ABSTRACT
The detection and avoidance of collisions in a congested space is a problem that all independently mobile agents must solve quickly and efficiently. A powerful method for solving the problem of conflict detection is to base navigation decisions on estimates of time to contact (Tc). We conducted a series of experimental investigations to determine if professional pilots flying a modern commercial airliner use Tc when given the authority to make decisions regarding routing and separation. Such authority would exist under current designs for the National Airspace System (NAS), namely the continuum of distributed control structures referred to as free flight. Ten currently certified commercial airline pilots, with 6000–24000 hours of flight time (average 9177 hours), flew 12 simulated en-route air traffic scenarios in our Boeing 757 glass-cockpit experimental platform. The pilots had the authority to make all decisions regarding routing and separation. The CRT displays were their only source of traffic information. From analysis of the pilots’ concurrent verbal reports and manoeuvres, we determined the separation (from the subject’s ownship) and relative velocity of each aircraft at the moment the subject identified the aircraft as a potential conflict. The data were plotted in a phase plane representation of the airspace which we call the risk space. The slope of a line in the risk space has units of time. Any line passing through the origin of the risk space (x=y=0) represents Tc. A least-squares linear regression analysis of the data resulted in a line with a slope of 3.0 minutes and an intercept of 3.6 nautical miles (r2=0.48, n=61). The intercept is statistically equivalent to the five-mile FAA criterion for minimum separation. The pilots detected conflicts when they were 3 minutes away from violating the FAA’s constraint on their behaviour. The least-squares fit to the data represents an invariant in the pilot-airspace interaction that triggers a consistent and task-relevant change in behaviour. The line defines the conditions that guide skilled detection of an impending en-route conflict. The risk space and its invariant elucidate the 230knowledge pilots invoke to identify impending conflicts, and could form the basis of a new decision aid tailored to free flight.
