AN IFR PA31 MISUNDERSTANDS ASSIGNED HDG AND CONFLICTS WITH AN ARRIVING C560 IN DSCNT TO ITH.

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PLT ACFT #1 REQUESTED IFR CLRNC (OVER ITH) AND WAS ISSUED CLRED TO HANCOCK 186019 AS FILED (SERIES OF VOR FIXES FOR AERIAL SURVEY) AFTER BEING RADAR IDENTED. I THEN POINTED ACFT #1 OUT TO SYR APCH AS FIRST FIX WAS ART 146042 AND COMMENCED HDOF TO ZNY. ACFT #2 WAS DSNDING TO ITH WITH BGM APCH APPROX 9000 FT DSNDING TO 6000 FT AND 25 MI SE. I COORD WITH BGM INITIALLY TO PUT ACFT #2 ON 270 DEG HDG TO AVOID ACFT #1 WHO SHOULD HAVE PROCEEDED NEBOUND. I CLBED ACFT #1 TO 8000 FT AND HDOF TO ZNY WAS COMPLETED. ACFT #1 DID NOT CHANGE TO ZNY AS INSTRUCTED. ACFT #2 CHKED ON FREQ DSNDING TO 6000 FT. ACFT #1 APPEARED TO BE SBOUND AND I THEN CALLED TO SEE IF HE WAS STILL ON FREQ AND HE ANSWERED SO I ISSUED A TURN TO 180 DEGS; AND THEN TURNED ACFT #2 TO 360 DEGS. BOTH ACFT COMMENCED A TURN AND COURSES WERE THEN PARALLEL. I QUERIED THE PLT AS TO WHY HE WAS SBOUND AND HE SAID HE THOUGHT HE WAS CLRED TO THE HNK FIX FIRST (CLRNC DEST). HE APPARENTLY MISSED THE 'AS FILED' PORTION OF CLRNC OR MISUNDERSTOOD. THE AIRSPACE IS AT 3 CORNERS OF APCH CTLS AND ZNY OVERHEAD; REQUIRING A GREAT DEAL OF COORD FOR THE SIMPLE ERROR OF ACFT #1 TURNING SBOUND INSTEAD OF NEBOUND.