Helgar, the light you are referring to is the "5 Tank Overpressure" warning.
When the Nimrod was built, consideration had to be given to mitigation against refuel valves failing to close when their associated tanks reached full while filling them up on the ground. Despite the presence of a ventilation pipe, a tank could rupture under the pressure of fuel being pumped in through the stuck open valve at a greater rate than the vent pipe could handle. Therefore, all the tanks are fitted with blow-off valves, whose diameters match the bore of the refuel pipes. This ensures that if a blow-off valve opens, it has the capacity, together with the vent pipe, to dump the fuel onto the ground at the same rate that it enters the tank, therefore protecting the tank from a rupture. This was designed for ground refuelling only. The No 5 tank is in the fuselage and its blow off valve was fitted just forward of the No 2 engine intake. This is not an issue when the engine is not running, which is always the case when refuelling on the ground.
When the AAR system was installed, it was recognised that a No 5 tank refuel valve might fail to close when the tank became full during AAR and that it's blow off valve might then send copious amounts of fuel down the intake of No 2 engine. Therefore, they installed a pressure switch which would activate the "No 5 Tank Overpressure Warning" light and simulataneously close the 5 tank refuel valve, by interupting its electricity supply.
Mick Smith, please take note:
It was later recognised that the warning system was inadequate: the pressure switch might not work in time to stop fuel passing through the valve, or the valve might be physically jammed open, so no electrical control would have any effect, and, lastly, the crew, having seen the warning, would not be given sufficient time to safely withdraw from the tanker before the fuel entered the engine intake. Further mitigation was then designed as follows:
A restrictor is now fitted into the refuel pipe for No 5 tank, such that the effective bore of the pipe at that point is less than the tank's vent pipe. This ensures that if the refuel valve jams open the vent pipe has the capacity to direct all of the surplus fuel to the vent outlets at the back of the wing at the same rate that it enters the tank. The blow off valve is now redundant and has been removed and blanked off. The warning light will never come on, so it has been "deliberately" removed.
In summary, the RAF/BAe have cleverly replaced a dubious warning system with a far safer arrangement that now completely avoids the possibility of overpresssuring the tank. It is a pain in the neck during AAR because the refuelling rate has slowed considerably, but we recognise the need for it. The modification was carried out in the late 80's.
Helgar, I hope this has helped. I will write another message about the flow rates.
Mick Smith, I urge you or your colleagues to pm me whenever you want to write anything in the Timesonline about anything technical on this aircraft. Helgar is clearly concerned about what has been written. This is yet another needless discussion.
Regards
Ed Sett