The Nimrod Review - Official Documents

Part II – Physical Causes
14. Hawker Siddeley 7 was responsible for (1), the original design flaws in the MR1 and R1. British
Aerospace 8 was responsible for (2) and (3), the subsequent design flaws in the MR2 and R1.
(1) Cross-Feed duct
15. First, the original fitting of the Cross-Feed duct to MR1s and R1s (1969-1975) was to enable the
distribution of Auxiliary Power Unit (APU) air, and then engine bleed-air, to the engines for ground
starting. It also enabled engines to be shut down in flight and re-started, using hot bleed-air routed
from another engine via the Cross-Feed duct. The Cross-Feed duct gave rise to a serious fire hazard,
particularly in No. 7 Tank Dry Bay. The Cross-Feed duct was located athwart the fuselage, in close
proximity to fuel pipes and outside any fire zone. It gave rise to two risks: (a) fuel leaking from
adjacent fuel couplings, or other parts of the fuel system onto exposed or vulnerable parts of the
Cross-Feed duct could ignite; or (b) a leak of hot air from the Cross-Feed duct could cause heat
damage to the fuel couplings in close proximity and ignite escaping fuel. (See further Chapter 4).
(2) SCP duct
16. Second, the addition of the SCP to MR2s (1978-1984) provided additional cooling for the extra
electronic equipment when MR1s were upgraded to MR2s. The SCP required bleed-air taken off the
Cross-Feed duct. The fitting of the SCP take-off duct significantly exacerbated the fire hazard posed
by the bleed-air system because: (a) the SCP duct was routed through the bottom of the starboard
No. 7 Tank Dry Bay, thereby increasing the chances of fuel from fuel couplings above dripping onto
it and causing a pool fire at the bottom of No. 7 Tank Dry Bay; (b) immediately aft of the elbow of
the SCP duct, an expansion bellows was fitted with a laced muff which was vulnerable to the ingress
of fuel; (c) the elbow was encased in a fairing proud of the fuselage hull which was vulnerable to
fuel tracking along the outside of the fuselage; and (d) the flow of air through the Cross-Feed duct
increased the duct temperature whenever the SCP was on. (See further Chapter 4).
(3) Air-to-Air Refuelling
17. Third, the fitting of the permanent AAR modification to MR2s (1982/1989) to enable refuelling in
flight to take place. The addition of AAR capability further increased the fire hazard posed by the
Cross-Feed/SCP duct in three ways: (a) first, the refuel gallery was now pressurised whilst airborne
and undertaking AAR, thereby increasing the chance of leaks in flight contacting the Cross-Feed/
SCP duct; (b) second, it gave rise to the risk that blow-off valves on the side of the fuselage of the
aircraft (which can automatically release excess fuel from tanks during refuelling to prevent fuel
tank over-pressure) might now operate in-flight, and lead to fuel tracking back along the aircraft
and re-entering the fuselage through ports, vents and gaps in the aircraft structure; and (c) third, it
gave rise to the risk that fuel would enter the air vent system during AAR in flight and leak (both (b)
and (c) are because of an existing design problem with asymmetric filling of No. 1 tank). Previously,
such blow-off valves and fuel entering the vent system did not pose a hazard as refuelling was only
conducted on the ground.
18. The AAR modification was initially fitted in May 1982 to MR2s (including XV230) and, later, R1s as
an Urgent Operational Requirement (UOR) for the Falklands Campaign (Mod 700). Because of the
imperative to enable the Nimrod to conduct AAR during the Falklands campaign the initial UOR
design did not meet all the requirements of Aviation Publication (AvP) 970 (subsequently Defence-
Standard (Def-Stan) 00-970). Therefore, a modified design, intended to meet this standard, was
fitted, beginning in 1989, to MR2s and R1s (Mod 715). The risk of fuel from the blow-off valve of
No. 5 tank on the Nimrod’s port side entering engine intakes was addressed in Mods 700 and 715,
but the risk from the blow-off valve of No. 1 tank on the starboard side was not. It is important to
note that the effect of fitting the AAR modification was to change the function of the refuel pipes
within No. 7 Tank Dry Bay port and starboard. Previously, these pipes had not been used in flight
7 Hawker Siddeley became part of British Aerospace on 29 April 1977.
8 British Aerospace became BAE Systems Plc in November 1999 on the merger with Marconi Electronic Systems.
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