PCT CTLR EXPERIENCED OPERROR; CORRECTING DEVELOPMENTAL CTLR ACTION BUT NOT IN TIME TO PREVENT LOSS OF SEPARATION BTWN 2 ACR'S ON VECTOR TO FINAL AT IAD; VA.

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THE DEVELOPMENTAL ISSUED A HDG WHICH WOULD PUT THE ACFT THAT WE WERE CTLING (ACR X) IN CONFLICT WITH AN ACFT IN ANOTHER SECTOR. I USED THE INSTRUCTOR OVERRIDE CAPABILITY AND ISSUED A HDG AND ALT THAT WOULD HAVE PREVENTED THE LOSS OF SEPARATION. THE PLT OF ACR X DID NOT RESPOND. I ISSUED ANOTHER INSTRUCTION WHICH THE PLT ACKNOWLEDGED BUT DID NOT READ BACK. THE PLT CONTINUED TO TRACK THE HDG ASSIGNED BY THE DEVELOPMENTAL; LEADING TO A LOSS OF SEPARATION. AFTER LISTENING TO THE TAPES; IT SOUNDS LIKE MY INSTRUCTIONS MIGHT HAVE BEEN PARTIALLY BLOCKED BY THE PLT OF ACR X. RECOMMENDATION TO PREVENT SIMILAR SITS (IF TECHNOLOGY ALLOWS): 1) CTLR'S INSTRUCTIONS WOULD BE HEARD IN THE COCKPIT EVEN THOUGH THE PLT IS XMITTING AT THE SAME TIME. THIS WOULD PREVENT THE CONFUSION THAT COMES FROM CTL INSTRUCTIONS THAT WE ISSUED IN QUICK SUCCESSION. THE PLT THINKS HE HEARD THE INSTRUCTION BUT HE IS ACTUALLY XMITTING AN ACKNOWLEDGEMENT AT THE SAME TIME THAT THE CTLR HAS ISSUED A NEW INSTRUCTION. 2) CONFLICT ALERT WOULD BE PROGRAMMED TO RECOGNIZE A FACILITY'S 'HOT SPOTS' AND SOUND AN ALERT SOONER IN THESE AREAS.