The Nimrod Review - Official Documents

The Future
Chapter 23 – Age Matters
23.42 For the future, it is right to emphasise that any decision to sample or replace seals on a periodic basis ( i.e. such
as the Nimrod FSRP) must balance the likely benefits with the potential disadvantages. (It should be noted that
because of the fact that: (i) the Cross-Feed air duct is now only used on the ground and the use of the SCP
has been prohibited entirely; and (ii) Air-to-Air Refuelling has been stopped, the fuel leak/ignition risk profile in
Nimrod aircraft has of course been very substantially reduced.)
23.43
23.44
It is also right to emphasise that the mere fact that sampling or periodic seal replacement may not be an easy,
straightforward or inexpensive exercise, (particularly in relation to ‘legacy’ aircraft’ which were not always built
for ease of maintenance), should not be seen as a reason to fail actively to consider the necessity for such proactive
fuel seal management measures in the light of e.g.: (a) current assessments of perceived risks; and/or (b)
evidence of an increasing leaks trend; and/or (c) life extensions to the assumed design life of aircraft. The Nimrod
FSRP represents the type of outcome that could result from a formal review of maintenance policy prompted by,
e.g., adverse trend data or an aircraft life extension in the future.
In my view, targeted seal replacement and/or seal sampling are sensible, measured, and proportionate approaches
to the problem of management of fuel and hydraulic seals, and should be key tools in the management of fuel
system reliability in the future, unless and until a better understanding of elastomeric lifing can be found.
23.45 In any event, it is vital in the future that active thought is always given to the appropriate management of all
elastomeric seals.
Generic Problems with some ‘aged’ and ‘legacy’ aircraft’
23.46
Generic problems arising in relation to the maintaining and repair of some aged and ‘legacy’ aircraft include
the following:
(1) They may have been designed and built to standards which would not be acceptable today;
(2) They may not have been designed with ease of aircraft access and maintenance in mind;
(3) There may be a diminishing pool of engineers with the requisite specialist ‘old-fashioned’ engineering
skills;
(4) Maintaining relevant ‘corporate knowledge’ and records becomes more difficult as experienced design
and maintenance personnel at the Design Authority and in Depth and Front Line maintenance retire;
(5) The availability of spare parts becomes more difficult as the number and interest of manufacturers dwindles
as the OSD looms and ‘robbing’ spares from other aircraft become less easy;
(6) Adding modifications and integrating new systems with old can become more difficult as an aircraft
ages;
(7) Different systems and components age at different rates;
(8) Not enough is yet known about the ageing of certain ubiquitous components (I explain above and in
Chapter 5 that elastomeric fuel seals do age for a variety of reasons and refer to research by e.g. RAPRA
Technology Ltd 22 that “many rubbers aged in the compressed condition, particularly in the hot climates,
have set to such an extent after less than 40 years that their ability to act as an efficient seal is very
doubtful” 23 and research by QinetiQ 24 that Thiokol can suffer long-term temperature-related degradation);
and
(9) Determining a ‘baseline’ of safety for such aircraft becomes more difficult the older an aircraft becomes.
22 Natural Ageing of Rubber, RAPRA Technology Ltd, 2000.
23 Ibid, page 10.
24 Nimrod Fuel Seals, Literature Survey, Issue 1, September 2007.
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