SIMILAR CALL SIGNS ACR XY CLB THROUGH OCCUPIED ALT TCASII RA HAD LTSS FROM ACR Y. SYS ERROR.

Narrative

IN THIS SIT; I WAS CAPT PERFORMING THE DUTIES OF PNF; ON A 2 PERSON CREW. WE WERE CRUISING AT FL290; AND REQUESTED CLB TO FL330. OUR CALL SIGN WAS ACR XY. IT IS IMPORTANT TO NOTE THAT ACR Y WAS ALSO ON FREQ. WE HAD ACKNOWLEDGED THE PRESENCE OF ACR Y AND ACR Y HAD ACKNOWLEDGED OUR PRESENCE WITH RESPECT TO LIKE CALL SIGNS. WHEN I INITIALLY REQUESTED CLB FROM FL290; ZDV RESPONDED THAT WE COULD EXPECT THE CLB; AND THAT SOME VECTORING WOULD BE REQUIRED. SOON; WE WERE CLRED TO CLB. A MIN OR SO LATER; ATC INSTRUCTED ACR Y TO TURN TO A HDG. ACR Y RESPONDED AND TURNED. WE SAW ACR Y ABOVE OUR ALT; AHEAD OF US; AND IN THE SAME DIRECTION OF FLT. AT LEAST 2 MORE INSTRUCTIONS WERE THEN DIRECTED TO ACR Y. TURN FURTHER TO A HDG; AND TO INCREASE THE RATE OF TURN. ALL RADIO TFC HAD SOUNDED VERY ROUTINE TO THIS POINT. THEN I NOTICED A YELLOW DIAMOND SYMBOL ON THE TCASII SCREEN (12 O'CLOCK; MINUS 300 FT) AT ABOUT THE TIME ATC TOLD ACR Y TO INCREASE RATE OF TURN. AT THAT MOMENT; I REALIZED THAT THE TURNS WERE ACTUALLY INTENDED FOR US ACR XY. I REQUESTED INSTRUCTIONS FROM ATC; BUT WAS BLOCKED. THEN OUR TCASII ISSUED AN RA. MY COPLT COMPLIED; INCREASING OUR RATE OF CLB. WE HAD VISUALLY ACQUIRED THE APCHING HEAD-ON TFC BY THIS TIME; AND MY COPLT TURNED TO THE R; TO INCREASE THE SEPARATION DISTANCE. THE RA CLRED BEFORE WE LEVELED AT 330. I AM VIRTUALLY CERTAIN THAT ATC ISSUED INSTRUCTIONS TO THE WRONG ACFT; BUT SIMILAR SOUNDING CALL SIGNS PLAYED A SIGNIFICANT ROLE IN THIS INCIDENT; IN SPITE OF THE FACT THAT BOTH AIRCREWS AND THE CTLR WERE WELL AWARE OF THE POTENTIAL HAZARD. I DON'T BELIEVE A MIDAIR COLLISION WAS IMMINENT; BUT TCASII PROBABLY PREVENTED THE INCIDENT FROM DEVELOPING INTO A NMAC.