CORP JET ON VECTOR FOR ILS RWY 17 AT JYO HAD AN NMAC WITH AN ACR DC9 CLBING OUT OF IAD. BUSY CTLR ISSUED TFC AND TURNS TO THE DC9 WHO TOLD CTLR THAT THE CORP JET WAS IN SIGHT. THE DC9 DID NOT RESPOND TO THE TURNS THE CTLR GAVE HIM.

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WE WERE BEING RADAR VECTORED BY DULLES APCH CTL TO INTERCEPT THE JYO RWY 17 ILS. ASSIGNED AN E HEADING AND CLRED TO DSND FROM 7000 FT MSL TO 5000 FT MSL. APCHING 6000 FT IN THE DSCNT APCH CTLR TOLD US TO IMMEDIATELY LEVEL OFF AT 6000 FT AND IN THE SAME BREATH TOLD AN AIRLINE DC9 TO TURN TO A 330 DEG HDG OR 340 DEG HDG; THAT WE EACH HAD MUTUAL TFC LESS THAN A MI. THE AIRLINE DC9 TOLD APCH CTLR THEY HAD US AND WILL KEEP SEPARATION. 4-5 SECONDS LATER THE DC9 TOLD APCH CTLR THEY HAD A VISUAL ON US. APCH CTLR ASKED THE DC9 IF THEY HAD A VISUAL ON US IN HIS FIRST RESPONSE. HE SAID NO; 'WE HAD THEM ON TCASII.' WE SAW THE DC9 AT OUR 10 'CLOCK POS LESS THAN 1/2 MI; HEADED RIGHT FOR US SLIGHTLY BELOW OUR ALT WITH A HIGH CLB ANGLE. THE DC9 PASSED SLIGHTLY ABOVE AND BEHIND US. THE CTLR WAS VERY BUSY. AFTER THE INCIDENT; HE VECTORED US OVER THE ARPT AND ASKED IF WE HAD IT IN SIGHT FOR A VISUAL (HE FORGOT WE REQUESTED AN ILS APCH). WE WERE ABLE TO MAKE A VISUAL APCH AND LAND; THEN CANCELED IFR ON THE GND USING THE RDO. CTLR WORKLOAD WAS THE CULPRIT; I BELIEVE; AND THE DC9 CAPT'S INACCURATE RESPONSE AND CONTINUATION OF A TURN AND CLB TOWARDS US. HE MADE NO ATTEMPT TO STOP HIS TURN SOONER TO PASS WELL BEHIND US; NOR FOLLOWED CTLR HDG INSTRUCTIONS.