The Nimrod Review - Official Documents

The Nimrod Review
88
would generate an IR). The majority of fuel leaks were not detectable in the air and would thus be
detected when ground crew examined the aircraft during post or pre-flight inspections, or during periodic
maintenance. Thus, a simple examination of IRs and SFRs would provide only a narrow perspective on
the true rate of fuel leaks within the aircraft.
5.68.3 There is no evidence that any of the previous incidents (see further Chapter 8) sparked any particular
thought as to the risks inherent in fuel coupling failures.
5.68.4 There is no evidence that anyone had formally addressed the apparent discrepancy between the FRS
coupling manufacturer’s original recommendation in 1968 (that FRS Series 1 elastomeric fuel seals
should be subject to a five-yearly inspection regime), and the actual maintenance regime operated in
practice from 1969 to 2006. 67
Avimo Seals – Quality problem: post-2000
5.69
I now turn to consider a major issue which has arisen in relation to Avimo seals. The Nimrod is fitted with 66
Avimo seals, of which one is located in the starboard No. 7 Tank Dry Bay immediately above the SCP elbow.
Avimo “Original” Couplings
5.70
5.71
The original patent for the Avimo coupling (obtained by the Review from the British Library) dates back to 1937
as a “Resiliant Pipe Joint” for pipes subject to vibration or small relative movements, such as those in internal
combustion engines. 68 The patent was acquired by Avimo Ltd (a company formed in 1937) and apparently
approved under CAA Airworthiness number E2594, although it has not been possible to locate this document.
The couplings were used extensively on World War II aircraft69 before De Havilland (later Hawker Siddeley)
selected the coupling for use on the Comet airliner and then subsequently for the Nimrod R1 and MR2 aircraft,
around 1969. 70
In general terms, the Avimo coupling was selected to join pipe work in the Nimrod’s refuel/de-fuel and fuel
tank venting systems, i.e. (broadly) the larger diameter pipes in the aircraft’s fuel system. More particularly,
the Nimrod IPT confirmed to the Review in October 2008 that there are a total of 66 Avimo couplings fitted in
the Nimrod R1 and MR2 fuel system, comprising: (a) 46 in the refuel/de-fuel system (no longer expected to be
pressurised in flight now that AAR has ceased); (b) four in the vent system; and (c) 16 in the fuel feed system
(which see a pressure of up to 28psi in flight, but all of which are contained within the wing fuel tanks). One of
these couplings is located in the starboard No. 7 Tank Dry Bay.
67 BOI Report, Exhibit 75.
68 Patent Specification 465,724 accepted on 7 May 1937.
69 Including the Spitfire.
70 DG AS/WYT/3/2/NIM IPT & DG&S(Air)(Wyt)508324/120/1.