1. A method of operating an electronic flight planning system for aircraft, the method comprising: receiving as data input to the system from one or more users a plurality of flight plans, each flight plan defining a flight by one of a plurality of aircraft; determining, for each flight by one of said plurality of aircraft, as defined in the respective received flight plan, a three-dimensional region of potential conflict, representative both of an uncertainty in the position of the aircraft and of a region of air exclusion appropriate for the aircraft or for the respective received flight plan; and determining, on the basis of the determined regions of potential conflict, whether any one of the received flight plans is in conflict with any one of the other received flight plans.