The Nimrod Review - Official Documents

7.25
Chapter 7 – Damage from Cross-Feed/SCP Duct Failure
In February 2005, BAE Systems published its Feasibility Study into the Introduction of a Bleed Air Leak Detection
System. 34 The study recommended a system which could be used both to detect leaks and automatically shut
down the cross-bleed air. The study also noted that the flow-limiting venturi, used to detect air leaks in the SCP
duct downstream of the duct that failed on XV227, would only detect leaks above a certain threshold level “thus
a high magnitude of leaking airflow would exist undetected” 35 . The ten page document was, however, a highlevel
feasibility study and noted that “the current main concern...rests with the detailed design solution further
required to achieve a fully workable system”. As discussed further below, the Nimrod IPT was in due course to
decide against fitting such a system, on the grounds that to do so was “not practical [when] factored against
the MR Mk2 OSD”. 36
7.26 On 3 March 2005, pursuant to PDS Task 16-3491, BAE Systems sent to the Nimrod IPT its “Provisions of Repair/
Recovery Advice” in relation to the failure of the SCP duct on XV227. 37 The work undertaken by BAE Systems
in this respect included an investigation into the residual strength of the lower rear spar in the area adjacent to
the failed duct which had been affected by heat. This was found to have been reduced by approximately 25%.
(In the event, the damage to the aircraft was so severe that it was deemed to be incapable of repair and XV227
was cleared for one flight only, in benign weather conditions, to join the MRA4 conversion programme). BAE
Systems’ advice of 3 March 2005 also referred to the fact that seals removed from pipe couplings adjacent to the
duct failure were found to have suffered significant deterioration, stating that it was “strongly suspected that
the deterioration of the polysulphide seal material has been caused by exposure to excessive temperatures”.
The decision to switch back on the SCP
7.27
In view of the corrosion detected on the duct that failed on XV227, the IPT decided that that particular duct
should be replaced on all Nimrod MR2s. On 26 April 2005, the Nimrod IPT issued a Routine Technical Instruction
(RTI) which permitted the SCP to be switched back on, and used in flight, once the relevant bellows section of
SCP ducting had been replaced, which work had to be carried out at the first suitable maintenance opportunity
after receipt of the replacement duct. 38 Pursuant to this Instruction, the replacement of the relevant section of
ducting duly took place on the MR2 fleet between April 2005 and April 2006. 39
Rationale of the Nimrod IPT’s decision to switch back on the SCP
7.28
The IPT’s decision to allow the use of the SCP following the XV227 incident was based on its belief that all hot air
leaks (with the exception of any from the SCP bellows that had failed on XV227 and which were to be replaced)
would be detected by the aircraft’s overheat detection system or other warning system (see further below). As
the SCP bellows had been replaced and, as the only example of that duct to fail (XV227’s) was over 20 years
old when it fractured, it was felt that there was a minimal risk of the same duct failing again. The logic of this
line of argument meant that the IPT could consider resuming use of the SCP before receiving the results of BAE
Systems’ further work into corrosion in other elements of the hot air system: even if a duct failed it would be
detected, enabling the crew to shut the system down before damage was caused to the aircraft. The IPT had
sought clarification from BAE Systems as to the capability of the hot air leak detection systems, but were told
“we are unable to comment on the capability of the existing system to detect such leaks without further study
and tasking.” 40 This would appear to demonstrate either: (a) a surprising lack of knowledge of the capabilities
of ‘as designed’ systems by the aircraft designer itself; or (b) a marked reluctance to disclose important, safety
related design information without being formally tasked and paid. In seeking clarification, the IPT was obviously
considering the possibility of the existing hot air warning system not having comprehensive coverage. It seems
it then decided to go ahead and undertake its own assessment of the position.
34 Report No. MBU-DES-R-NIM-210758. dated February 2005.
35 Ibid.
36 Out-of-Service Date.
37 RJO-NIM-05-003.
38 RTI/NIM/119, dated 23 April 2005; Nim(ES)Av(A).
39 The relevant section of ducting was replaced on XV230 on 4 July 2005.
40 E-mail dated 19 January 2005, titled “RE:Task 16-3468”, from BAE Systems and Nimrod IPT.
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