The Nimrod Review - Official Documents

6.57
Chapter 6 – Overflow or Pressure from Air-to-Air Refuelling
6.56.2 Second, BAE Systems argued that blown-off fuel would not be able to gain entry to the No. 7 Tank Dry
Bay because the lower panel is sealed with a bead of PRC mastic (a flexible sealant akin to bathroom
silicone filler) and this would have affected a liquid proof seal. However, on such a panel on the underside
of the wing, the most likely purpose of the mastic is to prevent fretting between the aircraft structure
and the lower panel, not to prevent fluid ingress. 31 Nonetheless, the Review obtained the engineering
drawings of the lower panel and these clearly show that the ‘bead’ is only required on the forward, rear
and outboard edges of the panel, not the inner which is the most likely edge for any blown-off fuel
to enter. When the blow-off valve operates, however, any blown-off fuel will be trapped close to the
fuselage by the airflow and reach the inner edge most easily. I therefore reject BAE Systems’ argument
that fuel could not enter the No. 7 Tank Dry Bay because of the PRC mastic.
6.56.3 Third, BAE Systems argued that blown-off fuel would not be able to gain entry to the No. 7 Tank Dry
Bay because drainage holes in the panel allowed liquid egress, but not liquid ingress, from outside. The
ingress path postulated, however, is through the panel’s inner edge, not the drain holes. BAE Systems’
argument thus somewhat misses the point.
6.56.4 Fourth, BAE Systems argued that the No. 7 Tank Dry Bay has a positive pressure when measured against
the pressure under the wing. Martin Breakell said in evidence to the Review: “...[D]ry bay 7 is one of the
main vent bays from the bomb bay to get air coming from the bomb bay, it’s vented out through dry
bay 7, you have always got a positive pressure differential, so it’s going to want to blow out, rather than
allow fuel to come in”
This latter argument appears to rely on the fact that the pressurised cabin discharges air into the rear of the
bomb bay and that the No. 7 Tank Dry Bay is the area from which this positive pressure is discharged; thus,
the bomb bay and the connected No. 7 Tank Dry Bays would have a higher pressure than the pressure under
the aircraft wing. BAE Systems suggested that this positive pressure differential would prevent the ingress of
liquid. However, evidence from the QinetiQ combustion study suggests that the pressure gradient may not be as
simple as propounded by BAE Systems. In simple aerodynamic terms, of course, the air pressure above the wing
surface is lower than that below it (otherwise the aircraft would not fly). The QinetiQ team noted that there
were significant gaps between the wing panels on the aircraft in the vicinity of the No. 7 Tank Dry Bay and that,
considering the overarching pressure differential just described, this would initiate a sympathetic movement of
air within the wing (i.e. from lower wing surface to upper wing surface). Any air exhausting from the bomb
bay would join this upward movement of air and escape through the gaps in the upper wing surface panels.
Therefore, it follows that the No. 7 Tank Dry Bays will not be pressurised.
Lack of recorded instances of blow-off in previous 25 years
6.58
6.59
A question which must be addressed is: Why had this phenomenon of blow-off during AAR not occurred
before? The Nimrod had been conducting AAR for 25 years prior to the accident, yet it appears to have been
an unknown phenomenon until the two occasions when it occurred in theatre prior to XV230’s loss (see above).
In my view, the explanation for this resides in the fact that, as shown by the BOI fuel model, a number of
conditions need to coincide to provoke blow-off: small variations in the rate of refuel; the amount of fuel taken
into the No. 1 Tank; and the attitude of the aircraft.
Further, for the first six years that AAR was conducted, the old uplift sequence (see above) would have prevented
blow-off. It should also be noted that many sorties were conducted with Victor or VC10 tankers, which had a
lower refuel rate than a Tristar. By contrast, AAR with a Tristar was comparatively rare with an average of nine
Tristar sorties per year. Further, for most of the 1990s, the Nimrod Zero Fuel Weight (weight without fuel) was
higher than it was in 2006, meaning that uplifting to full fuel capacity was not possible, as this would have
exceeded the maximum allowable total weight. There were also restrictions on the amount of fuel which could
be taken in relation to the Centre of Gravity position such that, once again, uplift to a full load was prohibited.
There were also many occasions when tankers were unable to dispense all the fuel the Nimrod could take. 32
31 Further, if the PRC mastic is used as a weather-proof seal, it will be used on the top surfaces of the panels only, in order to prevent rainwater
ingress.
32 BOI Report, Part 2, paragraph 40(d).
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