When a loss of separation is likely to occur or has occurred, the controller will issue instruction(s) to one or both aircraft. These instructions may conflict with an Airborne Collision Avoidance System (ACAS) RA, creating the potential for confusion and the possibility of an inappropriate response.

Such a conflict often arises because of the much more rapid update of relative position available to the flight crew via TCAS than to ATC with even the best radar refresh rates. Controllers must also: