The Nimrod Review - Official Documents

1989: Mod 715 Installation Changes
6.12
6.13
6.14
Chapter 6 – Overflow or Pressure from Air-to-Air Refuelling
In 1989 Mod 715 was introduced and the features developed as part of the AEW3 program were incorporated
on the MR2. In particular, the restricted No. 5 tank refuel valve was introduced. This complicated the refuelling
sequence as devised in 1987. To prevent longitudinal balance limits being breached, as No. 6 tank now filled
faster than No. 5 tank, the air engineer had to switch the former’s refuel valve on and off. No. 7 tanks would
reach full before the No. 1 tank and, therefore, on most occasions, the only refuel valves open near the end of
an AAR serial would be the two on the No. 1 tank and that on the restricted No. 5 tank.
The net effect of this was to increase substantially the instantaneous flow rate of fuel into the No. 1 tank during
AAR.
11 Formal trials were conducted to ascertain the functionality of the Mod 715 changes. However, the bowser
used to provide fuel for the practical test of the AAR system was unable to deliver fuel at more than 30psi. The
trials team extrapolated figures to calculate fuel flows at 50psi. A direct result of this was that the opportunity
possibly to observe No. 1 tank blow-off was lost. In their response to this suggestion, BAE Systems said that the
above report12 showed the bowser pressure at 50psi for the last two minutes of the 25 minute trial and therefore
that the opportunity to observe No. 1 tank blow-off was not lost. However, blow-off would be dependant on
the higher rate of refuel at 50psi being present for the majority of the refuel to induce asymmetric filling of
individual cells, which it was not in this case.
1989: Introduction of the Tristar Tanker
6.15
6.16
Until 1989, the Nimrod had refuelled primarily from the Victor and VC10, each with a single Hose Drum Unit
(HDU) capable of delivering up to 1,800kg per minute. In 1990, the Tristar was converted to be a tanker aircraft
and was cleared to refuel a variety of receivers, including the MR2. The Tristar’s twin HDUs each delivered fuel
at a greater rate (2,100kg per minute) than its predecessors because of the addition of two hydraulically driven
Carter pumps. Trials for the Nimrod to receive fuel from the Tristar were completed in 1989. A fuel flow rate of
2,100kg per minute was achieved for the Nimrod.
The additional delivery capacity of the Tristar Carter pumps represented another potential increase in the
individual tank refuel rates and increased further the potential for asymmetric filling of No. 1 tank.
Cumulative effect of changes ignored
6.17
The combined effect of these changes was to increase the flow rates at certain stages of AAR, and particularly
during Tristar AAR. As the BOI found, at no stage were the cumulative effects of successive changes to the
Nimrod AAR capability understood and analysed. As the RAF Kinloss Station Commander put it, at no point was
“a holistic view” taken of the incremental effect of the changes.
2008: Prohibition of AAR on Nimrods
6.18
A year after the loss of XV230, on 5 November 2007, Nimrod XV235 diverted into Kandahar airfield following
a leak in the bomb bay which occurred during AAR. The leak had been observed by the crew through the
periscope that is fitted to allow aircrew to survey the bomb bay area in flight. The leak could not be replicated
on the ground, despite many attempts. As it was no longer possible to state that leaks which might occur during
AAR would be detected on the ground, AAR on Nimrods was prohibited by AOC 2 Gp and this has now been
incorporated formally within the Nimrod’s Release to Service documentation.
11 British Aerospace Report HAS-MPP-T-801-0273 “Ground Test of the Air to Air Refuelling System”, dated May 1988.
12 Ibid, Figure 1 (footnote 7).
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