Optimizing the Next Generation Collision Avoidance System for Safe, Suitable, and Acceptable Operational Performance

Abstract

The Traffic Alert and Collision Avoidance System (TCAS) is mandated worldwide on large commercial aircraft and has been shown to substantially reduce the risk of mid-air collision. However, the logic used to select pilot advisories is difficult to modify and does not easily support new surveillance inputs. The next generation system, called Airborne Collision Avoidance System (ACAS X), currently addresses many of the design limitations of TCAS. ACAS X is optimized with respect to a cost function. The system was initially optimized to increase safety and decrease alerts. Recent work has focused on tuning ACAS X to also meet operational suitability and pilot acceptability performance metrics. An iterative tuning process reduced the operational impact on the air traffic system and improved acceptability of alerts. This paper summarizes a 15-month effort that resulted in substantial improvements. Compared with TCAS, ACAS X reduces collision risk by 59%, lowers the alert rate by 50%, and issues 28% fewer disruptive alerts. ACAS X also resolves encounters with simpler alert sequences and issues less than half as many reversal and altitude crossing advisories.