CL65 FLC INITIATE EVASIVE MANEUVER FROM CPR JET UNDER IAD CTL AS ALT IS QUESTIONED.

Narrative

WHILE ON THE JASEN 2 ARR TO IAD; ON A FLT FROM BNA TO IAD; WE CONTACTED DULLES APCH AND WERE GIVEN A 040 DEG HDG AND A DSCNT TO 8000 FT. UPON PASSING ABEAM THE LINDEN VOR; WE WERE GIVEN A HDG TO REINTERCEPT THE STAR AND TOLD TO DSND TO 5000 FT. WHILE AT 5000 FT AND PASSING OVER THE JASEN INTXN; BOTH THE CAPT AND I NOTICED AN ACFT ON OUR MFD (TCASII) SCREEN ALSO AT 5000 FT AND AT A DISTANCE OF 10 NM. WE QUERIED EACH OTHER THAT FIRST IT WAS ODD TO HAVE AN ACFT AT OUR ALT SO CLOSE AND THAT IF THINGS CONTINUED; WE WERE GOING TO GET A TA FOLLOWED BY AN RA. AT A DISTANCE OF ABOUT 5 NM; WE DID GET A TA FOLLOWED BY AN IMMEDIATE RA AND A CALL FROM DULLES APCH TELLING US TO MAINTAIN 6000 FT. WE TOLD HIM THAT WE WERE AT 5000 FT IN A CLB TO 6000 FT PER OUR RA AND DULLES APCH ALSO ISSUED US AN IMMEDIATE L TURN TO HDG 360 DEGS. FOLLOWING THE CONFLICT RESOLUTION; WE WERE GIVEN A HDG TO PROCEED DIRECT GILBY AND THE REMAINDER OF THE ARR. NO FURTHER INCIDENTS FOLLOWED. BOTH THE CAPT AND I WERE ON THE FREQ AT THE TIME THE DSCNT TO 5000 FT WAS ISSUED. WE SHOULD HAVE BEEN MORE SITUATIONALLY AWARE SINCE NORMALLY DULLES WOULD STOP US AT 6000 FT FOR DEPS OFF OF RWY 30. BOTH OF US READ BACK 5000 FT TO EACH OTHER AS I SET THE ALT IN THE PRESELECT WINDOW. WE ALSO SHOULD HAVE BEEN MORE ASSERTIVE IN QUERYING ATC ABOUT THE ACFT AT OUR ALT. TCASII AND VMC WERE INSTRUMENTAL IN THE SUCCESSFUL OUTCOME OF THIS EVENT. BASED ON THE TCASII PRESENTATION; WE WERE ABLE TO VISUALLY ACQUIRE THE ACFT AND MAKE THE NECESSARY CTL INPUTS TO AVOID THE FALCON. WE SHOULD HAVE QUERIED ATC AND MADE THOSE INPUTS SOONER WHEN THE CONFLICT WAS REALIZED.