The BOI report makes reference (2-34) to the fact that following a number of sorties, prior to the XV230 accident, fuel was seen dripping from the bomb bay and there was evidence of fuel in No.1 blow-off exit pipe. Also, a limitation was placed on the the amount of fuel held in No.1 tank at 15K. Does anyone, who was out there, know how these defects were carried (ADF/Lims) and who signed them off?
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DV
The BOI indicates that the concensus of opinion in theatre was that the No1 tank blow-off valve had operated during AAR. Blow-off occurs due to the rate of fuel flowing into the associated tank exceeding the rate of the air ventilating out of the tank. Pressure will build up and the blow-off valve will open to relief that pressure. The valve sits at the top of the tank and is assumed to be in the space occupied by the top 125 gallons (1000 lbs) of fuel. If a blow-off valve has operated at a high fuel level it is not recognised as a malfunction of the Nimrod fuel system. In fact, it can be argued that the Nimrod is working very well and protecting itself from a ruptured tank. Therefore, unless the blow off valve is suspected of operating early, it is not regarded as a system defect and no fault can be justifiably raised, deferred or signed off.
The cause of the imbalance between fuel flow in, and air flow out, can be attributed to one or more of the following factors: operator technique, fuel pump rates, refuelling rates in other tanks (this will effect the input into the blown-off tank and the demands on the central ventilation system) and in the case of fuel blow-off the most likely cause would be aircraft attitude and turbulent movement. Its quite possible for the blow-off valve to react to air pressure alone, and that would not be problematic (escaping air is not a hazard), but fuel escaping is another matter. So, by agreement between the aircrew, it was decided to attempt to obviate No1 Tank fuel blow-off by trying to ensure that the valve did not immerse in fuel during AAR. Hence the tank was not in-flight refuelled above 15K. The BOI report indicates that 120/3 was briefed on this slight and very sensible amendment to the procedure. The fact that the revised procedures, after the crash, now encompasses a reduced target quantity in the No1 tank is a solid endorsement of their decision.
Ed Sett