The Nimrod Review - Official Documents

The Nimrod Review
134
Causation
9. The Board of Inquiry was right to conclude that, whilst a Cross-Feed/Supplementary Conditioning
Pack duct failure could not be ruled out as a causal mechanism for the fuel source, it was much
less likely than the other two potential causes (fuel coupling leak or escape of fuel during Air-to-
Air Refuelling). In my view, a sudden major rupture of the bleed-air system would not have gone
unnoticed, let alone unremarked upon, by the crew of XV230. Moreover, such a failure would have
made it impossible for the Air Engineer to report the Supplementary Conditioning Pack overheat,
which he did at the start of XV230’s emergency. Further, a small hot air leak is much less likely
to have caused sufficient degradation unless coincidentally placed close to a coupling in the fuel
system; it would also require a significant amount of time to do so.
Introduction
7.1
In this Chapter, I first analyse early problems with the bleed-air ducting experienced in the 1980s and 1990s and
a later incident on XV227, before explaining my conclusion in relation to whether or not, a bleed-air duct failure
is in fact a likely cause of the fire on board XV230.
The Nimrod Bleed-Air System
7.2 The Nimrod bleed-air system and its function are described in detail in Chapter 4.
Risks of fire from failure of Cross-Feed/Supplementary Conditioning Pack duct
7.3
In my view, there was a clear risk of a catastrophic in-flight fire resulting from a failure or rupture of the Cross-
Feed/Supplementary Conditioning Pack (SCP) duct. This arose because of the high temperature (400ºC+) and
pressure of the air contained therein and the juxtaposition of the duct to numerous fuel couplings in the
starboard No. 7 Tank Dry Bay. A failure of the duct would have allowed the escape of hot bleed-air which might
damage the rubber seals leading to leakage of fuel onto very hot duct surfaces, either through pre-existing
gaps in the insulation or through new gaps caused by damage occurring during the original failure. The risks
were amply illustrated by the XV227 incident referred to below, when a duct within the SCP system fractured,
allowing hot air to escape and cause considerable damage to the aircraft.
Breach of applicable design standards
7.4 In my view, this scenario represented a “single point failure” and a breach of general “fire zone” safety standards
applicable to the original Nimrod MR1 and MR2 designs in 1969 and 1979, in particular AvP 970 Chapter 715. 2
As explained in Chapter 4, I do not accept the (frankly far-fetched) argument put forward on behalf of BAE
Systems that a failure of the Cross-Feed/SCP duct would somehow ‘blow all the fuel away’ and that an ignition
scenario was not credible. (For duct design issues, see Chapter 4 generally).
Maintenance policy
7.5
Prior to the loss of XV230, the Nimrod bleed-air system was maintained under a policy of ‘Corrective
Maintenance’. The hot air ducts between the engines (including the Cross-Feed ducts) were subject to a leak
test under pressure at Minor, Minor Star and Major maintenance. 3 Insulation muffs were removed during this
process to facilitate inspection. The Maintenance Policies (MP) for the inspection note that: “Examination is to be
both audible and visual, checking for leaks and cracks. Particular attention is to be paid to the ducting bellows
2 AvP 970, Volume 1, re-issue, 1 June 1960.
3 Using Maintenance Procedures (MP) 48-00 and 23-11/6.