The Nimrod Review - Official Documents

The Nimrod Review
6.60
6.61
6.62
6.63
Causation
6.64
6.65
122
Whether or not blow-off occurs is dependent upon a number of factors, as HAL’s computer model shows. These
factors are principally: (a) whether No. 1 tank is approaching full; (b) whether fuel flow rates into the tank are
high enough; and (c) whether the aircraft attitude is suitable (this is influenced by aircraft speed, which, in turn,
is a function of aircraft weight and the type of tanker). During AAR training it is unusual for a receiver to take
significant amounts of fuel and therefore blow-off is not possible.
It is quite possible that blow-off had in fact happened on earlier occasions, but no evidence of it was visible
post-flight. Any fuel which had gathered in the bomb bay or other panels could easily have drained away and,
even if found, would have resulted in standard leak checks which would have revealed no faults. An area like
the starboard No. 7 Tank Dry Bay could have had fuel in it but, unless it was seen to be dripping externally,
would not have been checked as it is not routinely disturbed on normal servicing. Over the remainder of a flight
normal manoeuvres would allow most of that fuel to drain away. It should also be remembered that the two
prior occurrences of blow-off in 2006 gave rise to a stain on the fuselage which led back to the No. 1 tank
blow-off valve. Without the build up of dust in theatre, however, there may have been no catalyst to draw
attention to the blow-off valve. It is worth noting, in this respect, that Nimrods operating from RAF Kinloss
are regularly taxied, post flight, through an automatic washing facility, which may further have disguised any
external evidence.
The further question arises: If blow-off did occur previously why was there no fire until that on board XV230?
The answer to this is likely to be the simple one that the SCP may not have been in use at the time that blow-off
occurred, as it was often not required for cooling in the Northern latitudes.
There are, therefore, many reasons why blow-off may not have occurred and, if it had, why it was not noticed
or did not result in a fire. I am satisfied that the absence of previously recorded incidents of blow-off occurring
can be explained.
I am satisfied, for the above reasons, that fuel ejected from No. 1 blow-off valve during AAR could well have
tracked back along the fuselage and entered the SCP elbow and the starboard No. 7 Tank Dry Bay, leading to
the initiation of the fire on XV230.
An issue nonetheless arises as to whether there would have been a sufficient flow of fuel entering starboard
No. 7 Tank Dry Bay to have caused pooling, and to allow the initial fire to take hold and become established
and self-sustaining. The fire mechanism advanced by the BOI depends upon the existence of a pool of fuel on
the horizontal lower panel of the No. 7 Tank Dry Bay, just forward of the point of ignition. Pooling could also
theoretically have taken place in the bottom of the curved fairing around the elbow against the ‘lip’ leading
to the bomb bay. The answer to this issue depends on when, during the sequence of AAR, any such blow-off
first started occurring. Extensive modelling and reconstruction of the fuel configuration and known events was
undertaken by the BOI. This showed that over a range of likely parameters the time for blow-off was around
about five minutes 40 seconds after the start of AAR. This coincided with the transcript evidence of the length
of time the AAR serial had lasted i.e. from 11:03:53 until 11:09:50 when the Air Engineer states “we’re full”.
Prior to this the SCP tripped off at 11:09:23, which may also have been an event related to blow-off.
6.66 The AAR of XV230 on 2 September 2006 took approximately six minutes, i.e. from 11:03:53 to 11:09:50. The
fire warning went off at 11:11:33. Blow-off is likely to have occurred towards the end of the AAR uplift. If it
occurred right at the very end of AAR, this would have left about one minute 30 seconds for the following
sequence of events to have taken place:
fuel tracking back along the fuselage;
fuel then entering starboard No. 7 Tank Dry Bay;
fuel then coming into contact with the Cross-Feed/SCP duct;
auto-ignition delay of around 50 seconds;