Airworthiness Directives; Aviation Communication & Surveillance Systems (ACSS) Traffic Alert and Collision Avoidance System (TCAS) Units

We are adopting a new airworthiness directive (AD) for certain ACSS TCAS units installed on but not limited to various transport and small airplanes. This AD was prompted by reports of anomalies with TCAS units during a flight test over a high density airport. The TCAS units dropped several reduced surveillance aircraft tracks because of interference limiting. This AD requires upgrading software. We are issuing this AD to prevent TCAS units from dropping tracks, which could compromise separation of air traffic and lead to subsequent mid-air collisions.