One proposed change to the national airspace system, designed to increase its capacity, involves allocating more authority over flight path selection to the individual pilot. This proposed increase in pilot authority has been termed free flight or user-preferred routing (Planzer & Jenny, 1995; Radio Technical Committee for Aeronautics, 1995; Remington, Johnston, Ruthruff, Gold, & Romera, 2000; Wickens, Mayor, Parasuraman, & McGee, 1998). Varying amounts of authority shift have been proposed, ranging from extreme (placing air traffic control totally into a monitoring role) to minor (allowing pilots more freedom during the en route phase of flight), or some compromise between the two. This shift in authority will present new challenges to the pilot in avoiding other traffic that is sharing the same airspace.
In order for these changes in authority to occur safely, pilots will need an accurate picture of the current and future predicted location of traffic that is close to their own aircraft. This awareness will be supported by providing pilots with a cockpit display of traffic information (CDTI), which will extend and enhance the current traffic situation display on the Traffic Alert and Collision Avoidance System (TCAS). The CDTI would conceivably be coupled with decision aids to recommend maneuvers that would avoid conflicts, analogous to the manner in which the current TCAS system advises maneuvers to resolve conflicts.
The current national airspace system already allows pilots some authority over where they choose to fly. For example, when operating under visual flight rules (VFR), pilots must maintain traffic separation by means of visually scanning the airspace around them. When a pilot encounters a potential conflict with another aircraft, Federal Aviation Regulation (FAR) 91.113 dictates that aircraft alter course using a lateral maneuver (e.g., turn right). These procedures rely heavily on pilots' adherence to learned procedures in order to maintain safe separation of aircraft. However, little research has been directed toward determining which maneuvers pilots naturally prefer and how different traffic conflict geometry, as visible on a CDTI, might modulate these preferences. It is important to fully understand these natural decision preferences or "population stereotypes" (Fitts & Posner, 1967; Smith, 1981) before policy makers begin to develop the freeflight procedures that pilots will need to follow when confronted with a traffic conflict. Careful attention should be paid so as to avoid creating procedures that conflict with pilots' stereotypes or natural preferences for certain types of maneuvers.
When pilots are faced with a potential traffic conflict requiring an alteration of the original flight plan, they can use many information sources--both external and internal--to choose an appropriate maneuver (Wickens, Helleberg, & Xu, 1999). Some of these sources are currently available to pilots, whereas others could be used by pilots in the near future if free-flight rules begin to be implemented. Furthermore, some of these are external (cue based) and others are internal (knowledge based). These current and future information sources (the number designations will be referred to in this paper), include the following:
1. Air traffic control (ATC) instructions and vectors.
2. TCAS traffic situation display and resolution advisories.
3. Federal Aviation Regulations, or accepted procedures such as FAR 91.113, which states that aircraft shall alter course to the right in the event of a head-on conflict.
4. On-board decision aids: Although current TCAS resolution advisories are considered decision aids, these are simple (climb or descend) and are offered only in the most severe cases when ATC has allowed a conflict to become critical. With a CDTI future developments of more complex algorithms are likely (Johnson, Battiste, & Bochow, 1999). …