The Nimrod Review - Official Documents

The Nimrod Review
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Figure 5.10: Avimo Seal Damage
QinetiQ’s subsequent examination of fuel seals removed from XV255 and XV244 confirmed the conclusions
expressed in its June 2008 report. It found that one seal from each of XV244 and XV255 was split in a similar
manner to that on XV240. As with the seals removed from XV240, none of the Avimo seals in the batch
examined by QinetiQ had been observed to be leaking. They were merely replaced pursuant to STI 922 as a
precautionary measure. Examination of the most damaged Avimo seal removed from XV255 (removed from the
lower coupling joining the starboard Rib 1 Y-branch to the pipe that goes to tank 7, i.e. in a wing root location
adjacent to the No. 7 Tank 7 Dry Bay), however, revealed that its inside surface had split open and it was “very
close to failure”. Although situated to the rear of the Rib 1 area, this seal is separated by a bulkhead from the
No. 7 Tank Dry Bay and is thus unlikely to have been the source of fuel for XV230’s fire (there is, though, one
Avimo coupling within the No. 7 Tank Dry Bay). In QinetiQ’s letter dated 25 July 2008 to Nim(ES)AWS, the lead
scientist concluded that it was another case of progressive mechanical failure (fatigue) in the rubber seal and
stated: “As far as I am aware, most of the attention to date has focused on FRS couplings. We now have what
appears to be an equally plausible fuel leak location in a branch pipe joined by Avimo couplings. Although both
FRS and Avimo seals exhibit signs of material ageing and/or stress relaxation, in the Avimo seals, the primary
mode of failure appears to be progressive mechanical failure (i.e. rubber fatigue). I know STI 922 is already
replacing Avimo seals. However, I suggest you should review and consider whether sufficient attention is being
given to the inspection and replacement of this type of fuel coupling across the fleet. Note that these are
generic couplings probably also used on other aircraft.”
In a subsequent e-mail to the Nimrod IPT dated 28 August 2008, following a further examination of Avimo seals
removed from XV236 and XV231, QinetiQ reiterated its view that: “We clearly have a serious problem with the
condition of this type of seal.”
In the ensuing months, during further maintenance or as a result of the Nimrod FSRP, a further eight Avimo seals
were found to have split in a similar manner and made the subject of an F760 (Narrative Fault Report) action.
In the light of which, and acting on QinetiQ’s advice, the Nimrod IPT ordered that all 23 seals from the Avimo
couplings within the Rib 3 starboard to Rib 3 port area were to be replaced under STI 922, and that all Avimo
seals (outside the wing tanks) were to be replaced by the end of March 200973 . It further tasked BAE Systems to
take the QinetiQ investigation forward together with the Avimo coupling equipment manufacturer (who by this
time, was Taunton Aerospace Limited (see below)), at an initial meeting on 23 September 2008.
Avimo seals now being replaced with new material
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The seal compound was found not to conform to the original specification. Following the discovery of the noncompliant
formula being used for the Avimo seals, the Nimrod IPT instigated a technical investigation and an
urgent programme to find and manufacture new material for Avimo seals. I turn to consider this below.
73 Annex A to Business Procedure (BP) 1301, dated 18 September 2008.
XV240 inboard seal split through
Seal exhibited deformation consistent with surrounding components
Failure occurred in the central band, between the pipe ends