The Nimrod Review - Official Documents

The Nimrod Review
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6. I have concluded that the balance of probabilities favours overflow during Air-to-Air Refuelling
being the most likely source of the fuel which caused the loss of XV230 for four main reasons:
(1) The recent evidence of the fault signal for XV235 which shows that fuel found in the
Supplementary Conditioning Pack elbow muff probably originated from No. 1 Tank Cell 4 rear
vent.
(2) The recent careful examination of the No. 1 Tank Cell 4 rear vent confirms the Board of Inquiry’s
view that it is a poor design for fuel and could leak when under fuel pressure. On the majority of
occasions, the vent system will simply carry air but during manoeuvres and Air-to-Air Refuelling,
fuel may enter it.
(3) 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 and No. 7 Tank Dry Bay.
(4) The fact that fuel could have emanated from the two sources (the blow-off valve and vent system)
simultaneously during Air-to-Air Refuelling, with both sources ending up in the starboard No. 7
Tank Dry Bay.
7. It is important to note that much of this evidence is new and was not available to the BOI, the AAIB
or the US Air Force Safety Center.
Deployed crew of XV230
8. The deployed air and ground crew of XV230 have nothing to reproach themselves for. The two
previous incidents of blow-off on XV230 were sensibly dealt with by imposing a limitation of 15,000
pounds on the No. 1 Tank. The assumption that there was no threat to the aircraft as a result of
blow-off operating was perfectly reasonable.
Post accident
9. Air-to-Air Refuelling of Nimrods was suspended following an Air-to-Air Refuelling incident on
XV235 in November 2007, and has not been resumed.
Introduction
History of Air-to-Air Refuelling modifications to the Nimrod fleet
6.1
It is important to have a clear and detailed understanding of the history of the Air-to-Air Refuelling (AAR)
modification to the Nimrod fleet. 1
1982: Original AAR modification fitted for Falklands campaign (Mod 700)
6.2
The Nimrod MR2 was converted to enable AAR for Operation Corporate (the Falklands campaign) during 1982,
as an Urgent Operational Requirement (UOR) under Modification (Mod) No. 700. The pressing operational
requirement to fit AAR meant that there was insufficient time to ensure that the AAR system design met all the
requirements of Aviation Publication (AvP) 970. 2 The modification was conceived, fashioned and installed in a
record 18 days. However, the refuelling hose from the AAR probe to the fuel system passed through the aircraft
cabin requiring the crew to step over it. The Nimrod fuel system was originally fitted with blow-off valves in all
but two fuel tanks (the 4A tanks), to ensure that tanks could not be over-pressurised to the point of structural
1 Annex L of the BOI Report contains a summary of the history of AAR which this Chapter expands upon.
2 Subsequently Defence Standard (Def-Stan) 00-970.