BE02 CREW TAKES EVASIVE ACTION TO AVOID CLOSE TFC IN IAD AIRSPACE.

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FLT FROM SWF TO IAD; WITH APCH CTLR AT DULLES ASSIGNED 4000 FT ALT; 190 DEG HDG EXPECTING VISUAL FOR RWY 1R. ACFT POS: DOWNWIND ABEAM ARPT R DOWNWIND AIRSPD APPROX 180 KT; 1-1 1/2 MI FROM RWY. NOTICED TARGET ON TCASII SAME ALT AT 10 O'CLOCK POS APPROX 3-5 MI; BUT HAD NO VISUAL ON TARGET. WITH TARGET CONVERGING AT APPROX 2 MI (STILL NO VISUAL) WAS PREPARING TO QUERY APCH ABOUT TFC. AT THAT TIME APCH CTLR YELLED; 'DSND TO 3000 FT IMMEDIATELY' (REPEATEDLY WITH A LOUD SCREAMING VOICE). I IMMEDIATELY REDUCED PWR; AND TOLD THE FO TO 'DSND NOW.' BY THE TIME ALL OF THIS TOOK PLACE TFC WAS PASSING UNDERNEATH US LESS THAN 50 FT AND IT WAS TOO LATE. IF I MAY POINT OUT; THE CTLR DIRECTED US TO DSND TO 3000 FT MSL WHICH WOULD HAVE CAUSED A SERIOUS ACCIDENT IF WE HAD RESPONDED WITHIN THE APPROX 3 SECOND WINDOW OF EVERYTHING THAT TOOK PLACE WHICH WAS IMPOSSIBLE. WE LANDED WITH NO INCIDENT RWY 1R (IAD) AND TWR GAVE US TELEPHONE NUMBER FOR AN EXPLANATION OF THE EVENT. TWR TOLD US THAT THE CONFLICTING TFC WAS MIL UNDER THE CTL OF REAGAN NATIONAL DEP CTL. I FILED A 'NEAR MISS' RPT WITH ATC SUPVR (IAD) OVER THE TELEPHONE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATES THAT THE OTHER ACFT WAS A MIL 2 ENG TURBOPROP; AND WAS UNDER CTL OF REAGAN NATIONAL DEP CTL. HE DID NOT SEE THE ACFT; BUT THE FO DID. RPTR STATES THAT THEY DID NOT FILE A WRITTEN NMAC RPT. SUPPLEMENTAL INFO FROM ACN 427126: OUR TCASII SYS DOES NOT PROVIDE EVASIVE MANEUVER DIRECTIVES. OUR APCH CTLR WAS VERY BUSY; AND IT WAS DIFFICULT TO 'PLACE A WORD' TO QUERY ABOUT INCOMING TFC/SWITCH OVER TO NEXT CTLR. A TIME WINDOW OF 3 SECONDS OR LESS OR SO OCCURRED BTWN THE TFC ALERT; THE CTLR'S DIRECTIVE AND MY EVASIVE MANEUVER AS I SAW THE PLANE. WE HAD NO TIME AT ALL.