Aviation Safety

Users Differ in Views of Collision Avoidance System and Cite Problems Gao ID: RCED-92-113 March 16, 1992

For more than 30 years, the Federal Aviation Administration (FAA) and the aviation industry have been working to develop a system to prevent mid-air and near mid-air collisions. After evaluating several systems, FAA decided in 1981 to develop and deploy the Traffic Alert/Collision Avoidance System, an airborne, aircraft-to-aircraft system that scans surrounding airspace, warns of potential intruders, and recommends evasive maneuvers. This report discusses (1) pilots' and air traffic controllers' views on the system; (2) FAA efforts to resolve problems with the system; and (3) key aspects of FAA's software engineering approach, including FAA's plans to verify and validate the system.

GAO found that: (1) FAA officials believe that TCAS has increased the margin of safety in aviation travel, but some problems have emerged that prevent the system from reaching its full potential; (2) pilots and air traffic controllers with TCAS experience expressed concern regarding such TCAS problems as resolution advisories that have caused pilots to unnecessarily leave assigned airport approaches, go around airports, and reenter landing patterns, excessive altitude deviations pilots have made in response to TCAS, and unnecessary TCAS advisories issued while pilots were following established air traffic control procedures; (3) before installing TCAS in commercial aircraft, FAA verified that TCAS performed in accordance with its specification, but did not validate a key element of TCAS, the collision avoidance requirements; (4) FAA plans to develop system-level specifications and perform full TCAS verification and validation by the end of 1992; (5) opinion within the aviation industry is sharply divided on whether FAA should fully verify and validate TCAS and the modifications before implementing the modifications, since neglecting current problems could reduce pilots' confidence and present a greater risk; and (6) FAA plans to implement the modifications before completing verification and validation.

Recommendations

Our recommendations from this work are listed below with a Contact for more information. Status will change from "In process" to "Open," "Closed - implemented," or "Closed - not implemented" based on our follow up work.

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