The Nimrod Review - Official Documents

The Nimrod Review
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However, the IPT’s assessment proved to be imperfect and was based on an overly optimistic understanding of
the capabilities of the centre section overheat detection system. The Nimrod’s overheat detection system has 28
detectors (temperature sensitive switches) that operate warning lights on the air engineer’s panel if they detect
a temperature rise above a pre-set level. The detectors are connected in five groups, operating at either 150°C
or 230°C depending on their installed position. One group is positioned in each Rib 2 (between each pair of
engines) and one group in each wing leading edge. The remaining group forms a “centre section” overheat
detection system, as described in the Aircrew Manual Book 1 (Technical). 41 However, this description is, to some
extent a misnomer: this element of the system in fact consists of ten sensors in two groups either side of the
aircraft fuselage. The actual locations of the sensors are: one in the No. 7 Tank Dry Bay, one just in the bomb
bay (adjacent to the No. 7 Tank Dry Bay) two in Rib 1 and one in Zone 2 of the inboard engine.
In view of the detection temperature of the centre section overheat sensors, it is unlikely that they would detect
anything other than a leak in their close vicinity, meaning that a significant section of the Cross-Feed pipe as
it traverses the bomb bay is effectively unmonitored. Further, in the case of XV227, the No. 7 Tank Dry Bay
had been filled with enough hot air to cause significant and widespread damage. One of the centre section
overheat system’s detectors is in that bay and yet there is no record of it being activated. Unfortunately, there is
no record of whether or not this sensor was subsequently tested for serviceability. It was worth remembering,
however, that in the case of XV230, the BOI recorded that the centre section overheat warning was not reported
as activated by what was, without doubt, a large fire within the aircraft’s starboard No. 7 Tank Dry Bay, the
heat from which reached into the bomb bay. The BOI did note that there could be two explanations for this:
(1) the electrical power to the system could have been disrupted by the fire before it could register the rise in
temperature; or (2) another possibility was that the centre section overheat was triggered at about the same time
as the other alarms, but the air engineer chose not to mention it on intercom as it was, by then, superfluous. 42
There is nonetheless, in my view, sufficient evidence to conclude that the centre section overheat sensors would
not necessarily detect a leak in the Cross-Feed pipe as it traverses the bomb bay.
This was, of course, a weakness that had existed in the aircraft since its inception, but a more detailed analysis
should have picked it up. Indeed, the XV227 UI had noted this weakness: “there is no hot air leak warning
system for the ruptured duct or the cross-bleed air duct as it passes through the bomb bay”. One of the
UI’s recommendations was that a leak-detection capability should be fitted for the “ruptured duct and crossair
bleed ducts” (emphasis added), although this was subsequently rejected as it could not be fitted in the
remaining life of the aircraft. The UI report was published in July 2005, some two to three months after the
decision was taken to switch the SCP back on in April 2005. As noted above, the BAE Systems study into a
leak detection system had also identified a weakness in the current warning system, i.e. that the venturi/under
pressure warning system would only detect relatively large leaks. However, at no stage does anyone appear to
have attempted to reconcile the conflicting views of the IPT, BAE Systems and the UI over the efficacy of the
aircraft’s hot air leak detection system.
It was suggested at the Inquest by Air Commodore George Baber that the decision to allow the SCP to be
switched back on was not simply an engineering decision, but was further influenced by pressure from the
aircrew community: “And in conjunction with discussions with our operating crews, and it wasn’t just a sole IPT
decision, this was done in conjunction with the operating crews, the decision was made that this was a sensible
thing to do”. 43
Whilst a number of Nimrod IPT witnesses confirmed to the Review that the operator community was keen
to regain the use of the SCP (particularly as summer approached in the Gulf), there is no evidence to suggest
that the decision to switch the SCP back on after the XV227 incident was improperly affected by operational
pressures. On the contrary, the aircrew at RAF Kinloss relied on the engineering judgment of the IPT to provide
them with an aircraft that was safe to operate.
41 AP101B-0503-15A, Part 2, Chapter 8.
42 With a bomb bay fire alarm, an underfloor warning activated and reports of smoke coming from two underfloor bays, reporting the centre section
overheat would have added nothing to the crew’s understanding of the situation.
43 Inquest transcript, 13 May 2008, page 227.