ATC LOST STANDARD SEPARATION BTWN THE RPTR'S MLG DSNDING ON A VISUAL ARR AND A BEECH JET WHO WAS INSTRUCTED TO TURN AWAY FROM RPTR.

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WE CHKED ON WITH DCA APCH ON 119.85. WE WERE INITIALLY GIVEN 'MAINTAIN 10000 FT.' WE WERE THEN GIVEN 'DSND TO 6000 FT. SLOW TO 210 KTS AND FLY 030 DEG HDG OFF OJAAY FOR MT VERNON VISUAL APCH'. NO APCH CLRNC WAS GIVEN NOR WE TOLD TO JOIN THE R-189 OFF DCA. WE WERE GIVEN DSCNT TO 3000 FT THEN 2500 FT. APCH THEN ASKED 'ARE YOU JOINING THE APCH?' WE TOLD THEM 'NO WE WERE NOT GIVEN APCH CLRNC.' WE WERE THEN TOLD TO MAINTAIN 6000 FT. WE WERE ALREADY THROUGH 5800 FT AND DSNDING. WE LEVELED AT 5700 FT AND BEGAN A CLB BACK TO 6000 FT. WE THEN GOT A TCASII TA WHICH SHOWED TFC AT -600 FT. WE WERE IN BROKEN CLOUD. WE WERE ALSO TOLD TO TURN L TO 340 DEGS (?). A BEECH JET WHO WE BELIEVE WE WERE IN CONFLICT WITH WAS INSTRUCTED TO TURN AWAY. WE BELIEVE ATC SEPARATION WAS LOST DURING THIS ENCOUNTER. WE WERE THEN GIVEN 'FLY 270 DEGS AND MAINTAIN 2500 FT' AND TOLD TO FOLLOW A CITATION. WE SAW ACFT LIGHTS IN A POS CLOSE IN AND LOW WHICH WE WERE ABOUT TO LOSE UNDER THE NOSE SINCE WE WERE STILL ABOVE 5000 FT DSNDING. WHEN WE LOST THE LIGHTS UNDER THE NOSE WE TOLD THE CTLR TO 'TAKE US BACK OUT AND DO IT RIGHT.' THIS REQUEST WAS MADE BECAUSE OF LOSS OF SEPARATION AND VISUAL CONTACT ON OUR TFC AND THE FACT THAT WE COULD NOT GET DOWN AND EXPECT TO FLY A STABILIZED APCH (5000 FT AT 10 MI). AT THIS TIME ANOTHER CTLR CAME ON FREQ AND SORTED OUT A VERY BUSY TFC SIT. WASHINGTON APCH SEEMS TO ISSUE VISUAL APCH CLRNCS IN A NON STANDARD MANNER. THEY FREQUENTLY ISSUE VISUAL APCH CLRNCS WITHOUT INSURING ACFT HAVE THE FIELD IN SIGHT OR EVEN GIVING SPECIFIC CLRNC ACCORDING TO THE .65. THEY JUST SORT OF EXPECT US TO FLY THE VISUAL WITHOUT SPECIFIC CLRNC.