ZID CTLR EXPERIENCED POTENTIAL SEPARATION LOSS DURING ACFT COM PROB.

Narrative

THIS SIT WAS SCARY BUT IN A WAY CTLED AT ALL TIMES. ACFT #1 WAS CLBING WBOUND FROM COLUMBUS; OH. ACFT #1 WAS ASSIGNED FL230. ACFT #1 RESPONDED TO CLRNC. ACFT #1 WAS HANDED OFF AND SWITCHED TO MY FREQ. ACFT #1 MIKE QUIT FUNCTIONING. RESPONSE TO FREQ CHANGE NOT HEARD ON PREVIOUS FREQ. PREVIOUS CTLR TRIED TO SWITCH ACFT #1 SEVERAL MORE TIMES. PREVIOUS CTLR CALLED ME AND ASKED WAS I TALKING TO ACFT. THINKING ACFT MIGHT BE SITTING ON MY FREQ BUT NOT CHKED ON; I CLB ACFT #1 TO FL280. I GET NO RESPONSE. I ASSUME ACFT #1 STILL CLRED TO FL230. I NOTICE ACFT #1 LEAVING FL245 SO I START CALLING HIS CALL SIGN -- NO RESPONSE. NOT KNOWING HOW HIGH ACFT #1 IS GOING; I TURN ACFT #2 FROM ABOUT 190 DEG HDG TO 070 DEG HDG. ACFT #2 MUCH SLOWER AND HAVING TROUBLE GETTING OUT OF ACFT #1'S WAY. COMMOTION ON FREQ CAUSES ACFT #1 TO REALIZE THAT I DON'T HAVE CTL OF EVENTS. USING A DIFFERENT MIKE; WE RE-ESTABLISH COMS. HE ASSURES ME HE IS ONLY GOING TO FL280. POSITIVE SEPARATION? PLT ALSO MAKES REF TO HAVING PROBS WITH MIKE IN PAST. I SHOULD NOT CHK FOR ACFT BY ISSUING CLRNC THAT REQUIRES READBACK. IF PLT KNOWS EQUIP MAY NEED MAINT; PLT SHOULDN'T USE IT.