Flight Safety Digest November 1993

The U.S. Air Traffic Control System Wrestles with the Influence of TCAS Although TCAS II has been credited with averting inflight aircraft collisions, its implementation continues to cause disagreements between pilots and controllers. by V.J. Mellone and S.M. Frank A significant increase in incident reporting related pilot-controller interactions and to suggest strat- to Traffic Alert and Collision-avoidance System egies that could promote a more harmonious (TCAS II) implementation prompted a closer ex- human-technology interaction. amination of how TCAS II is affecting pilots and air traffic controllers. The study was limited to a randomly selected data set of 174 ASRS reports from the January 1992- In July 1992, the U.S. Federal Aviation Administra- March 1993 period. The data set specifically in- tion (FAA) and the U.S. National Transportation cluded pilot and controller reports that involved Safety Board (NTSB) asked the U.S. National Aero- traffic advisory (TA) and resolution advisory (RA) nautics and Space Administration’s (NASA) Avia- incidents. tion Safety Reporting System (ASRS) to complete a data base analysis of TCAS II incident reports. A TA identifies nearby traffic meeting “certain minimum separation criteria,” according to an FAA The ASRS research team identified evidence of advisory circular (AC) issued in August 1993. An increasing air traffic controller consternation with RA provides flight crews with aural and display and resistance to the implementation of TCAS II information advising whether or not a particular technology into the U.S. airspace system. There maneuver should be performed to maintain safe were also strong indications from the data set that separation from a threat aircraft. the aviation community, government agencies and industry may have unwittingly underestimated the The research addressed four issue areas: Were there influence of TCAS II avoidance maneuvers on air any significant changes in TCAS II reporting dur- traffic controllers and flight crews. ing the past three years? Do TCAS II avoidance actions influence the air traffic control (ATC) sys- The study’s objectives were to analyze the ef- tem? Is there evidence of contention between pi- fects of TCAS II avoidance applications on lots and controllers because of TCAS II applications? FLIGHT SAFETY FOUNDATION • FLIGHT SAFETY DIGEST • NOVEMBER 1993 1 Are there effective strategies that could enhance system. In 1987, the U.S. Congress mandated the pilot-controller cooperation? installation of TCAS II on all air carriers with 30 or more passenger seats by the end of 1993. To The ASRS reports were randomly drawn from a date, more than 4,000 air carrier and business air- pool of 780 TCAS II incidents from the same pe- craft are TCAS II equipped. riod involving TAs and RAs. The 174 reports ac- counted for approximately 11 percent of the 1,522 TCAS II is an aircraft-based airborne collision TCAS II full-form reports in the ASRS data base. avoidance system that provides information, inde- For reasons of maximum currency, the data set pendent of the ground-based ATC system, of the included 55 unprocessed reports from the January- proximity of nearby aircraft. It alerts pilots, March 1993 period. There were a total of 109 pilot visually and aurally, to potentially threatening situ- reports, 51 controller reports and 14 reports where ations (intruders) by monitoring the position, clo- both a pilot and controller submitted reports on the sure rate, and altitude of nearby transponder-equipped same TCAS II incident. aircraft. TCAS II offers the pilot both TAs and RAs. RAs are limited to vertical avoidance ma- A coding instrument was developed to extract per- neuvers if an intruder comes within approximately tinent information from the records. The coding 25 seconds to 30 seconds of closure. instrument addressed several areas. The key points are described in Table 1. Initial testing of TCAS II in 1984-85 by two major air carriers, in a limited operational environment, Table 1 determined that “TCAS II is safe and operationally effective, [and] operates harmoniously in the air traffic Key Traffic Alert and control system.” The testing data indicated that “ap- Collision-avoidance System (TCAS II) proximately 50 percent of the alarms will be ‘pre- Coding Points ventive’ and the pilot will not deviate from his flight P1 Pilot Use of TCAS II Advisory path. The average deviation will be approximately P2 Pilot-initiated Avoidance Actions 300 feet (91 meters) based on a 1,500 foot-per-minute P3 Air Traffic Control (ATC)-initiated (457 meters-per-minute) climb or descent.”1 Avoidance Actions P4 Traffic Alert (TA)/Resolution Advisory (RA) The reporting of TCAS II incidents increased sig- Causal Factors nificantly from 1990-1992 (Figure 1). The increase P5 Pilot Communications to ATC about TA/RA P6 ATC Reactions to TA/RA P7 Pilot Comments About TCAS II Incident Traffic Alert and Collision-avoidance P8 TCAS II Application: Prevented-caused System (TCAS II) Incident P9 Evidence of Pilot/Controller Contention Reporter Breakdown 900 Source: Batelle/U.S. National Aeronautics and Space 800 Controller Administration, Aviation Safety Reporting System 700 Pilot Development of the coding form required a number 600 of iterations of trial codings and comparisons to 500 validate the instrument. A team of experienced ASRS 400 pilot and air traffic controller analysts was assigned to analyze and code the 174 reports, and the com- 300 pleted coding forms were entered into a data base 200 Number of Incident Reports program for statistical analysis. The data was re- 100 viewed, tabulated and summarized. 