The Nimrod Review - Official Documents

The Nimrod Review
7.20
138
When XV227 landed on the day of the incident, the aircrew had not noticed any abnormalities, but were
advised by the ground crew that there was a hole in the fairing of the SCP duct. Examination by the ground
crew indicated that the expansion bellows had fractured, allowing the hot air to escape, both to atmosphere
and within the No. 7 Tank Dry Bay. Because of the potentially catastrophic nature of the event, a Unit Inquiry
(UI) was convened. I discuss the findings of the UI further below.
Cessation of use of the SCP following the XV227 incident
7.21
31 On 26 November 2004, the Nimrod IPT issued an Urgent Technical Instruction (UTI) that required, within seven
days, a visual fleet inspection of the duct that had failed on XV227 (looking for evidence of hot air leakage)
and of the adjacent structure (looking for any signs of scorching and discolouration). Given the proximity of the
failed duct to the No. 7 fuel tank, UTI/NIM/026, issued on 10 December 2004, further required that the SCP
should be electrically isolated for “Nil use” across the fleet pending further advice from the IPT.
Investigations following the XV227 incident
7.22
7.23
7.24
The IPT’s initial report of the incident on XV227 (Annex A to BP 1301) recognised that it might have wider
implications for airworthiness and referred to the need to investigate whether any similar ducts posed a risk to
the airworthiness or the flight safety of the Nimrod fleet. BAE Systems was accordingly commissioned by the
Nimrod IPT to carry out a formal fault investigation of the failed duct and to undertake a study to identify similar
bleed-air ducts that might be vulnerable to a similar failure (PDS Task 16/3468).
32
BAE Systems provided its conclusions in relation to PDS Task 16/4368 to the Nimrod IPT on 7 January 2005,
identifying those sections of ducting which incorporated bellows which were subjected to high pressures
and temperatures. BAE Systems noted that “the duct failure presents a significant risk to the continued safe
operation of the Nimrod aircraft” and recommended that a feasibility study be conducted to develop a bleed air
leak detection system. The IPT duly tasked BAE Systems to carry out such a study.
On 10 January 2005, following laboratory analysis, BAE Systems reported the results of its fault investigation
of the failed duct. 33 BAE Systems concluded that the ducting contained a number of locations along its length
where pitting corrosion had occurred, in particular at the enclosed ends adjacent to the welded joint securing
the retaining ring to the ducting. The strength of the duct had also been weakened by cracking in the area of
the corrosion. A few days later, on 14 January 2005, BAE Systems sent to the Nimrod IPT its Fault Investigation
report (MOD Form 761) into the incident. That report contains a number of noteworthy comments, as follows:
7.24.1 The narrative report included that “it was suspected that fuel contamination had occurred due to the
proximity of fuel tank 7 to the hot air leak.” (The fuel contamination referred to was discoloration of
the residual fuel in the No. 7 fuel tank.)
7.24.2 The section on fault trend analysis noted: “For reference only: A bleed air duct failure of this nature,
in this aircraft zone, is recognised in the Nimrod Baseline Safety Case hazard ref. NM/H73.” (It is
regrettable that further analysis in relation to the hazards identified in the Nimrod Safety Case (NSC)
was not undertaken; see further Chapters 10 and 11).
7.24.3 The fault investigation and conclusions noted that “the failed Flexible Duct was manufactured in
1980 and it is believed that it was installed on to XV227 in 1986. This duct is not a lifed item although
a report was published by BAE which recommended the lifing of this type of duct (1982) ... It is
not apparent whether any of the information in this report was adopted into the Nimrod Servicing
Schedule.”
7.24.4 It was noted in the recommendations that the Nimrod IPT had placed an order enabling a fleetwide
replacement of these ducts. Pending the introduction of the new items or any favourable
outcome of the follow on work (i.e. the investigation into the bleed air detection system) BAE Systems
recommended that the SCP remained isolated as directed under UTI/NIMROD/026.
31 UTI/NIM/025, dated 26 November 2004.
32 Reference: MBSY/WRB/06015, dated 7 January 2005.
33 “Investigation into failure of Nimrod duct assembly (AFT/NIMROD/657)”, Document No: CHD-TFM-R-ISA-MB1383, dated 10 January 2005.