Traffic Collision Avoidance System (TCAS) is a flightdeck-based technology aimed at helping aircraft avoid proximate traffic. TCAS information has traditionally not been presented to the air traffic controller. A 2002 German midair collision was triggered, in part, by incompatible air traffic control (ATC) and TCAS clearances. Largely in response to this accident, attention has focused in recent years on the potential benefits of “downlinking” to the controller TCAS Resolution Advisories (RAs) in near real time. Such presentations, it is thought, could benefit situation awareness and joint decision making between controller and pilot. A cognitive task analysis (CTA) was recently conducted into the present-day and future RA Downlink (RAD) operational concepts. On the basis of functional task description and cognitive walkthroughs, CTA assessed the impact of various specific non-nominal events (e.g. pilot reports RA, but does not initiate an evasive maneuver). Finally, a set of cognitive elements and potential error mechanisms was identified.