The instructor said later that you can jettison on the ground the same way as you can pull the fire handle and discharge the bottles even if the aircraft is only battery supplied. In this case (aircraft with ext power on), you press the jettison pushbuttons and the STBY fuel pumps starts running, all x valve are open and there is no protection to stop you. the conlcusion: there is no logic, but you can jettison on the ground.
But this my conclusion only, I will ask an A340 engineer and then I will post again. Thank for your opinions mates!
