Evaluation of Proposed Changes to the ACAS Modified Tau Calculation

Modified tau is a parameter computed by ACAS to estimate the earliest time at which a collision could occur should an intruder aircraft accelerate toward the own aircraft. A concern with the modified tau calculation has been raised in a class of encounters where intruders are already close and converging slowly. In these problem cases, ACAS may induce a Near Mid-Air Collision by generating RAs with inappropriate timing or initial sense or failing to reverse sense when necessary. Performance in some problem encounters is greatly improved when using several proposed changes to the modified tau equations. These changes are outside CP112E, which focuses only on RA reversals. Although changes to modified tau resolve some problem encounters, aggregate risk-ratio results do not support implementing the existing proposals. There remains a concern about mid-air collision risk due to vulnerability in the existing modified tau equations, yet a robust solution to the problem has not been developed.