupling seals seemed to many, heretical. The RAF Kinloss Station Commander supported the BOI
recommendation that a life for FRS Series 1 fuel seals be determined, but rightly said “such a life should be
based on an understanding of the failure mode and factors that promote degradation as these will determine
when a seal is not fit for purpose rather than simply installed or shelf life”. 13 The Air Member for Materiel took
the view that applying a finite life to seals might not yield improvements in safety because of e.g. associated
disturbance to other systems and the fuel system itself, and said primacy should be afforded to taking action to
mitigate any consequential hazard from a leak. 14 In November 2007, DE&S Safety & Engineering Air-CASD-AVI
Hd carried out a detailed review of the MOD’s lifing policy for aircraft system seals, which concluded that there
was no requirement for an MOD-wide single policy for lifing seals, but this depended on “design requirements
being met throughout the life of the aircraft and best practice maintenance policies with regard to seals being
applied”. 15
Eaton Aerospace’s post-XV230 decision to revise its Declaration of Design and Performance (DDP) for the FRS
seal to recommend replacement after 25 years was rejected by the MOD. This was primarily because Eaton was
unable to offer any evidence of analysis to support its recommendation, as was the case when they presented
to the Review. There appears to be little evidence of other manufacturers or operators following a similar path.
Fuel seals are still regarded to some extent as a product to be fitted and forgotten. This is not a view unique
to the MOD or the UK, but appears to be common among other nations’ military and, indeed, civilian aircraft
fleets. However, it is clear that, on a few occasions, certain fuel seals have been lifed.
13 See Comments on BOI Report by Station Commander, paragraph 9.
14 See Comments on BOI Report by Air Member for Materiel, paragraph 3.
15 See DE&S Safety & Engineering Air-CASD-AVI Hd: Review of the Lifing Policy for Aircraft System Seals, dated 22 November 2007.
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