Statistics reveal that the vast majority of mid-air collisions in the past years have occurred during daytime under visual meteorological conditions in the vicinity of airports and at low altitudes. From the description of possible proximity warning or collision avoidance systems it follows that it is just this type of collision that causes technically the greatest difficulties for an operationally and economically feasible solution because of the multiple intruder problem, mutual interference, ground echoes, curved flight paths, and changes in aircraft attitude with resulting changes in aircraft speed. Under these conditions, the automatic pilot, the general introduction of which (for transport aircraft) has been recommended in the interest of collision avoidance, would be only of minor importance as it is common practice to switch off the automatic pilot whilst flying in the aerodrome traffic pattern.