ACR MLG; ON VISUAL APCH; CONFLICTED WITH UNKNOWN; UNREPORTED; ACFT ON FINAL APCH COURSE TO RWY 12.

Narrative

WE DSNDED UNDER IFR TOWARD DULLES RWY 12 AND CALLED THE RWY IN SIGHT ABOUT 12-14 DME FROM THE ARMEL VOR AT 5000-6000' MSL. WE WERE CLRED FOR THE VISUAL APCH; REQUESTED AND RECEIVED APPROVAL FOR S-TURNS TO LOSE ALT. NO RESTRICTIONS OR TFC WAS GIVEN IN REGARD TO THE S-TURNS. ON A SOUTHERLY HDG AT ABOUT 3800' MSL; APCH GAVE US AN IMMEDIATE TURN TO 090 DEGS AND IMMEDIATE DSCNT TO 3000' RE: TFC AT OUR 1 O'CLOCK POS AT 4000'. WE CALLED THE TFC IN SIGHT; BUT WERE UNABLE TO ACCURATELY DETERMINE HIS HDG OR SPD IN RELATION TO OURS ON ACCOUNT OF THE DARKNESS. IN OUR TURN BACK TO THE ARPT; WE HAD LESS THAN 1 MI SEP ACCORDING TO APCH. AS FAR AS WE KNEW; WE WERE THE ONLY AIRPLANE IN THE AREA. APCH SHOULD HAVE ISSUED TFC OR APPROVED TURNS TO THE NE SIDE OF FINAL APCH COURSE IF HE WANTED TO KEEP US AWAY FROM THE OTHER AIRPLANE. I UNDERSTAND THAT THE CTLR WORKLOAD IS HIGH; BUT IF I AS A PLT COULD BE AWARE OF A LARGER PICTURE THAN MY OWN COCKPIT; I COULD PROBABLY HELP THE SITUATION WITH LESS COM INSTEAD OF FEELING LIKE I'M BEING LED BLINDLY AROUND; NOT KNOWING THE CTLR'S PLAN OF ATTACK.