The Nimrod Review - Official Documents

The Nimrod Review
72
Description of Fuel Couplings
5.5
5 The Nimrod fuel system contains approximately 400 fuel couplings, of which the majority are of the FRS type.
The remaining 66 are Avimo couplings. Both FRS and Avimo couplings have been in existence for many years
and are widely used in aerospace applications. 6
FRS coupling
5.6
An FRS coupling is shown below (see Figure 5.5). It consists of an inner and outer metal sleeve, a flexible rubber
seal, two split collars, two circlips and a locking wire (not shown in the diagram). The elastomeric seal is used
to create a pressure-tight seal between two pipes. The seal is chamfered at each end and has moulded grooves
which fit over a bead formed near the end of each fuel pipe. The split collars are fitted at each end of the seal
and retained by circlips. Assembly is completed by screwing together the inner and outer metal sleeves, after
which the locking ring is applied. Each sleeve has a shoulder formed at one end, which abuts the outer face of
the adjacent split collar, thus applying pressure to the split collar (and hence the elastomeric seal) to ensure an
effective joint. The most recent Declaration of Design and Performance for the FRS 110 coupling notes that the
maximum pipe angle deflection for the installation is 1º. 7 Figure 5.5 shows an FRS coupling prior to assembly,
while Figure 5.6 portrays an FRS coupling assembled. The elastomeric seals are currently manufactured from
a Nitrile Series 4 compound (Series 1 Polysulphide rubber “Thiokol” was used until 2004, 8 but this material is
now obsolete).
Split Collar and Circlip Fuel Seal
Inner and Outer Metal Sleeves
Figure 5.5: FRS Coupling Assembly (parts shown separated)
5 Used to connect pipes to other pipes, and pipes to fuel tanks.
6 FRS couplings were fitted to Spitfires, Lancasters and Vulcans; and the original patent for the Avimo Original coupling dates back to the 1930s.
7 Eaton DDP-FRS110-Series-1, dated 7 June 2007. The original Declaration of Design and Performance issued in 1968 likewise stated under
the heading of ‘Special Limitations’ “Maximum pipe deflection 1° in any direction with or without axial pipe restraint.” See XV230 BOI Report
Exhibit 75.
8 Change of material notified at FR-HiTemp Service Information Letter SIL/CS/1/03 9.