The Nimrod Review - Official Documents

The Nimrod Review
50
Shrouds
4.32
be reached with high power settings). 41 The chance of fuel actually reaching and remaining on this potential
point of ignition in sufficient quantity to cause a fire is, however, relatively low. During a test to determine the
potential leak paths for fuel, should it leak from the aircraft vent system couplings above the unlagged bellows,
none touched the bellows. 42 This was, of course, a static test and could not reproduce the variations in pitch
and bank that would occur during flight. There is no doubt though, from the information calculated by RAE
and subsequently utilised by BAE Systems, that the unlagged bellows could reach the AIT for AVTUR and,
therefore, by BAE Systems’ own criteria, should have been lagged. As discussed below, however, the addition of
an insulation muff or shroud could have exacerbated the risk of fire in the event of leaking fuel making contact
with the bellows. Whether or not the risk was properly considered at the Nimrod ‘acceptance conference’
during August 1968 is a moot point (see further below).
There are two metal shrouds covering the Cross-Feed expansion bellows in the fuselage centre section; these
shrouds appear to be the device used within the MR1/R1 design to insulate expansion bellows. They consist of
two half-shells, each comprising two layers of stainless steel, separated by a layer of insulation. One of these
shrouds was involved in the (fortunately short-lived) fire discovered on Nimrod XV249 on 28 January 2008. An
accelerant43 had entered the shroud (thus it is clearly not liquid-proof) and been ignited; the ignition process was
complex, but it is believed that fuel soaked into a Cross-Feed duct mounting block, ignited and in turn ignited
fumes emitting from the fluid in the shroud. 44 The investigation suggested that the accelerant had pooled within
the shroud because it had been closed such that the join between its two halves was in a horizontal alignment; it
was postulated that, if the shroud had been rotated through 90 degrees, the accelerant would have drained out
between the join. However, prior to this incident, there was no requirement to fit the shrouds in this manner and
no indication that not doing so could prove a fire hazard; furthermore, there are no drain holes in the shroud
to allow drainage.
Laced flexible muffs
4.33
4.34
During the conversion process from MR1 to MR2, the addition of the SCP also brought with it the need to
insulate the new duct from the surrounding structure, including the external fairing aft of the No. 7 Tank Dry
Bay. The majority of the insulation on the SCP pipe is Refrasil but the expansion joints were covered by flexible
muffs laced up with wire along a longitudinal seam (see Figure 4.8 below). The selection of the laced muff was
driven by the need to minimise its surface area to allow it to fit into the cramped confines of the (new) SCP
fairing. 45 Produced initially by Bestobell, the specification of these muffs describes the end finishing as “draw
wire or cord in the hem”, “to prevent the ingress of fluid” and that “all external unproofed stitching or cloth
is sealed against fluid contamination”. The muffs are faced with a metallic lacquer, which provides resistance
against absorption. BAE Systems’ understanding was that these muffs would provide a liquid-proof barrier. In
a BAE Systems document discussing the need to remove or replace some of the muffs, which had asbestos
content, a Chapter is entitled: ‘Lagging and Insulation Muffs Design Philosophy.’ 46 This Chapter notes that: “If
flammable liquids were not contained and drained, then the hot ducts would require lagging to bring surface
temperatures below the ignition point of any possible leaking flammable fluids or vapours. The latter design
philosophy applies in the APU and pannier bays on the Nimrod aircraft”. 47
In an obvious contradiction of the design philosophy expressed above, the muff immediately aft of the SCP
elbow is not long enough both to cover the exposed section of duct and to fit over the end of the Refrasil, to
form a liquid tight seal (see Figure 4.8 below). In any event, the rear end of the muff abuts against a clamp
which is not insulated. Moreover, the movement of the expansion bellows relative to the aircraft will always
41 BOI Report, paragrapth 42(c) [2-38].
42 NAEDIT/1505/09/14/Task, dated 1 September 2009.
43 The investigation was unable to determine whether the accelerant was aviation fuel or hydraulic fluid.
44 QinetiQ report /08/01561/5.0 dated 29 January 2009. An additional report is being compiled but was not available at time of publication of the
Review.
45 Evidence from Martin Breakell of BAE Systems, May 2009.
46 Report MBU-DES-R-NIM-210565 dated September 2003 “Removal of Asbestos Insulation Muffs in the APU Bay PDS Task 16-3381”, page 3.
47 Report MBU-DES-R-NIM-210565 dated September 2003 “Removal of Asbestos Insulation Muffs in the APU Bay PDS Task 16-3381”, page 3.