AN ACR MD88 FLC WERE VECTORED DOWNWIND THEN IMMEDIATELY TURNED TOWARD THE ARPT THEN TURNED BACK TO DOWNWIND. LATER; THEY WERE CHARGED WITH CAUSING AN NMAC.

Narrative

WE WERE ON AN EXTENDED APCH TO DCA IN VERY POOR IMC WX (MODERATE TURB; ICING; RAIN) AND ZERO FLT VISIBILITY. DCA APCH CTL HAD NUMEROUS ACFT IN THE RADAR PATTERN FOR THE ILS TO RWY 36. WE WERE GIVEN SEVERAL VECTORS (HDGS) BY ATC; THEN FINALLY A VECTOR (R TURN S) TO A 180 DEG HDG; PRESUMABLY FOR DOWNWIND. JUST AS WE COMPLETED THIS TURN TO 180 DEGS; THE CTLR GAVE US A L TURN ASSIGNMENT TO 090 DEGS; BASICALLY TOWARD THE ARPT ITSELF. DURING THIS TURN THE CTLR ASKED IF WE WERE TURNING TO 90 DEGS. THE FO RESPONDED IN THE AFFIRMATIVE (WE HAD ALMOST COMPLETED THE TURN); AND THEN ASKED THE CTLR: 'IS THERE A CONFLICT?' THE REVERSE VECTORING (R TURN TO L TURN) SEEMED UNUSUAL TO BOTH OF US. APCH CTL NEVER RESPONDED TO THIS QUERY. ESTABLISHED ON THE 090 DEG HDG; WE NOTICED TCASII TFC S OF US AT 4-5 MI; AT OUR ALT. WE DID NOT GET A TA NOR AN RA WARNING ON THE TCASII. WE WERE SOON VECTORED S (180 DEGS) AGAIN ON DOWNWIND FOR 26 MI BEFORE BEING TURNED ON THE ILS TO RWY 36 AND LNDG AT DCA. LATER THAT EVENING AT OUR LAYOVER HOTEL; OUR BASE DUTY PLT CALLED ME AND ASKED IF WE HAD BEEN NOTIFIED OF ANY CONFLICT OR 'NEAR MISS' AT DCA. I REPLIED THAT WE HAD NOT; THEN REMEMBERED THE UNUSUAL VECTORING AND RELATED IT TO HIM. WE WERE; UNTIL THIS TIME; NEVER NOTIFIED OF ANY TFC PROB. DURING THE ENTIRE APCH WE WERE ON RADAR VECTORS GIVEN BY DCA; AND ON ASSIGNED ALT(S). AT NO TIME DID WE RECEIVE A TCASII ADVISORY OR ALERT; AND UNTIL THE DUTY PLT'S PHONE CALL; WERE UNAWARE OF ANYTHING OUT OF THE ORDINARY; SAVE THE UNUSUAL VECTORING.