TCASII RA FOR LGT FLT IN IMC. CAPT OBSERVED TFC. UNABLE TO CONFIRM WITH APCH CTLR ACCOUNT FREQ CONGESTION. FOLLOWED TA TO DSND. MISSED TFC BY 600. IAD APCH INVESTIGATING THE CONFLICT.

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ON IAD APCH CTL AT 11000 FT DEVIATING R OF COURSE FOR WX WITH APCH CTL CONCURRENCE. WX RADAR ON; 40 NM RANGE; TCASII ON TA/RA. SAW TCASII TARGET AT 10 MI; 1100 FT BELOW; CLBING. SWITCHED TO 10 NM RANGE; TARGET 7 MI 900 FT BELOW; CLBING. TOLD FO TO ASK APCH CTL ABOUT TARGET. FO UNABLE TO GET THROUGH BECAUSE OF OTHER RADIO TFC. I FELT A DANGEROUS SITUATION WAS DEVELOPING AS TARGET WAS AT 5 MI; 700 FT BELOW; CLBING. TARGET AT ALL TIMES WAS AS 12 O'CLOCK. WE STARTED 30-40 DEG L TURN; LEVEL; TOLD FO TO TELL APCH CTL. APCH SAYS; 'I'VE GOT AN SMT IN THAT AREA; NOT TALKING TO HIM'. SMT PLT CHKED ON 'AT 11000 FT'. TCASII GAVE 'TFC' MESSAGE; IMMEDIATELY FOLLOWED BY 'DSND; DSND'. VERT SPD INDICATOR ARCS WERE ALL RED EXCEPT APPROX 1600-2000 FPM DOWN. PUT NOSE DOWN TO GET VERT SPD INDICATOR IN GREEN. FO SAYS 'DEVIATING ACCOUNT TFC'. TCASII SAYS 'CLR OF TFC'. LEVELED OFF NEAR 1000 FT; 300 IAS. I THEN TOLD APCH THAT I HAD EXERCISED MY EMER AUTHORITY BECAUSE OF A TCASII RA ALERT. APCH'S ONLY RESPONSE WAS 'CLRED TO 5000 FT; DIRECT DCA'. MADE LDA DME APCH AND LANDED AT DCA. IAD IS INVESTIGATING. THEY SAY CLOSEST POINT WAS 600 FT VERT; 2.64 NM HORIZ. YES; BUT THAT IS BECAUSE OF MY TURN AND TCASII PROMPTED DSCNT. I BELIEVE WITHOUT TCASII AND ABSENT ANY WARNING FROM APCH (AND THERE WAS NONE) THE VERT CLRNC WOULD HAVE BEEN 0 AND LATERAL VERY LITTLE OR NONE. I STRONGLY SUGGEST PROJECTION OF RADAR TRACKS WITH NO TURNS TO ESTIMATE TRUE GRAVITY OF SITUATION AND DISSEMINATION OF THIS INFO TO AIR CARRIER CREWS TO ENHANCE CREDIBILITY OF TCASII; CURRENTLY NOT HIGH BECAUSE OF BREAK-IN PROBLEMS.