The Federal Aviation Administration’s (FAA’s) Traffic Alert and Collision Avoidance System (TCAS) Program Office is developing an advanced Airborne Collision Avoidance System (ACAS X) to meet the needs of both manned aircraft and Unmanned Aircraft Systems (UAS). ACAS Xu, the UAS variant, provides vertical Resolution Advisory guidance in most situations and horizontal RA guidance for cases where either surveillance quality or vehicle performance does not support vertical maneuvers. ACAS Xu is fully interoperable with TCAS II and also features new passive (ADS-B based) collision avoidance maneuver coordination techniques. To evaluate the effectiveness of ACAS Xu for collision avoidance as well as to inform interoperability requirements for integration with self-separation systems, the FAA, in conjunction with NASA, General Atomics Aeronautics Systems, Inc, and Honeywell Aerospace, conducted a proof-of-concept flight test in November 2014 with both manned and unmanned intruder aircraft. This paper describes the ACAS Xu system as well as the flight test and provides a summary of observed system performance.