IFR ACR EXPERIENCED TCAS RA CONFLICT EVENT AT 6000 INBOUND TO IAD; LISTED BY FAA AS OPERROR.

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CHKED IN WITH SECOND POTOMAC APCH FREQ 125.8 AT 6000 FT. POTOMAC ACKNOWLEDGED WITH 'ROGER.' ABOUT 1 MIN LATER; WE GOT A 'TFC' FROM THE TCAS. TFC WAS AT 1 O'CLOCK POS; 1200 FT ABOVE AND DSNDING. WE WATCHED THE TFC ON THE TCAS DISPLAY DSND THROUGH 1000 FT RELATIVE ALT WITH A CONSTANT BEARING. WE WERE IMC IN CLOUDS. WITHIN A FEW SECONDS; THE TA BECAME AN RA WITH 'MONITOR VERT SPD' COMMAND AND VERT SPD INDICATOR SHOWING RED FROM 0 UP TO 2000 FPM CLB. WE WATCHED THE RELATIVE ALT DECREASE TO 300 FT AND THE TARGET MERGED WITH OUR ACFT SYMBOL. THE TCAS THEN COMMANDED 'DSND' WITH GREEN VERT SPD AT 1500 FPM TO 2500 FPM. DURING THE ESCAPE MANEUVER; POTOMAC CALLED AND CLRED US TO 3000 FT. I ADVISED HIM THAT WE HAD AN RA AND WERE ALREADY IN A DSCNT. I THINK THE CTLR MIGHT HAVE BEEN OVERLOADED. USUALLY WHEN CHKING IN AT 6000 FT YOU ARE ASKED TO DSND TO 5000 FT OR 4000 FT. THERE WERE TSTMS THROUGHOUT THE AREA ALL DAY. I CAN'T REMEMBER IF IT WAS THIS CTLR OR THE PREVIOUS ONE; BUT HE WAS DISTR WITH A GA PLT TRYING TO GET INTO A LCL ARPT. THEY WERE DISCUSSING HOLDING OR TRYING AN APCH.