Pilots' use of the traffic alert and collision avoidance system (TCAS II) was evaluated in simulated air carrier line operations. Sixteen three-person airline flight crews currently flying the Boeing 727 served as subjects. Each crew flew eight flights with or without TCAS as part of the full-mission simulation. Their performance of the avoidance maneuvers and evaluation of the system were measured.
The crews were trained on the aircraft differences and the TCAS II. The second day consisted of a 10 hour duty day of normal line operations. All communications, navigation, and cockpit procedures were carried out according to the standards of their particular airline. The crews were under full air traffic control along with the other aircraft in the airspace. All crews were exposed to the same traffic conflicts under the same conditions of high/low traffic densities, high/low workload, high/low visibility. Pilot flying and dusk/night lighting were counterbalanced.
To ensure safe separation of aircraft, the TCAS II collision avoidance system commands a climb, a descent, or a reduction in the rate of climb or descent. Aircraft separation was significantly improved when the system was in use. No aircraft came within 200 ft vertically and 1000 ft horizontally in 96 flight segments. Without TCAS, in 32 flight segments, there were four instances in which minimum aircraft separation was less than 1000 ft horizontal and 200 ft vertical.
All maneuvers in response to the resolution advisories issued by TCAS were made in the correct direction. The time to initiate a maneuver in response to a resolution advisory was within the 5 seconds allowed by the system except in 1 out of 59 instances, which did not result in unsafe separation.
Pilots with TCAS exceeded the altitude changes required for separation. The average required altitude change from level was 652 ft. The average pilot response was 852 ft. In 9 of 19 cases, pilots changed altitude unnecessarily when a preventive advisory was issued.
The probability of visual acquisition of conflicting aircraft did not vary as a function of the presence of TCAS. Pilots did not maneuver differently to a resolution advisory when the traffic was in sight. There were no differences in maneuvers made in visual and instrument meteorological conditions.
The TCAS logic may command a maneuver toward another aircraft's present altitude when one or both aircraft are climbing or descending. It was found that pilots responded significantly more slowly to crossing maneuvers, and demonstrated higher peak vertical velocities, and greater altitude changes. No learning effects were observed.
Pilots who had displays of conflicting traffic used the displays to maneuver to avoid unseen traffic before a resolution advisory was issued by the TCAS.
The pilots who flew the traffic encounters without TCAS tended to be less abrupt in their maneuvers than the TCAS-equipped crews, as would be expected since the non-TCAS crews maneuvered only enough to insure visual separation. They were not able to see, and thus to maneuver to avoid, nine of the 33 conflicting aircraft that would have triggered a resolution advisory.
Pilots who were exposed to TCAS were asked to rate the effectiveness of the TCAS equipment on a 10 point scale. TCAS was rated as useful for reducing the risk of midair collisions (mean=8.0) and useful in aiding visual contact (mean=7.3). The evasive maneuvers prescribed by TCAS were rated 7.7 from unacceptable (1) to acceptable (10). Pilots rated their overall satisfaction with the TCAS as 7.6.
The results of tills experiment represent pilot behavior in using the traffic alert and collision avoidance system under simulated conditions. These results indicate that (1) TCAS II, properly used, is effective in ameliorating the severity of the simulated traffic conflicts presented in this study, and that (2) pilots are able to utilize TCAS II correctly and within the response times allocated by the system.
