The Nimrod Review - Official Documents

6.111
Chapter 6 – Overflow or Pressure from Air-to-Air Refuelling
(5) The blow-off valve is only 2.1 metres directly forward of the fairing in way of starboard No. 7 Tank Dry
Bay.
(6) There is ample empirical evidence of the likelihood of blow-off fuel migrating back along the fuselage and
entering panels. 52
(7) Gaps in the fairing could allow fuel to ingress the starboard No. 7 Tank Dry Bay, come into contact with
the SCP elbow, and pool.
(8) One is certain to get fuel in the vent lines before blow-off occurs.
(9) There is a recorded incident of fuel leaking from the No. 1 tank vent connections in 2006. 53
(10) The recent XV235 fault signal shows that No. 1 Tank Cell 4 rear vent connection was the probable cause
of fuel wetting the SCP elbow muff below. 54
(11) Recent experiments have confirmed that fuel from No. 1 Tank Cell 4 rear vent drops onto and pools on the
panel at the bottom of the starboard No. 7 Tank Dry Bay.
(12) The No. 1 Tank Cell 4 rear vent pipe connection was designed for air not fuel, is a poor design, and could
have leaked under even low fuel pressure. 55
(13) No. 1 Tank Cell 4 rear vent could have been weakened by blow-offs on previous XV230 sorties.
(14) Fuel could also have leaked from the couplings in the vent pipe connections.
The following reasons militate against overflow during AAR being the source of fuel:
(1) There have been only three known incidents of blow-off on Nimrods in 25 years.
(2) It is not straightforward for blown-off fuel to pool on the lower panel in the starboard No. 7 Tank Dry
Bay.
(3) The crew were operating a 15,000lbs limitation in relation to Tank No. 1.
(4) No venting from XV230 was observed at the time by the Tristar tanker.
Balance of probabilities of overflow during AAR being the source of fuel
6.112
I have carefully weighed up all these factors, and all the evidence I have heard and read, and have concluded
that the balance of probabilities favour overflow during AAR being the source of the fuel which led to the fire
in the starboard No. 7 Tank Dry Bay which caused the loss of XV230 (i.e. (b) above). In my view, four factors tip
the balance in favour of overflow during AAR being the cause:
6.122.1 First, the recent evidence of the fault signal for XV235 which shows that fuel found in the SCP elbow
muff probably originated from No. 1 Tank Cell 4 rear vent.
6.122.2 Second, the recent careful examination of the No. 1 Tank Cell 4 rear vent shows it is a poor design for
fuel and could leak when under fuel pressure.
6.122.3 Third, the recent evidence of the dye experiments on MRA4 which show the likelihood of fuel tracking
straight back along the fuselage and entering the SCP elbow of the MR2.
6.122.4 Fourth, the fact that fuel could have emanated from the two sources simultaneously during AAR and
both ended up in the starboard No. 7 Tank Dry Bay, i.e. both: (i) fuel blown off from No. 1 blow-off
valve and tracking two metres down the starboard fuselage into the fairing over the SCP elbow; and (ii)
fuel simultaneously entering the vent system and leaking out of No. 1 Tank Cell 4 rear vent and pouring
down onto the SCP elbow muff and panel at the bottom of No. 7 Tank Dry Bay below. An initial splash
52 AEW3, MRA4 experiments, XV249, XV232 etc.
53 Air Incident Report KIN/59/06 (BOI Report, Exhibit 57).
54 Serious Fault Signal 181300 May 09, raised on 20 April 2009, during maintenance of XV235 at RAF Kinloss.
55 See QinetiQ’s Fault Report: QINETIQ/MS/SES/TR0902158/1, dated 2 July 2009, during the Ageing Aircraft Audit conducted on Nimrod XV236.
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