If the lift-generated wakes of aircraft were not hazardous, and did not persist for several minutes, runways could be safely located close each other and re-used within time intervals based only on air-traffic management constraints rather than on aerodynamic ones that relate to flight safety. 1 Because lift-generated wakes of aircraft pose a hazard, studies have been conducted on ways to reduce the effect of vortex wakes on airport capacity. One such method considered the transport and decay of vortex wakes as a function of time as a means to shorten the time required for single runways to become vortex free for re-use. 2-11 It was found that the capacity of a single runway might then be increased by as much as 10% by judicious use of weather information and aircraft timing. Although beneficial, such an improvement does not accommodate the factor of two or three increase in traffic volume that is expected during the next 20 years. It was therefore reasoned that improvements in the use of a single runway would not achieve the desired goal, and that the number of runways at each airport will need to be increased. An increase in the number of runways at most airports is not possible if the runways are built with conventional lateral spacings of 4300 ft (or 1311 m) so that they can be operated independently. Because available land area at or near existing airports is already in short supply, current research has focused on the use and addition of closely-spaced parallel runways, which are often spaced parallel to each other at distances of 750 ft (or 230 m) or more. The runways are then too close to operate independently, because wakes of preceding aircraft might intrude into the air space of a following aircraft. Management of aircraft flight paths will then require more planning due to proximity of aircraft to one another, and due to the higher density of aircraft traffic in the air and on the ground. The research reported addresses the development of a reliable method for estimation of the rate at which liftgenerated vortex wakes of subsonic transport aircraft move and spread due to the wind and turbulence in the atmosphere along the flight path of arriving aircraft, and due to self-induced spreading mechanisms by the vortex wake. Previous studies have examined the details of various aerodynamic mechanisms that cause the vortex wakes of subsonic aircraft to spread and to move. 12-20 This paper first presents an overview of the results of the research