The Nimrod Review - Official Documents

4.27
4.28
4.29
4.30
4.31
Chapter 4 – Cross-Feed/SCP Duct
The BOI determined that, whilst the un-insulated bellows were a possible source of ignition, it was much less
likely than the SCP bellows for very similar reasons to BAE Systems’ points at (1), (2) and (4) above. The BOI did,
however, note a greater risk of fuel leaking from the vent system, because of a poorly designed vent connection
and the potential for overflow during AAR. 32 As to point (5), I would have expected that if insulation had been
required, a product which would not form a ‘loose article’ hazard would have been used, i.e. it would be
fastened in such a way that it could not detach from the duct, fall onto the floor of the bay and/or potentially
become entangled in flying controls etc. The better point, in my view, is that had a muff been fitted, it might
have given rise to just the same risk of leaking fuel being held in contact with a hot duct that we can see in the
case of XV230’s SCP muff at a lower level in the No. 7 Tank Dry Bay.
Point (3), however, requires closer examination: would the AIT for AVTUR be above the surface temperature of
the duct? The surface temperature of the duct is quoted in a BAE Systems paper33 as 435°C, which corresponds
to the figures quoted by the XV230 BOI and the Air Accident Investigation Branch (AAIB). The BAE Systems
paper was produced as the result of the need to replace the insulation muffs around the SCP (and other) ducts,
because they had a high asbestos content. It considered whether the aircraft could be operated without muffs
fitted to these areas whilst replacements were manufactured. The paper stated that the muffs could only be
removed if duct temperatures were 100°C below a “worst case ignition temperature of 305°C”. 34 This ‘worst
case’ temperature was calculated for a hydraulic fluid contacting a three inch diameter pipe within a chamber
having an ambient temperature of 79°C to 90°C. 35 The only possible difference between these criteria and the
situation of the unlagged bellows is that the ambient temperature around the bellows might be lower. However,
even allowing for a significantly reduced ambient temperature of 35°C, the AIT would be 420°C. Thus, by BAE
Systems’ own criteria (admittedly produced many years after the bellows were initially left unlagged) these
bellows should actually have been insulated.
Nonetheless, it is worth considering whether, in absolute terms, the unlagged bellows actually could reach AIT
for AVTUR. In the 1960s and 1970s the Royal Aircraft Establishment (RAE) conducted laboratory experiments36 in
an attempt to determine the AIT of a number of fluids in differing circumstances. In some of these experiments,
pipes of known temperature were placed in spheres of uniform temperature. This arrangement will “quite
closely approximate to real situations”, but is “a more severe condition than is often met in practice, where a
more open environment of forced ventilating flows can give additional relief”. 37 Nonetheless, one of the papers
notes that “the extent of such relief is extremely difficult to assess and unless directly applicable data is available
it may be considered safer to use the relevant limiting ignition temperature as quoted”. 38 In other words, the
laboratory experiments provide a ‘worst case’ and it is best to use these, unless data specific to a given situation
shows the figures to be too pessimistic. This is, of course, exactly what BAE Systems did in its paper discussed
above.
The RAE experiments conducted in controlled environments indicate that the AIT of AVTUR aviation fuel vapour
on a 76mm diameter pipe (the approximate diameter of the SCP pipe) is 325°C, 39 while that of AVTUR liquid is
390°C40 (using ‘worst case’ examples as noted above). The graphs from which these figures are obtained are the
same as those cited in the BAE Systems’ report above and the ambient temperatures used are the same.
Although BAE Systems apparently provided a logical rationale and explanation for the lack of insulation on
the bellows within the No. 7 Tank Dry Bay, it appears clear that, on occasions, the temperature of the exposed
ducting could reach the AIT for AVTUR (the BOI noted that temperatures in the order of 420°C would only
32 BOI Report, page 2-33, paragraph 40(d)(i), and page 2-49, paragraph 65f(3).
33 MBU-DES-R-NIM-210565 Issue 1 dated September 2003. The temperature is for the hot air duct within the SCP external fairing. The unlagged
bellows will, if anything, be of a higher temperature as they are closer to the engines.
34 Ibid, page 5.
35 Both pipe diameter and ambient temperature can have a significant effect on the AIT for a given liquid.
36 Royal Aircraft Establishment Technical Report 72059, dated May 1972 and Royal Aircraft Establishment Technical Report 67162, dated July 1967,
Figure 41. Some of the data from these form the basis of the BAE Systems report discussed above.
37 Royal Aircraft Establishment Technical Report 72059 dated May 1972, page 11.
38 Ibid.
39 Interpolated from Royal Aircraft Establishment Technical Report 67162, dated July 1967, Figure 7.
40 Interpolated from Royal Aircraft Establishment Technical Report 67162, dated July 1967, Figure 41.
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