A question has been asked about routine and operational AAR. By its very nature all AAR is operational, except where it is for training only. So, there is no such thing as "routine" AAR. AAR training does not need fuel transfer to occur, thus all aircrew can be trained without actually refuelling, per sec. Mandatory "minimum time in contact" requirements are achievable with dry prods, although some fuel does travel down the hose and into the receiver. However, once the receiver's pipes are full, the volume of which which will vary from type to type, no more fuel transfer will occur unless the receiver opens a refuel valve. This is standard knowledge among aircrew familiar with AAR.
A question has been asked about how the blown-off fuel can travel up to the 7 Tank dry bay. The answer is this: the dry bay boundaries, for the want of a better term, are arranged like this: the front wall is the rear spar. The rear wall is the front wall of the 7 tank. The top is the wing fillet panel and the bottom is the panel that is mainly flat but is curved at its inner rear corner to provide a fairing for the front portion of the SCP pipe. The BOI suspects that fuel would probably have travelled along the bomb bay skirt from the No 1 tank blow off outlet and run into the gap between the front edge of the dry bay lower panel and come into contact with the hot SCP pipe and auto-ignited. The non-drained panel's ribbed sections can hold approx 300ml of fluid (in this case fuel). Once a small amount of the fuel has ignited the remaining portion of the 300ml then also ignites and creates a very strong fire which rises straight up inside the bay and plays onto the 7 tank pipework and the 7 tank wall as well. The pipe seals would have been the first components to fail and then we have more fuel to keep the fire self-propagating.
It doesn't take a lot of blow-off in this scenario.
Blow-off occurs when the tank over-pressures (not necessarily overfills), but if it is fuel that pushes the blow-off valve open, the size of the blow-off valve plate increases in diameter when open and it therefore requires a lower pressure to keep it open, thus ensuring that the valve does not cycle open/shut/open/shut, etc. It simply remains open until it is no longer covered by the fuel in the tank. The valve sits on the top of a stack pipe in the tank, so the blown off fuel exits at the bottom of the tank. This is usually achieved only by stopping the refuel of the tank.
The BOI has determined that the design of the No 1 tank is not compatible with the flow rates that can be achieved during AAR, thus blow-off is very likely and a lot of fuel will escape towards the SCP fairing, which only serves to make the auto-ignition easier.
Regards
Ed Sett