Narrative:

Air carrier X was on a STAR from scy at 11000 ft. He was approximately 3 mi from the STAR termination fix at 11000. Small transport Y was VFR overflt at 11500 (radar showed 11600). Traffic was issued to air carrier X twice but he could not see small transport Y. Traffic was issued to small transport Y and he saw and was advised to 'maintain visual separation' from air carrier X. When the 2 aircraft crossed, air carrier X advised that he was descending for a mandatory TCASII resolution. He descended approximately 500 ft, then later returned. When questioned, air carrier X advised that the TCASII showed the VFR aircraft 500 ft above him and TCASII resolution advised to descend (it was mandatory for him). Problem area: had there been an IFR departure under air carrier X at 10000 ft (which is standard for departures) and air carrier X descended even 200 ft, then there would have been an IFR separation error to miss a VFR. 500 ft is legal and standard separation between a VFR and IFR (except heavy jets) within a TCA. This is the norm and not the exception in an approach control 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 approach airspace or maybe even disregard TCASII resolutions within the terminal airspace. I believe the TCASII is a great 'aid' for the pilots but should not be used for separation (pilots) or second-guessing a controller.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.