The Nimrod Review - Official Documents

Section One: Ignition Source
CHAPTER 4 – CROSS-FEED/SCP DUCT
Contents
Chapter 4 addresses the ignition source, the Cross-Feed/SCP duct. It answers the following questions:
What was the history of the fitting of the Cross-Feed/SCP duct?
What was the purpose of the Cross-Feed/SCP duct?
How and why did the Cross-Feed/SCP duct pose a risk to the Nimrod?
Chapter 4 – Cross-Feed/SCP Duct
Was the Cross-Feed/SCP duct in breach of design standards and regulations applicable of the time?
Who was responsible for any breaches of design standards and regulations?
Summary
1. The Cross-Feed duct was part of the original specification of every Nimrod MR1 and R1. 1 The
Supplementary Conditioning Pack (SCP) duct was added to the upgraded Nimrod MR2s. 2 The
combined duct is known as the Cross-Feed/SCP duct.
2. The purpose of the Cross-Feed/SCP duct was to allow high pressure, high temperature (around
400 o C+) “bleed-air” to be transferred between the engines and to the SCP.
3. It was always intended and understood that the Cross-Feed/SCP duct would be used regularly by
crews in-flight.
4. The Cross-Feed/SCP duct posed a potentially catastrophic fire risk to the Nimrod fleet from the very
beginning.
5. The Cross-Feed/SCP duct posed a serious fire risk to the aircraft for four separate reasons due to
design flaws:
(a) its location at the bottom of a bay closely packed with fuel pipes and couplings;
(b) the design of the No. 7 Tank Dry Bays, which allowed fuel pooling;
(c) inadequate insulation, which was vulnerable to fuel ingress; and
(d) the absence of fire protection in those bays.
6. The Cross-Feed/SCP duct gave rise to two main fire risks to the aircraft. First, the Cross-Feed/SCP
duct was vulnerable to fuel and/or hydraulic oil coming into direct contact with its very hot 3 metal
surfaces as a result of leaks from couplings or other sources, leading to auto-ignition. Second, the
Cross-Feed/SCP duct itself posed a direct threat to the fuel system because an escape of hot air could
degrade fuel seals in close proximity in the No. 7 Tank Dry Bays, leading to the escape of fuel and
auto-ignition.
1 MR1s and R1s were delivered by Hawker Siddeley to the RAF between 1969-1975.
2 MR2s were delivered by British Aerospace to the RAF between 1979-1984.
3 The AAIB report into the loss of XV230 shows that air in the duct at the point it leaves the engine could be at up to 510ºC (depending on
altitude and external air temperature) and would have cooled by about 10 deg by the time it reached the No. 7 Tank Dry Bay area. At the time
of the initiation of XV230’s fire, the air in the ducts within the bay would probably have been at approximately 420ºC or higher.
39