In areas of sufficiently high traffic density, air traffic control in some form will always be the primary mode of ensuring aircraft separation. Any traffic control system is concerned with separating aircraft in the future; therefore, the nominal separation criteria are determined by the accuracy of prediction of future movement of the aircraft and by the ability of the control system to correlate flight path predictions. The utilization of an airspace of a given capacity is determined by the distribution or flow of traffic.

The accuracy of prediction depends on track-keeping and maintenance of planned ground speed. Since improvements in track-keeping will be limited for some time by past I.C.A.O. decisions on short-range navigation aids, attention is directed in this paper to maintenance of flight-planned ground speed. The effect of this on the operating cost of a modern jet airliner is examined and compared with the cost of flying at minimum cost mach number at a less than optimum altitude. It is concluded that in a crowded airspace, speed maintenance is preferable in short-range operations.

The increase in space utilization by flow control is illustrated for a simple model, showing that speed control by A.T.C. increases utilization of a space capacity, which has already been increased by speed maintenance.