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

We propose to adopt a new airworthiness directive (AD) for various aircraft equipped with certain ACSS TCAS units. This proposed AD would require upgrading software. This proposed AD results from 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. We are proposing this AD to prevent TCAS units from dropping tracks, which could compromise separation of air traffic and lead to subsequent mid-air collisions.