An air traffic control system, for use by a human controller controlling a plurality of aircraft held vertically separated in a stack above a minimum stack level, the system comprising at least one processor, and a display device for the human controller, controlled by said at least one processor; further comprising: means for periodically inputting a value representative of local terrestrial air pressure conditions; means for periodically inputting an aircraft flight level reading representing an altitude defined by a reference air pressure measured on the aircraft; means for periodically generating a display on said display device comprising a plurality of flight levels vertically arranged; means for indicating in said display said plurality of aircraft, arranged in a vertical list ranked by flight level; said at least one processor being arranged, on reception of a new said value, to redetermine said minimum stack level and to vary said display so as to indicate changes to said minimum stack level.