The Nimrod Review - Official Documents

6.78
Fuel bag tank extension
Gap between extension
and small diameter
section
Chapter 6 – Overflow or Pressure from Air-to-Air Refuelling
Figure 6.9: No. 1 Tank Cell 4 Vent Connection
Small diameter section
Location of ‘Vent’ label
tape
Large diameter section
37 In order to assist the Review, a trial was carried out at RAF Kinloss to investigate where fluid would track if
released in the area of the No. 1 Tank rear vent connection (see Figure 6.8 and 6.9). Although unable to replicate
the exact pitch attitude of XV230, the results showed that fluid would track back into the No. 7 Tank Dry Bay
and some would pool on the lower panel. Larger quantities of fuel (several litres) were required in order to cause
the fuel to disperse widely over the panel. In addition to the ‘on aircraft’ trial tracking the movement of the
fluid, the connection was also subjected to a bench test under pressure. Leaks were not apparent up to 5psi on
the test specimen with the jubilee clip on the wider part of the vent pipe. However, as noted above by QinetiQ,
the jubilee clip had potential to slip rearwards over the narrower section if located over the weld. Given the
difference in the two diameters this would result in a large leak, as can be seen from the photograph above.
Vent pipe couplings
6.79 There is also evidence that, in addition to the possibility of a leak from the direct vent connections to Cells 1
and 4 of No. 1 Tank, fuel may also leak from the FRS couplings in the main vent system. In 1998 a technical
improvement38 was suggested to the IPTs predecessor, the Support Management Group. In proposing a
Modification to the high-level float switches of No. 1 tank an engineer at RAF Kinloss was trying to deal with
a recurring problem of minor fuel spills from the No. 1 Tank. In his explanation of the problem, he wrote: “Of
course fuel doesn’t always flow freely through the vent lines, often it finds a way to leak through these lines at
various connections and soaks either Rib 1 port or starboard. This alone causes a large quantity of fuel to soak
all the equipment there” (emphasis added).
6.80
6.81
Several of the FRS connections in the fuel vent pipes are directly above the No. 7 Tank Dry Bay and fuel dripping
from them would drop onto the lower panel and pool.
I am satisfied, in the light of the above, that there are several ways in which fuel can migrate into the No. 7 Tank
Dry Bay from the aircraft vent system and pool on the lower panel.
Venting could occur without blow-off
6.82
The incident on XV235 shows the potential for fuel to enter the vent system in the event of the No. 1 tank Cell
4 overfilling, allowing fuel to leak into the area of the No. 7 Tank Dry Bay. This could happen without reaching
an internal tank pressure sufficient to cause blow-off. It is therefore possible that a leak from the vent system,
37 NAEDIT/1505/09/14/Task 01, dated September 2009.
38 SMG/512712/16/SM60a1, dated 18 November 1998.
125