The Collision Avoidance Problem Requires a Mix of Partial Solutions Available to Purchase

The objective of collision avoidance is to assure the separation of aircraft during flight operations — to avoid the situation occurring where two or more aircraft could occupy the same point in airspace at the same time. The problem centres on how best to avoid such a spatial conflict—through ground‐derived data and air traffic control, by means of air‐derived data and pilot judgment, or by some combination of the two? In the quest for a solution, many kinds of systems have been studied and prototype hardware built and tested. Some provided only partial answers; some might meet the safety needs but create implementation difficulties; several have demonstrated the potential for world‐wide application. It is with the last group that we are concerned here. Any collision avoidance system (CAS) selected must be one that meets international requirements; this implies that protection can be offered to all aircraft, that the CAS is compatible with other existing systems and that costs would not be excessive. It goes without saying that reliability must be very high to achieve international acceptance. Several approaches are in development in the United Kingdom and in the United States that could meet these criteria, according to their proponents. However, a controversy continues over which approach is best — one depending heavily on ground‐computed data, the other on air‐derived data. And, there are associated concerns with the presentation of data in the cockpit and the evasive‐manoeuvre capability offered. Finally, with regard to a developmental system favoured by both the United Kingdom and the United States, there appears to be serious divergence of opinions among critics of the system concept related to the impact of radio‐spectrum utilization on existing ICAO secondary surveillance radar standards. Because there is honest technical disagreement on what the optimum collision avoidance system should be, we have made the following pages available as a forum for discussion of the subject of those representing differing sides of the argument: air traffic control, CAS developers, the airlines, the air transport pilots, general aviation and radio frequency allocation. As is usual in the presentation in ICAO Bulletin, the views expressed herein are those of the authors and do not imply in any way acceptance or rejection of those views by ICAO.