The impact of a quantitative specification of a Well Clear Boundary on pilot displays for self separation

The basis for a conflict probe display is the function which computes the 3-D locations where a conflict exists if ownship were to travel in that direction. The conflict space that is computed by the probe depends on the definition of its boundary. Initial implementations of conflict probe displays used a spatial separation threshold such as, for example, the assured normal separation distance as the criterion for the conflict space boundary. Recent research conducted within the unmanned aircraft domain has proposed that the concept of well clear be quantified in terms of acceptable likelihood that a Well Clear Violation will turn into a collision hazard. Both spatial and spatial-temporal definitions of the Well Clear Boundary have been proposed. With such a definition, the concept of a conflict probe display can be used to visualize the well clear boundary. This paper discusses the rationale for choosing the spatial threshold given a specified collision risk, identifies limitations of a spatial-only approach to the definition of a well clear threshold and illustrates how a modified-tau based approach can be used to minimize the likelihood of TCAS Resolution Advisories outside the Well Clear Boundary.