Narrative:

Flight from swf to iad, with approach controller at dulles assigned 4000 ft altitude, 190 degree heading expecting visual for runway 1R. Aircraft position: downwind abeam airport right downwind airspeed approximately 180 KT, 1-1 1/2 mi from runway. Noticed target on TCASII same altitude at 10 O'clock position approximately 3-5 mi, but had no visual on target. With target converging at approximately 2 mi (still no visual) was preparing to query approach about traffic. At that time approach controller yelled, 'descend to 3000 ft immediately' (repeatedly with a loud screaming voice). I immediately reduced power, and told the first officer to 'descend now.' by the time all of this took place traffic was passing underneath us less than 50 ft and it was too late. If I may point out, the controller directed us to descend to 3000 ft MSL which would have caused a serious accident if we had responded within the approximately 3 second window of everything that took place which was impossible. We landed with no incident runway 1R (iad) and tower gave us telephone number for an explanation of the event. Tower told us that the conflicting traffic was military under the control of reagan national departure control. I filed a 'near miss' report with ATC supervisor (iad) over the telephone. Callback conversation with reporter revealed the following information: the reporter states that the other aircraft was a military 2 engine turboprop, and was under control of reagan national departure control. He did not see the aircraft, but the first officer did. Reporter states that they did not file a written near midair collision report. Supplemental information from acn 427126: our TCASII system does not provide evasive maneuver directives. Our approach controller was very busy, and it was difficult to 'place a word' to query about incoming traffic/switch over to next controller. A time window of 3 seconds or less or so occurred between the traffic alert, the controller's directive and my evasive maneuver as I saw the plane. We had no time at all.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.