OPPOSITE DIRECTION TFC. A VISUAL DSCNT THROUGH OCCUPIED ALT IS GIVEN BY IAD TRACON. A NEAR POTENTIAL CONFLICT IS SENSED BY TCASII RA. PIC OF ACFT X FAILS TO HEED THE RA COMMAND AND CONTINUES DSCNT USING VISUAL FLC JUDGEMENT OF TFC SPACING. PIC ACFT X CITES LACK OF SITUATIONAL AWARENESS RELATED TO SPECIFICS OF DSCNT CLRNC.

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ATC FACILITY: DULLES TRACON. LOCATION: 20 NM W AML VORTAC/IAD. I WAS PF AND MAKING A FINAL PA TO PAX WHEN FO RECEIVED QUERY AND CLRNC TO MAINTAIN VISUAL SEPARATION FROM A B757 AT 14000 FT MSL. WE WERE AT 15000 FT MSL. HE STATED TFC IN SIGHT AND WE WERE GIVEN CLRNC TO EXPEDITE DSCNT THROUGH HIS ALT DOWN TO 12000 FT. I GOT OFF THE PA; HAVING HEARD A DSCNT CLRNC; BUT I DID NOT KNOW THE SPECIFICS. FO ADVISED WE NEEDED TO HURRY DOWN SO I STARTED A SMOOTH BUT AGGRESSIVE DSCNT TO GET THROUGH THE TFC ALT. WELL INTO THE DSCNT AND AT LEAST 4000 FPM; GOT A TCASII ADVISORY TO CLB. TFC AT THAT POINT WAS IN SIGHT AND IN MY JUDGEMENT IT WAS A FASTER AND SAFER MANEUVER TO CONTINUE THE AGGRESSIVE DSCNT THROUGH HIS ALT AND OUT OF THE WAY. APPARENTLY; THE TCASII COMMANDED HIM TO DSND ALSO WHEN I DID NOT FOLLOW MY UNIT'S ADVISORY AND THAT COMPLICATED THE EVASION. I CONTINUED DOWN TO ASSIGNED 12000 FT ALT QUICKLY AND SMOOTHLY WITHOUT INCIDENT. CLOSEST POINT OF APCH PROBABLY 4 MI BUT AT COMMON ALT NO CLOSER THAN 6 MI. LESSONS LEARNED ARE: CONSIDERATION ON MY PART TO MAKING MY PERSONAL STERILE PERIOD FL180 AND BELOW IN BUSY TERMINAL AREAS. LESSON #2: BOTH PLTS SHOULD ALWAYS COORDINATE AND AGREE BEFORE ACCEPTING VISUAL SEPARATION CLRNC. LESSON #3 AND MOST IMPORTANT --IFR FLT PLAN. LET ATC CTLRS SEPARATE; LET PLTS FLY. WE ALL HAVE BEST INTENTIONS TO HELP THE SYS WORK; BUT IF OFTENTIMES WINDS UP HELPING NO ONE -- CTLR OR PLT -- TO CUT CORNERS ON THE SYS. SUPPLEMENTAL INFO FROM ACN 353243: WE WERE AT 15000 FT. TFC WAS AT 14000 FT. APCH CALLED TFC AT 2 O'CLOCK AND 20 MI.