ACR B190 FLC TAKES EVASIVE ACTION WHEN OBSERVING A MIL ACFT TURN IN CLOSE PROX OF THEIR FLT PATH WHEN BEING VECTORED FOR A VISUAL APCH RWY 1R; IAD. MIL ACFT UNDER CTL OF ANOTHER TRACON SECTOR.

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FLT FROM SWF TO IAD (PAX TRANSPORTATION). DULLES APCH CTLR ASSIGNED US A HDG OF 190 DEGS AND AN ALT OF 4000 FT WHICH BROUGHT US TO A R DOWNWIND FOR RWY 1R. SHE SAID TO EXPECT VISUAL RWY 1R. WE NOTICED TARGET 10 O'CLOCK POS SAME ALT MOVING TOWARDS US ABOUT 3-5 MI; BUT NO VISUAL ON THE PLANE. WITH TARGET CONVERGING TOWARDS US AT 2 MI; PIC WAS ABOUT TO QUERY CTLR. AT THAT MOMENT CTLR YELLED IN A FRENZY 'ACR X; DSND TO 3000 FT IMMEDIATELY' (WITH A LOUD VOICE AND A TONE OF PANIC). AT THAT MOMENT; I CAUGHT A GLIMPSE OF THE CONVERGING PLANE COMING TO US AT A 45 DEG ANGLE TO THE L OF OUR NOSE AND PULLED ON THE YOKE TO PICK UP SOME ALT AND SAW THE PLANE ZOOM UNDERNEATH US AT ABOUT 50 FT BELOW. THE PIC ORDERED ME TO DSND AND PULLED THE PWR TO GO DOWN TO 3000 FT; WHICH I DID WHILE I EXPLAINED TO HIM THAT I SAW THE ACFT AND AVOIDED IT ALREADY AND THAT THERE WAS NO MORE DANGER OF COLLISION. HAD WE DSNDED IMMEDIATELY AS DIRECTED BY THE CTLR; WE WOULD HAVE MOST PROBABLY COLLIDED WITH THE INCOMING PLANE. I WAS FORTUNATE TO HAVE A LAST SECOND VISUAL ON THE INCOMING PLANE AND TOOK WHAT I FELT WAS THE ONLY EVASIVE ACTION POSSIBLE; WHICH WAS TO PULL UP; AS I FELT THE PLANE WAS SLIGHTLY BELOW OUR PATH. WE LANDED WITH NO INCIDENT ON RWY 1R IN IAD AND OBTAINED A NUMBER FROM TWR CTL TO RELATE THE EVENT. WE SUBSEQUENTLY LEARNED THAT THE INCOMING PLANE WAS MIL; UNDER THE CTL OF WASHINGTON NATIONAL ARPT (REAGAN) DEP CTLR. THE PIC FILED A 'NEAR MISS' RPT WITH ATC SUPVR (IAD) AND AN OCCURRENCE RPT WITH OUR COMPANY. IF I HAD TO SPECULATE; I WOULD SAY THAT DUE TO THE HIGH AIR TFC DURING THAT HR AROUND DULLES; APCH CTL MIGHT HAVE FORGOTTEN TO HAND US OVER TO APCH CTL S; WHICH WOULD HAVE BROUGHT US DOWN TO 3000 FT OR LOWER. WE WERE ABEAM THE NUMBERS ON A R DOWNWIND FOR RWY 1R; AND YET WE STILL WERE WITH THE CTLRS HANDLING THE N SECTOR OF DULLES. I STILL CAN'T EXPLAIN WHY TFC FROM WASHINGTON NATIONAL WAS FLYING SO LOW; 4000 FT; OVER A CONGESTED ARPT LIKE DULLES. OUR TCASI 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.