The Nimrod Review - Official Documents

The Nimrod Review
7.47
Causation
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valves). Amongst other recommendations in the report, BAE Systems recommended that the ducting adjacent
to Rib 2 should be replaced as soon as practicable, as these ducts were considered most likely to fail; the Cross-
Feed duct should also be replaced as soon as practicable and the SCP should remain permanently mechanically
isolated. The report concluded: “The conclusion from this report confirms that defects continue to be found in
ducts as previously reported, in particular from the ECU to Cross-Feed cocks. 59 The ducts are past their published
safe lives and corrosion damage is prevalent. Consequently, it is recommended by the [Design Authority] that
the above recommendations concerning duct replacement and provisioning, increased protection and increased
inspections are adopted by the IPT at the earliest opportunity.”
As a result of this report, in June 2008 the Nimrod IPT issued a Special Technical Instruction (STI 926) regarding
a fleet-wide duct replacement programme. This programme required the replacement of the high pressure hot
air ducts between the ECU and the Cross-Feed valves, and also those that formed the Cross-Feed system. The
Nimrod IPT determined that, for the risk to the aircraft to remain ALARP, the replacement programme should be
completed by 31 March 2009 and that aircraft that had not had their hot air ducts replaced by this time would not
be flown until the work had been completed. Although it had originally been planned that the programme would
be completed by 31 March 2009, delays in the provision of replacement parts rendered this target unachievable;
as a result a number of Nimrod aircraft were not flown after this date until the new ducts were fitted.
Relevance of XV227 and the Earlier Duct Corrosion Problems to the loss of XV230
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The UI into XV227 concluded that the expansion bellows had failed due to corrosion and caused a leak of very
hot bleed-air, for up to 40 minutes. Significant damage had been caused to the adjacent airframe structure and
systems. In particular, the fuel seals in adjacent pipework were shrivelled almost to the point of destruction (see
below). The fuel from the No. 7 tank had been used at an early stage in the flight and there was no evidence of
any leakage of the small amount of residual fuel remaining in that tank. However, when fuel was subsequently
transferred from the No. 7 tank, damaged seals in surrounding FRS couplings were observed to leak. One of
the damaged seals is shown in Figure 7.1. Figure 7.2 shows an undamaged/unused seal. Furthermore, the
minimum static reserve factor of the aircraft’s rear spar had been reduced by approximately 25%, i.e. below
the level which “is normally accepted for safe unrestricted operation of the aircraft”. 60 Indeed, as stated above,
the damage to the aircraft was so serious that it was subsequently cleared for one flight only “in clear and calm
conditions”, 61 with restrictions applied to its operating envelope, in order to re-locate to BAE Systems Woodford,
to join the MRA4 programme.
Figure 7.1: Shriveled fuel seal from XV227 Figure 7.2: Normal Fuel Seal
showing effects of severe heating
59 More properly referred to as ‘air valves’, not ‘cocks’.
60 Air Incident Report KIN/97/04, page 5.
61 DLO(Strike)(WYT)/512725/17, dated 7 June 2005.