The Nimrod Review - Official Documents

6.3
Chapter 6 – Overflow or Pressure from Air-to-Air Refuelling
failure. The blow-off valves operate when tank pressure exceeds 2.7 psi and relieve pressure by releasing fuel
to the atmosphere.
During the original fitting of Mod 700 in 1982, the risk of fuel from No. 5 tank blow-off valve (located forward
of the engines on the aircraft’s port side) entering No. 2 engine intake was appreciated. This potential hazard was
addressed by the fitting of a pressure switch in the vent line of No. 5 tank, to close the refuel valve should fuel
enter the vent line. No similar switch, however, was fitted to the vent line of either of the remaining fuselage tanks.
Although the Review has been unable to determine the rationale behind this decision, it was probably for similar
reasons to those quoted in subsequent reports3 regarding the formal incorporation of AAR in the Nimrod design,
namely that the No. 5 tank blow-off outlet is in front of the engine air intakes, whereas the others are behind
them. It should be noted that the refuelling procedure for the original AAR installation required the refuel valves of
all the tanks needing fuel to be opened as refuelling commenced and closed individually as each tank filled. This
had the effect of distributing the fuel to a number of tanks simultaneously, with a relatively low rate to each one.
No. 5 Tank Blow-Off
Valve
Figure 6.1: Location of Nimrod Fuel Tanks
1983-1985: AEW3 project included fitting of permanent AAR capability
6.4
No.1 Tank Blow-Off Valve
(on underside of wing)
The development of the Nimrod AEW3 in the mid-1980s allowed the MOD to seek a Nimrod AAR solution that
would meet Defence Standard (Def-Stan) 00-970 requirements. Some of the work involved in this redesign can
be traced through a series of documents detailing trials undertaken by the then British Aerospace PLC (now BAE
Systems) in furtherance of MOD instructions.
6.5 In March 1985, BAE Systems issued a Report entitled “ALL NIMROD AIRCRAFT REPORT ON INTEGRITY OF TANKS
1 AND 5 BLOW OFF VALVES DURING AIR TO AIR REFUELLING (P.D.S. TASK NO. 0351)” 4 . Whilst the Report was
produced pursuant to Post Design Service (PDS) Task No. 0351 as part of the AEW3 project, it was entitled “ALL
NIMROD AIRCRAFT REPORT…” and clearly intended to be applicable to the whole Nimrod fleet. The report
investigated the possibility of removing the blow-off valves within Nos. 1 and 5 fuel tanks, to prevent the chance
of blown-off fuel being ignited by aircraft engines. The report recommended that the blow-off valve from No.
5 tank be removed and replaced with a flow restrictor, to prevent tank over-pressure. This modification was
subsequently enacted. It was decided, however, that a similar arrangement within No. 1 tank would prolong
AAR unreasonably and that the threat of fuel entering and/or igniting in the jet efflux (rather than entering the
intake as with the No. 5 tank blow-off) was minimal. The report did, nevertheless, note that:
‘... [T]here may be some cause for concern with regard to the wetted surfaces caused by
a discharge of fuel and it is suggested that tests are made in flight using a coloured dye
to study the behaviour of liquid in relation to the various ports and intakes, particularly
the tail-pack pre-cooler in the bottom of the rear fairing. If the liquid is found to enter
3 See subsequent discussion.
4 BAE-MPP-R-AEW-0063 (BOI Report, Exhibit 66).
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