The Nimrod Review - Official Documents

History of leaks in Nimrod fleet 1983-2006
Introduction
5.59
Chapter 5 – Leak from Fuel Coupling
56 The Nimrod fleet experienced a four-fold increase in fuel coupling leaks during the period 1983–2006. Although,
in absolute terms, the increase was not large, it should have been detected by reviews of maintenance policy.
The relatively slow increase, over a period of decades, was, however, below a level at which it was immediately
apparent to those operating the aircraft. Although the leak rate was broadly comparable with leak rates from
some other RAF aircraft, no comparison appears to have been made until after the loss of XV230. No single
cause for the increase in leaks has been determined. It is likely to be due to a combination of interlinked factors,
including inaccurate alignment of couplings and fuel pipes, exposure to high temperature, and mechanical
movement or vibration. Although seals manufactured after 2000 used in Avimo couplings were not of the
correct specification, and were found to have degraded (see further below), none of the couplings from which
they were extracted was itself found actually to be leaking.
Wing Tank Leaks
5.60
The Nimrod, as with other types employing similar design, suffers from leaks from the wing fuel tanks. The
design of the Nimrod is based on the Comet. When the Comet was designed in the 1940s the ability to make
large wing panels did not exist and a large number of joints were required for the Nimrod in order to fasten
together the relatively small wing skin panels. 57 As fuel leaks are most likely to occur at structural joints, the
susceptibly of the Nimrod to fuel leak incidents was seen as an ‘inevitable’ consequence of the basic design. 58
Because leaks from certain areas of the wings will enter the airflow and be dispersed behind the aircraft, as long
as they are within prescribed limits, they can be accepted and the aircraft will fly with them. This is, however,
subject to certain procedures: a careful engineering assessment and the recording of each leak in the aircraft’s
documentation (Form 700). Further, the aircraft captain will examine this document before deciding whether
or not to accept the aircraft for flight. The addition of electric operational equipment to the hard points under
the Nimrod’s wings from the early 1990s meant that the areas of the wing in which leaks were ‘acceptable’
was reduced. However, in practice, ground crew found it difficult to prevent recurrence of leaks in these areas
and this led to a reduction in aircraft availability: the aircraft were not allowed to fly until the leaks had been
cured. Significant time and effort was devoted to this recurrent problem straining ground crew resources. In
2006, QinetiQ was tasked to investigate the issue and determine whether it was possible to reduce the leak rate
from wing tanks, to improve aircraft availability. The subsequent QinetiQ report59 highlighted that, although the
tank repair work on the Nimrod MR2 examined at RAF Kinloss was to a ‘high standard’ there were a number of
improvements that could be made. In particular, QinetiQ highlighted the following: “The working conditions of
RAMS60 are ... less than ideal” and “the temperature and humidity variations are unlikely to be conducive to the
correct curing of sealant materials”; “Finally, it was noted that the facilities and previous hangar environmental
conditions were not fully conducive to working efficiently with fuel tank sealant ...” to allow the curing of tank
sealant”; “the APs in use by both RAMS and NSG were found to be both out-of-date and of insufficient depth
to be of practical value”, “the bolt replacement has been a random process according to where the leaks occur
and is not covered by preventative maintenance or inspection routines”. QinetiQ also noted that there was poor
sharing of data between organisations, poor analysis of available data, the use of outmoded procedures and
documentation and the use of particular sealants, when more appropriate sealants were available. This episode
thus does not reflect very well on the then current processes and procedures for dealing with wing leaks. But
it is right to point out that the report made it clear that, when faced with a wing tank leak that posed a threat
to aircraft safety, the RAF simply did not allow the aircraft to fly. The QinetiQ report was prompted by the
operational impact of unavailability of aircraft. Nonetheless, it is a matter of concern that these unsatisfactory
aspects of maintenance were found.
56 BOI Report, Part 2, paragraph 39(b).
57 “Nimrod Fuel Leak Study”, QinetiQ/D&TS/AIR/RF051726/14, dated 17 March 2006, page 10.
58 Ibid.
59 “Nimrod Fuel Leak Study”, QinetiQ/D&TS/AIR/RF051726/14 dated 17 March 2006.
60 Repair and Maintenance Section.
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