The Nimrod Review - Official Documents

The Nimrod Review
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5.27
to have been that, once fitted, fuel seals could be left in place indefinitely. For example, the Defence Equipment
& Support “Review of Lifing and Maintenance Policy for Aircraft Systems Seals” dated 22 November 2007 refers
to anecdotal evidence of 55-year old Nitrile seals which had remained in oil for their fitted life being “in perfect
condition.” 23 Whilst this may be true, it must be balanced against the equally valid fact that clearly some fuel
seals do not last anything like 55 years. The focus of any concern should be on early and medium failures, not
just those that survive to a ripe old age.
RAPRA Technology Ltd conducted research in 2000 into the effect of age on elastomeric material performance,
but only up to the age of 40 years. 24 Although this paper concluded that: “none of the materials (elastomers
exposed to differing climates over a period of 40 years) has deteriorated to such a degree as to be totally
unusable”, 25 it also noted 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”. 26
A study by QinetiQ27 in 2007 of the relevant literature concerning Thiokol (the material used in the FRS Series
1 fuel seals before its replacement with Nitrile) suggested that it could suffer temperature-related degradation;
the DDP for FRS110 Series 1 seals specifies a maximum temperature of 70oC. 5.28 This conclusion was reiterated by QinetiQ in its later report in 2007 into the “Removal and Examination of Fuel
Seals from Nimrod XV236”. 28 QinetiQ considered that two different ageing mechanisms were likely to occur
during the life of a polysulphide seal: the first being ambient temperature ageing, such as will occur in store or
when used in benign temperature conditions; and the second being elevated temperature ageing, which will
occur when the seals are exposed to high temperature environments. QinetiQ’s conclusion was that ageing
effects and thermal degradation initially cause hardening and lack of resilience, and that continued exposure to
heat (especially to temperatures above 70°C) can cause significant softening of the seal.
5.29
5.30
Work by the US Defence materials experts, Advance Materials Manufacturing and Testing Information Analysis
Center (AMPTIAC) 29 in 2003 on the service life of elastomeric seals, also suggests that the chemical structures,
and thus the physical properties of elastomers, degrade over time. Ageing can be inhibited or accelerate,
depending on the environmental conditions surrounding an elastomer. Ageing usually takes place under such
conditions as heat, sunlight, oxygen, ozone, moisture or stress; and the rate of ageing is dependent on the
amount of exposure to these factors. Such conditions will degrade the properties either by causing additional
cross-linking or by chain scission. Ageing is an irreversible process. 30
The inconvenient truth is thus that elastomeric seals do age. The problem is that this process is not yet well
enough understood.
(2) Misalignment
5.31
31 A number of leaking seals from Nimrods have been subject to detailed analysis since the loss of XV230. In the
case of the FRS Thiokol and Nitrile seals, the principal cause of degradation appears to be stress relaxation (i.e.
where the stress required to subject a material to a fixed deformation decreases with time), possibly caused by
a combination of high temperature and coupling misalignment. Specific limitations are detailed for the angular
displacement between pipes connected by couplings and for the distances between pipes and the coupling
assembly. 32 In many instances where couplings have been observed to leak, the assembly has been found to be
outwith the specified limits.
23 DE&S(WYT)/365/8/2/CASD, page 7.
24 Natural Ageing of Rubber, RAPRA Technology Ltd, 2000.
25 Ibid, page 8.
26 Ibid, page 10.
27 Nimrod Fuel Seals, Literature Survey, Issue 1, September 2007.
28 QinetiQ/EMEA/S&DU/CR0703772/2.0 dated November 2007.
29 AMMTIAC Report dated April 2003: pages 21 to 25 deal with the service life of elastomeric seals.
30 Ibid, paragraph 5.12: Ageing.
31 QinetiQ Reports: Examination of Fuel Seal from Nimrod XV260, dated August 2007; Removal and Examination of Fuel Seals from Nimrod XV236,
Issue 2, dated 2007 ; and Examination of Original Avimo Fuel Seals from Nimrod XV240, Issue 1, dated June 2008.
32 Chapter 41-20 of Air Publication (AP) 101B-0503-1DK dated August 2002 and AP 106D-4402-13A. See also the revised DDP published by Eaton
Aerospace after the XV230 incident dated 18 October 2007.