SYS ERROR SUGGESTED IN THIS NMAC RPT.

Narrative

OUR FLT DEPARTED DCA. AFTER CONTACTING DEP CTL (DULLES DEP); WE WERE GIVEN A SERIES OF HDG; AIRSPD; AND ALT CHANGES. OTHER ACFT WHICH DEPARTED DCA WERE ALSO SUBJECT TO THIS MANEUVERING. IT WAS APPARENT THAT DEP CTL WAS DEALING WITH A LARGE NUMBER OF ACFT. DEPARTING ACFT WERE APPARENTLY BEING VECTORED IN-TRAIL THROUGH A SERIES OF CLBING RIGHT HAND TURNS TO FACILITATE AIRWAYS SEQUENCING. AT THE TIME OF THE INCIDENT; OUR ACFT WAS IN IMC HDG 300 DEGS AT 13000 FT. BOTH THE COPLT AND I SAW AN AMBER TARGET AT THE 3 O'CLOCK POS OF OUR TCASII INDICATORS. ALMOST IMMEDIATELY; WE HEARD THE AURAL WARNING 'DSND NOW!' THE TCASII TARGET WAS AT OUR ALT AND APCHING OUR POS! THE COPLT; WHO HAD BEEN FLYING ON THE AUTOPLT; DISCONNECTED THE AUTOPLT; SMOOTHLY LOWERED THE NOSE OF THE ACFT AND BEGAN AN IMMEDIATE DSCNT. I CALLED DULLES DEP AND TOLD THEM WE WERE LEAVING OUR ASSIGNED ALT IN RESPONSE TO AN ACTUAL RA ON TCASII. WE WERE GIVEN AN IMMEDIATE CLRNC TO 12000 FT. WHILE DSNDING THROUGH ABOUT 12500 FT IN BROKEN CLOUDS; WE CAUGHT A GLIMPSE OF WHAT APPEARED TO BE A B-727 XING ABOVE US. IT COULD NOT BE DETERMINED IF THE ACFT WAS CLBING. THE ENTIRE EVENT CONSUMED NO MORE THAN 15 OR 20 SECONDS. IT APPEARS THE PROB AROSE DUE TO A SATURATION OF THE AIRSPACE AND CTLING CAPABILITY OF THE ATC AGENCY. CONTRIBUTING FACTORS MUST BE THE LARGE NUMBERS OF ACFT TRANSITING THIS AIRSPACE. THE PROB WAS DISCOVERED UTILIZING THE ON-BOARD TCASII. CORRECTIVE ACTIONS WERE MERELY THOSE PRESCRIBED BY THE TFC RA MODE OF TCASII. I AM OF THE OPINION THAT THE COPLT'S ACTIONS OF IMMEDIATELY DISCONNECTING THE AUTOPLT AND POSITIVELY CHANGING ALT RESULTED IN THE ADEQUATE AMOUNT OF ALT SEPARATION WHICH WE ACHIEVED. THE VISUAL SIGHTING OF THE TARGET ACFT CERTAINLY GAVE POS REINFORCEMENT OF THE EFFICIENCY OF OUR ON-BOARD TCASII SYS. UNFORTUNATELY; THE INCIDENT ALSO HIGHLIGHTS THE DEFICIENCIES AND INADEQUACIES INHERENT IN THE ATC RADAR ENVIRONMENT UNDER THE PRESSURES OF HIGH TFC VOLUME AND CTLR WORKLOAD.