What happened
A DHC-8, registration N680MA, was traveling northbound in radar control approximately 8 NM from Anchorage International Airport. At the time, the aircraft was assigned visual separation from a B-737, registration N673MA, which was traveling northeast on approach to Elmendorf Air Force Base.
The B-737 was flying within clouds when the advisory was issued. During this period, the pilot of the DHC-8 mistook another B-737 for the aircraft requiring visual separation.
The situation escalated when the TCAS (Traffic Collision Avoidance System) alerted the pilot of the first B-737 to close proximity with approaching traffic at his 2 o'clock position. In response, the B-737 pilot initiated a climb while in IMC (Instrument Meteorological Conditions). Simultaneously, the DHC-8 pilot initiated a rapid descent after observing the B-737 emerging from the clouds. These evasive maneuvers took place approximately 20 seconds before the flight paths of the two aircraft would have crossed.
At the time of the conflict, both aircraft were being managed by the same air traffic control position controller on separate frequencies. The pilot of the B-737 was neither advised nor aware that the DHC-8 was providing visual separation from his aircraft, as per FAA Order 7110.65G, until the TCAS pull-up alert occurred and the traffic was visually sighted passing in front and underneath the aircraft.