MIL ACFT HAD LTSS FROM COMMUTER Y. SYS ERROR.

Narrative

I WAS WORKING IAD S ARR AND HANDED OFF A C-12 TO THE N ARR/FINAL CTLR AT 4000 FT ON A VECTOR OF 010 DEGS TO PARALLEL IAD FINAL AND DEP CORRIDORS. C-12 WAS SE OF ARPT AT TIME OF HDOF. JUST PRIOR TO FREQ CHANGE; THE C-12 REQUESTED DIRECT AML. I TOLD THE ACFT TO MAKE THE REQUEST ON THE NEXT FREQ AND CHANGED COM. I TOLD N ARR/FINAL THAT THE C-12 WAS HER CTL. COMMUTER ACFT Y WAS DEPARTING RWY 19L AND WAS ISSUED A R TURN HDG 330 DEGS CLBING TO 4000 FT. THIS TURN PUT THE ACFT INTO MY RWY 12 ARR AIRSPACE; BUT DEP WAS USING PRE-COORDINATED CLB PROCS ('LOOK AND GO' REF ANY RWY 12 ARRS). THIS PROC REQUIRES THAT HE QUICK LOOK MY POS. N ARR/FINAL CLRED C-12 DIRECT AML; DEPART AML HDG 330 DEGS; VECTORS FOR A VISUAL APCH AT JYO. C-12 WAS LEFT AT 4000 UNTIL PASSING AML DUE TO TFC HIGH ON APCH TO RWY 19R. THEN N ARR/FINAL ISSUED DSCNT CLRNC. WHEN PENDING CONFLICT WAS NOTED; THE N ARR/FINAL CTLR INSTRUCTED THE C-12 TO LEVEL OFF. DEP ACFT ALSO LEVELED OFF. N ARR/FINAL AND DEP COORDINATED; ISSUED DIVERGING HDGS AND CONTINUED THE RESPECTIVE CLB AND DSCNT. DEP DID NOT SEE THE C-12 OVER AML BECAUSE IT WAS ON AN N TAG (WOULD NOT APPEAR IF HE QUICK LOOKED ME (S)) AND BECAUSE C-12 WAS OVER RADAR MAIN BANG. HIS DEP ALSO WAS WITHIN MAIN BANG AREA.