The Nimrod Review - Official Documents

23.4
23.5
23.6
Chapter 23 – Age Matters
Manufacturers seek to reduce ‘infant mortality’ problems by good design and ‘stress testing’ to identify design
weaknesses and specific assembly and materials problems. 2 The term ‘Military Specification’ is sometimes used
to describe systems, usually life-critical or system-critical, in which the ‘infant mortality’ section of the bathtub
curve has been ‘burned out’, or removed by means similar to accelerated stress testing.
It is not possible to eliminate all risk of ‘infant mortality’: there will always be initial design or manufacturing
problems that manifest themselves early on in the life of a new piece of equipment, e.g. initial software
problems in on-board computers in modern ‘fly-by-wire’ aircraft or the vulnerable ‘square’ window design on
early Comets. Equally, those charged with assuring and ensuring the airworthiness of aircraft need to grapple
firmly with issues which will inevitably arise in the ‘wear-out’ period, i.e. incipient failures as structures, materials
and systems begin to enter their ‘wear-out’ phases. The airframe failure cases, such as the Aloha Flight 243
accident in 1988, first highlighted the need to consider, e.g., the effects of long-term corrosion and interaction
with shortcomings in design, maintenance or manufacture of aircraft structures. Since this case, there has
been a great deal of work done on aged aircraft structures and structural failures in aircraft due to stress and
corrosion; and, as a result, fatigue and failure rates and modes in relation to aircraft fuselages and structures
are, for the most part, now well understood, documented, and predictable.
Much less is known, however, about aircraft systems and the effects of age and long term use on many of the
systems contained within airframes, comprising as they do a wide variety of different designs, functions, and
materials. 3 There has been considerable work in certain areas which have manifested problems, e.g. ‘Kapton’
wiring. The approach to aged systems generally, however, has been piecemeal and reactive. Work, research,
and knowledge need to increase in this field, since the numbers of ‘legacy’ aircraft in operation are growing.
In a 2002 paper COM(Air) highlighted that the average age of the MOD fleet was 21 years, and the majority
of MOD aircraft had exceeded the 15-year threshold which initiated the Ageing Aircraft Audit management to
preserve and sustain airworthiness (see further below). 4
Nimrod and ‘Bath tub’ curve
23.7 In the late 1990s, the Nimrod fleet was already beginning to be described as “old” and reaching the end of the
‘bathtub’ curve. In February 1996, a Nimrod ‘Health Check’ carried out by HQ Logistics Command itself reported:
“The Nimrod fleet is old and its systems appear to be reaching the wear-out phase of the reliability ‘bath tub’
curve.” In July 1998, the Nimrod Airworthiness Review Team (NART) referred to the demands “symptomatic of
keeping an old aircraft flying”. 5
23.8
A decade later, a number of witnesses to the BOI raised concerns in 2007 regarding the age of the Nimrod fleet
and its position on the ‘bathtub’ curve:
23.8.1 Witness 33 said: “The Nimrod is certainly towards the end of the “bath tub” curve”. We are starting to
see things we may not have seen before and these take time to investigate. There are some advantages
in having an ageing aircraft. Compared to something like Typhoon it is a relatively simple aeroplane. The
things that go wrong are the mission system but the bits that keep it in the air are very tried and tested.”
23.8.2 Witness 34 said: “Do you understand the bathtub? To my mind you have an aircraft which is beyond its
sell by date and is an ageing aircraft.… Your maintenance should change to more inspection or forced
replacement of critical components…I think it is wrong to have these components on condition.”
23.9 As I explained in Chapter 13, I do not believe that, in the period 1998-2006, there was the sort of “highly
attentive management, closely attuned to the incipient threat to safe standards if [Nimrod] airworthiness is to
be safeguarded” that the NART called for. 6 For instance, as the BOI pointed out, there was insufficient fuel leak
trend analysis. I do not believe that there was sufficiently attentive airworthiness management generally in the
MOD during this period.
23.10
There were, nevertheless, Life Extension Programmes and Ageing Aircraft Audits applied to the Nimrod fleet.
2 E.g. HALT (Highly Accelerated Life Test) or HAST (Highly Accelerated Stress Test).
3 There has been considerable work in certain areas, such as ‘Kapton’ wiring. The approach to aged systems has been piecemeal and reactive rather
than pro-active.
4 COM(Air)/105/02 paper.
5 Paragraph 30 of Nimrod Airworthiness Review Team Report, dated 24 July 1998.
6 Paragraph 30 of Nimrod Airworthiness Review Team Report, dated 24 July 1998.
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