If one aircraft is flying from city A to city B and another from city B to city A, If comparisons of the temporal distance of the first plane over city A with that of the second plane over city B and that of the second plane over city A with the first plane over city B lead to a separation of 4 minutes or less at any time during their flights, or If comparisons of the topical altitude of the first plane over city A with that of the second plane over city B and then the altitude of the second plane over city A with that of the first plane over city B give a separation of 30 flight units (nautical miles) or less at any time during their flights.