ACR X TCASII TA RA CLB NON ADHERENCE TO ATC CLRNC HAD AIRBORNE CONFLICT WITH VFR ACFT Y. PLTDEV. EVASIVE ACTION CLB TAKEN

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ACR X FLYING ON A 330 DEG ASSIGNED HDG TO DOWNWIND AT 7000 FT MSL FOR RWY 20R AT BNA. MY ACFT RECEIVED A TCASII TA WHICH SHOWED AN ACFT AT OUR 0130 O'CLOCK AND LESS THAN 5 MI. EARLY MORNING HAZINESS MADE IT DIFFICULT TO VISUALLY ACQUIRE THE ACFT. SECONDS LATER; THE TCASII RA SOUNDED 'CLB; CLB!' I IMMEDIATELY CLBED AT 2000 FPM TO SATISFY THE INDICATED CLB COMMAND OF 1500 FPM. AS I COMMENCED THE CLB I SAW THE TARGET VISUALLY AT 2 O'CLOCK AND 200 FT BELOW MY ALT (CORROBORATED BY THE RA SYMBOL MINUS 200 ON TCASII). I CLBED TO 7600 FT MSL BEFORE BEGINNING DSCNT BACK TO 7000 FT MSL WHEN CLR OF CONFLICT SIGNAL FROM TCASII SOUNDED. I BELIEVE THE TARGET ACFT Y PASSED OFF MY R BY 1 1/2 MI. AFTER LNDG; THE CTLR SUPVR CALLED ME TO EXPLAIN THAT THERE NEVER WAS A LOSS OF SEPARATION DURING THE EFFECTIVE ENCOUNTER. HE STATED THAT THE ACFT Y WAS A VFR SMA AT 6500 FT MSL HDG SW. I TOLD HIM THAT I FOUND IT HARD TO BELIEVE THE TARGET ACFT'S ALT SINCE THE TCASII SHOWED MINUS 200 FT AS I DEPARTED 7000 FT MSL. THE CTLR ALSO EXPLAINED THAT THE COMPUTER ASSIGNED ALTS WERE A PART OF THE TCASII EVENTS FROM HIS POINT OF VIEW. THE TCASII RA I RECEIVED WAS VALID AND HELPED ME AVOID A POSSIBLE COLLISION. THE TCASII UNIT WORKED EXACTLY AS DESIGNED!