The Nimrod Review - Official Documents

The Nimrod Review
20
Nimrod Technical Developments
2.30
2.31
The modification of the design of the Comet to allow the birth of the Nimrod involved a significant redesign
and the addition of new fuselage elements as mentioned above. The conversion of the Nimrod MR1 to the MR2
was driven principally by a need to upgrade the aircraft’s sensor systems. However, as a result of the additional
electronics of the MR2, the aircraft’s air conditioning system had to be supplemented to ensure additional
cooling. AAR was introduced to the Nimrod as part of the urgent response to the Falklands crisis as noted
above.
The following paragraphs set out the timeline of those changes in design which are relevant to the loss of
XV230 many years later.
Conversion from Comet to Nimrod MR1
2.32
2.33
The replacement of the Comet’s Avon powerplants with the Spey 250 engine necessitated the introduction of
an air starter system. To provide this, APU was installed in the tail area and a duct used to take hot air from the
APU to the engine starter turbine. The endurance of the Nimrod could be increased by shutting down up to
two engines. As and when it was necessary to re-start engines during flight, hot, pressurised air could be routed
through the Cross-Feed duct from engines on one side of the aircraft to re-start the engines on the other side.
Therefore, there were occasions, both on the ground and airborne, when the Cross-Feed duct would contain
very hot air and consequently it was insulated with a fibreglass covering contained within a stainless steel outer
layer. This insulation was primarily designed to prevent heat damage to the surrounding structure.
In order to further extend the endurance of the MR1, additional fuel tanks, No. 5 and No. 6, were added below
the fuselage cabin floor, in what were previously baggage compartments, and two further tanks, No. 7 tank
port and starboard, were attached on either side of the fuselage within the wing root area. The Cross-Feed duct
passed in front of the new No. 7 tank, running through an area known as the No. 7 Tank Dry Bay on the port
and starboard side.
Conversion from MR1 to MR2
2.34
The principal reason for the conversion of the Nimrod from MR1 to MR2 was to upgrade the aircraft’s sensor
systems. The heat generated by the new electronic equipment on board required additional air supplies for
cooling. For this purpose, a Supplementary Conditioning Pack (SCP) was fitted to the Nimrod MR2 in the rear
of the aircraft. The SCP was supplied with engine bleed-air from a branch taken off from the Cross-Feed duct.
This air was ducted through a pressure regulating valve in the bomb bay and then ducted rearwards outside the
bomb bay. At the point where the SCP leaves the bomb bay, it is routed through the lower part of the starboard
No. 7 Tank Dry Bay. With engines at high power at low altitude, the temperature of the air in the SCP duct
could be in the region of 470ºC. Thus, this addition to the already existing Cross-Feed duct placed a potential
source of ignition in the aircraft, the significance of which was not recognised until over 25 years later, with the
investigation into the loss of XV230; this is discussed in more detail in Chapter 4.
Development of the AEW Mk3 and AAR Capability
2.35
2.36
As explained above, the original Nimrod AAR installation was fitted during the Falklands conflict in 1982.
Subsequently, the MOD decided to upgrade the AAR system and move the refuelling pipes, for the most part,
out of the cabin and into the bomb bay. In 1985, in the course of the AEW3 programme, which was also
required to have an AAR capability, the AAR system design was refined, to enable its incorporation as a formal
modification to the aircraft design.
During the initial incorporation of AAR into the AEW3, one of the fuel system design features which was
considered by British Aerospace was the effect of the fuel tank blow-off valves. These valves are fitted to all, bar
two, 23 of the aircraft’s fuel tanks and operate as pressure relief valves: should the pressure in a fuel tank exceed
23 The No. 4A tanks.