The Nimrod Review - Official Documents

The Nimrod Review
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on its own, could have provided the fuel for the fire on XV230. However, when blow-off occurs, an internal
tank pressure of 2.7-2.9psi occurs and this pressure will also be felt in the vent pipes and at the vent connection
under discussion. This could increase the tendency of the vent connection to leak or even to become detached.
Thus, it is possible that fuel could both be blown-off and leak from the vent line connections.
In June 2009, STI/Nimrod/944 was issued by the Nimrod Integrated Project Team (IPT) to improve the integrity
of the connection between the No. 1 tank fuel vent stub pipe and the aircraft fuel vent system. This entailed
manufacturing a raised bead on the wider part of the vent stub pipe and introducing a mark to ensure that the
bag extension overlapped the bead by a set amount.
Fuel venting through air vent system was known problem
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Causation
It is clear the phenomenon of fuel venting through the air vent system was a well-known and long-standing
problem. Minutes dated 18 February 2003 referring to BAe’s Proposals for the Nimrod Life Extension Programme
state: “ … [A]t BAe’s presentation in Dec 92 on their latest proposals for a Nimrod Life extension to meet AR(A)
420, it became apparent that BAe were not fully aware of all our problems, or of the work being carried out in 3
specific areas: the fuel system, engine intakes and the wing/fuselage attachments. We have therefore produced
the briefing notes at Annex A ….”. 39
The briefing notes refer to two problems which needed to be addressed as part of the Nimrod Life Extension
Programme: the first was leaking of bag tanks; the second related to fuel venting. The briefing notes stated:
“Fuel venting has been a long standing problem. There are 2 causes: the failure of the high level float switches
to cut off the fuel when the tanks are full, and thermal expansion. The latter is a common occurrence when
operating in hot climates. Both problems result from inadequate system design and are controlled in service
by limiting the fill level of the tanks. Thus the BAe life extension proposal should address these built-in design
faults.”.
Thus it is clear that there was a known problem with the float switches which allowed fuel to enter the vent
system from which it might leak. It is not clear what steps were subsequently taken, if any, to address the
problem.
Causation of vent pipe
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I am satisfied that the vulnerability of No. 1 tank to asymmetric filling and fuel entering the vent system, and
the inadequate design of the vent pipe connection for the No. 1 tank to cope with fuel, could have led to a leak
during a blow-off occasioned by AAR. I am also satisfied that, because of the position of the vent connection
above the rear spar, it was also possible that leaking fuel could have migrated to the lower panel of the No. 7
Tank Dry Bay and into the SCP elbow muff.
In the event of fuel being pressed up in No. 1 tank into the vent pipe system, the vent pipe connection could
have experienced up to six times the pressure at which QinetiQ found it leaked. I am satisfied that it was quite
possible that the connection would have failed, allowing a significant quantity of fuel to escape. It was also
possible that the vent connection was weakened by the previous blow-off events.
Overflow: joint effect of blow-off and vent pipe
6.89 There could, therefore, have been two sources of fuel reaching the No. 7 Tank Dry Bay during blow-off, namely:
(1) fuel blown-off tracking along the fuselage; and (2) fuel leaking from a vent pipe connection. Both of these
could have ended up in No. 7 Tank Dry Bay simultaneously and these two sources might jointly have been
sufficient both to cause initiation of a fire as well as the sustaining of a fire. This is the joint phenomenon which
the BOI termed ‘overflow’.
39 Loose Minute dated 18 February 2003 referring to BAe’s Proposals for Nimrod Life Extension to Meet SR(A) 420.