0 1990 1991 1992 Since 1956, the aviation community has attempted, Year in the face of widely spaced, but successive com- Source: Batelle/U.S. National Aeronautics and Space mercial aircraft midair collisions, to conceive and Administration, Aviation Safety Reporting System implement an airborne collision avoidance Figure 1 2 FLIGHT SAFETY FOUNDATION • FLIGHT SAFETY DIGEST • NOVEMBER 1993 included a total of 99 TCAS II incident reports from controllers. This is the largest Pilot Use of Traffic Alert and number of controller reports to ASRS on a Collision-avoidance System (TCAS II) Advisory single topic since the August 1981 air traffic controllers’ strike. The total breakdown of None 11 188 citations from controller reporters on TCAS II incidents a total of 174 reports* included 37 reports from en route control- Evasive Turn 11 lers, 58 reports from terminal radar controllers and four reports from tower controllers. Establish Visual Contact 29 The overall increase in TCAS II incident reporting can also be attributed to the number of air carrier and business aircraft that Change Altitude 137 are installing TCAS II equipment in accor- dance with the requirements of U.S. Federal 0 20 40 60 80 100 120 140 Number of Citations Aviation Regulation (FAR) 121.356 and, pro- *Note: The number of citations may be greater than the number spectively, FAR 91.221 and 135.180. The of reports because categories are not mutually exclusive. reporting trend has continued upwards as Source: Batelle/U.S. National Aeronautics and Space Administration, Aviation Safety Reporting System, flight crews and controllers experience vari- ous TCAS II avoidance advisory situations. Figure 2 Figure 2 depicts the actions taken by pilots in con- Reporter narratives also confirm that ATC work- junction with a TCAS II RA based on the data set. load has increased. Pilot actions to avoid one The majority of pilot actions were precipitated by conflict sometimes precipitated a loss of separa- RAs (92 percent). tion or airborne conflict with another non-intruder aircraft in the same vicinity (Figure 6, page 5). Altitude changes ranged from 100 feet (30 meters) to 1,600 feet (488 meters), as indicated in Figure 3, There were indications in the reporter narratives with an average altitude change of 628 feet (191 that the process of assimilating TCAS II into the meters). In contrast, the designers of TCAS II ex- ATC system was affecting pilot-controller interac- pected altitude deviations to average 200 feet to 300 tions. Intensive coding analysis of the 174 reports feet (61 meters to 91 meters). Initial TCAS II simu- lation testing did not reveal that some aircraft would Pilot-initiated Altitude Change deviate by 1,000 feet (305 meters) or more and in- 20 19 trude upon other occupied altitudes. Average Altitude Change 18 628 Feet As shown in Figure 4 (page 4), the pilot informed 16 ATC after reacting to the RA in the majority of 14 13 13 13 incidents (93). In 15 incidents, the pilot did not 12 notify ATC about the RA maneuver. In 26 incidents, 10 8 the pilot either forewarned ATC (9) or informed ATC 8 7 while complying with the RA (17). A number of 6 6 controller reports indicate that this response pattern Number of Incidents 4 4 is a major source of their concern because of the 2 222 impact of the unanticipated avoidance deviations on 2 1 1 1 1 1 1 the controllers’ air traffic situation. 0 100 150 200 250 275 300 400 500 600 700 800 It is here that the dual impacts of TCAS II applica- 1,000 1,100 1,200 1,300 1,400 1,500 1,600 tions can be seen. Analysis of the reporter narra- Feet tives verify that TCAS II has been instrumental in Source: Batelle/U.S. National Aeronautics and Space Administration, Aviation Safety Reporting preventing many near midair collisions (NMACs) System and other conflicts. (Figure 5, page 4). Figure 3 FLIGHT SAFETY FOUNDATION • FLIGHT SAFETY DIGEST • NOVEMBER 1993 3 TCAS II has definitely prevented or re- Pilot Communications with Air Traffic Control duced the threat of airborne conflicts as (ATC) on Traffic Alert and Collision-avoidance reflected by the majority of favorable System (TCAS II) Resolution Advisory (RA) ASRS reports and evidenced by the fol- Ignored RA per ATC instructions 2 lowing statements from ASRS reports: Unable to notify ATC because 4 of frequency congestion 291 citations from Other 8 a total of 174 reports* • “It is the captain’s opinion that TCAS Forewarned ATC before complying 9 II saved about 100 lives today.

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