The Nimrod Review - Official Documents

The Nimrod Review
54
4.42
Gaps
4.43
not the interpretation of the maintainers, both military and civilian, in a variety of ranks, over many years. The
Refrasil in poor condition is not confined to a few isolated areas, but is relatively widespread throughout the
fleet. It is probable that the gradually deteriorating condition of the Refrasil became accepted as normal, over a
period of years, by those charged with its maintenance. There is no evidence that this view was ever challenged
by any supervisory organisation – civilian or military. The deteriorated condition of the ducting appears to have
been the accepted norm. There were very few, if any, brand new ducts manufactured with which comparison
could be made.
The deterioration in the Refrasail should, in my view, have been observed and corrected. There is no doubt that,
under the Corrective Maintenance policy for the hot air system, the Refrasil condition deteriorated. I thus agree
with the observation of the BOI that a “lack of guidance” on the allowable condition for hot air duct insulation
contributed to its gradual deterioration in some areas. 59
Whilst the deteriorated condition of the insulation did present a potential ignition risk, in my view, the most
significant risk of ignition, as demonstrated by incidents following the loss of XV230, was the presence of gaps
within the insulation associated with the muffs, which could allow fuel to enter the muff and be effectively
trapped between the surface of the duct and the inner surface of the muff. These gaps were there as a
result of inadequate design. It is design which is the predominant factor which allowed the ignition of fuel on
XV230. Nonetheless, as noted above, the maintenance policy for the hot air system did not prevent a significant
deterioration of the hot air duct insulation, inured personnel to its worsening condition, and may have prevented
a more questioning attitude to the effectiveness of that insulation.
No fire detection and suppression system
4.44
Despite the fact that fuel reaching the outer surface of the metal ducting is likely to ignite, there is no fire
detection or suppression system in either port or starboard No. 7 Tank Dry Bays. 60 There is a heat probe61 in both
No. 7 Tank Dry Bays close to the point where the Cross-Feed duct exits the bay. There is also a fire detection
wire (fire-wire) which runs round the bomb bay past openings between the No. 7 Dry Bays and the bomb bay.
But in no sense can either be considered a fire detection system in starboard No. 7 Tank Dry Bay, still less a fire
detection and suppression system.
Cross-Feed/SCP duct posed a serious fire risk
4.45
The Cross-Feed/SCP duct posed a serious fire risk on the Nimrod MR2 for four reasons: (1) location; (2) design of
the No. 7 Tank Dry Bay; (3) inadequate insulation; and (4) lack of fire protection (see Figures 4.11-4.13 below).
(1) Location
4.46
The location of the Cross-Feed/SCP duct across the centre section of the fuselage and within the No. 7 Tank Dry
Bays made it a potential ignition source for any fuel which might escape from the numerous fuel, hydraulic and
vent systems all around:
its location in starboard No. 7 Tank Dry Bay was very close to fuel pipes and couplings (there are several
fuel couplings within 12 inches);
the location of the SCP duct at the lowest point of starboard No. 7 Tank Dry Bay made it particularly
vulnerable to fuel either leaking from above or tracking along the fuselage; and
the location of the SCP duct in an external fairing only 2.1 metres aft of the No. 1 tank blow-off valve
made it particularly vulnerable to fuel tracking back from the blow-off valve during AAR.
59 BOI Report, paragraph 31(b) [2-22].
60 BOI Report, paragraph 43 [2-39.]
61 A Kiddie-Graviner temperature sensitive switch (part of the centre section overheat detection system).