A B777-200 CREW ON DSCNT INTO IAD TURNED OFF COURSE IN RESPONSE TO A TCAS TA TO AVOID CONFLICT WITH VFR TFC OPERATING IN IMC.

Narrative

CLRED TO DSND TO AND CROSS DRUZZ AT 11000 FT; 250 KTS. DSNDING THROUGH 13000 FT ABOUT 10 MI W OF DRUZZ; WE SAW TCAS TFC JUST E OF DRUZZ CLBING THROUGH 8000 FT. WE QUERIED ZDC AND HE REPLIED IT WAS VFR SQUAWKING 1200. WE THOUGHT THAT STRANGE SINCE WE HAD BEEN IN IMC SINCE 16000 FT. THE TFC SEEMED TO LEVEL AT 9500 FT WHICH WOULD HAVE BEEN OK SINCE WE WERE TO LEVEL AT 11000 FT. AT ABOUT 7 MI FROM US JUST AS WE LEVELED AT 11000 FT; HE STARTED TO CLB OUT OF 9500 FT. WE IMMEDIATELY TURNED 40 DEGS TO THE R TO AVOID A CONFLICT. WE GOT A TCAS TA BUT NOT AN RA. THIS TURN WAS SELF-INITIATED ALTHOUGH WE ADVISED ATC. THE CTLR ADVISED CLR OF TFC AND SAID HE PASSED 1 1/2 MI TO OUR L AND CONTINUED CLB TO 11500 FT. IT WAS TOTAL IMC FROM 16000 FT TO BELOW 5000 FT. BOTH ZDC AND POTOMAC APCH SAID THE ACFT WAS IN CONTACT WITH NO ONE AND DIDN'T KNOW WHO IT WAS OR WHERE HE HAD DEPARTED FROM. THIS WAS POTENTIALLY A VERY UNSAFE SITUATION CREATED BY A FLAGRANT VIOLATION OF MANY FARS ON THE PART OF THIS PLT FLYING IN IMC IN A VERY BUSY CORRIDOR.