Here in the states we had an issue come up with the float sw's for fuel valve shut-off.apparently this all stems from the TWA 800 accident.The first step of the process in"fixing" this problem was to de-activate the top-off float sw.'s.When these sw.'s are de-activated, the center tank can only be filled to a pre-determined maximum, well short of actual capacity for obvious reasons.The next step is to replace the wiring into the tank and add an upgraded type float sw.Once this is accomplished, normal operation is restored.Hope that helps.