ACR X NON ADHERENCE TO ATC CLRNC TCASII RA DSCNT FROM ASSIGNED ALT EVASIVE ACTION TAKEN FROM SMT Y. PLTDEV.

Narrative

ACR X WAS ON A STAR FROM SCY AT 11000 FT. HE WAS APPROX 3 MI FROM THE STAR TERMINATION FIX AT 11000. SMT Y WAS VFR OVERFLT AT 11500 (RADAR SHOWED 11600). TFC WAS ISSUED TO ACR X TWICE BUT HE COULD NOT SEE SMT Y. TFC WAS ISSUED TO SMT Y AND HE SAW AND WAS ADVISED TO 'MAINTAIN VISUAL SEPARATION' FROM ACR X. WHEN THE 2 ACFT CROSSED; ACR X ADVISED THAT HE WAS DSNDING FOR A MANDATORY TCASII RESOLUTION. HE DSNDED APPROX 500 FT; THEN LATER RETURNED. WHEN QUESTIONED; ACR X ADVISED THAT THE TCASII SHOWED THE VFR ACFT 500 FT ABOVE HIM AND TCASII RESOLUTION ADVISED TO DSND (IT WAS MANDATORY FOR HIM). PROBLEM AREA: HAD THERE BEEN AN IFR DEP UNDER ACR X AT 10000 FT (WHICH IS STANDARD FOR DEPS) AND ACR X DSNDED EVEN 200 FT; THEN THERE WOULD HAVE BEEN AN IFR SEPARATION ERROR TO MISS A VFR. 500 FT IS LEGAL AND STANDARD SEPARATION BTWN A VFR AND IFR (EXCEPT HVY JETS) WITHIN A TCA. THIS IS THE NORM AND NOT THE EXCEPTION IN AN APCH CTL AREA. SINCE AIRSPACE IS LIMITED WITHIN THE TERMINAL AREA; THIS IS USED A LOT. MAYBE WE NEED TO LOOK AT THE TCASII RESOLUTION PARAMETERS AND CHANGE THEM TO 400 FT WITHIN THE APCH AIRSPACE OR MAYBE EVEN DISREGARD TCASII RESOLUTIONS WITHIN THE TERMINAL AIRSPACE. I BELIEVE THE TCASII IS A GREAT 'AID' FOR THE PLTS BUT SHOULD NOT BE USED FOR SEPARATION (PLTS) OR SECOND-GUESSING A CTLR.