Narrative:

Air carrier X was northbound at assigned altitude of 3000 ft. 2 VFR aircraft nwbound in trail at 2500 ft, and an unknown VFR aircraft sbound at indicated altitude of 3500 ft. Air carrier X was passing 500 ft above, and slightly behind the second nwbound aircraft. Air carrier X started a climb and advised me she was responding to the TCASII. I again verified the altitude of the aircraft at 2500 ft. Air carrier X told me her TCASII showed the aircraft at 2800 ft. I then called the sbound traffic at 12 O'clock, 5 mi, at 3500 ft, and turned air carrier X nwbound. I instructed her to maintain 3000 ft. Air carrier X started a descent and told me she was responding to the TCASII for the 3500 ft traffic. I observed air carrier X descending through 2600 ft toward an area where my MVA is 2400 ft, this area contains a tall antenna. I advised air carrier X about the antenna and again told her to maintain 3000 ft. It is apparently company policy to respond to all TCASII advisories, even when the aircraft are separated from other aircraft by legal separation standards. Also, in this instance, air carrier X's TCASII provided an erroneous altitude report for the VFR aircraft. When air carrier X chose this course of action they not only put another aircraft in danger (climbing toward the sbound aircraft), they put their aircraft and their passenger in danger by descending toward the antenna.

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Original NASA ASRS Text

Title: ACR X TCASII RA DSND NON ADHERENCE TO ATC CLRNC UNAUTH DSCNT FROM ASSIGNED ALT INTO TERRAIN HAD POSSIBLE CONFLICT WITH ACFT Y.

Narrative: ACR X WAS NBOUND AT ASSIGNED ALT OF 3000 FT. 2 VFR ACFT NWBOUND IN TRAIL AT 2500 FT, AND AN UNKNOWN VFR ACFT SBOUND AT INDICATED ALT OF 3500 FT. ACR X WAS PASSING 500 FT ABOVE, AND SLIGHTLY BEHIND THE SECOND NWBOUND ACFT. ACR X STARTED A CLB AND ADVISED ME SHE WAS RESPONDING TO THE TCASII. I AGAIN VERIFIED THE ALT OF THE ACFT AT 2500 FT. ACR X TOLD ME HER TCASII SHOWED THE ACFT AT 2800 FT. I THEN CALLED THE SBOUND TFC AT 12 O'CLOCK, 5 MI, AT 3500 FT, AND TURNED ACR X NWBOUND. I INSTRUCTED HER TO MAINTAIN 3000 FT. ACR X STARTED A DSCNT AND TOLD ME SHE WAS RESPONDING TO THE TCASII FOR THE 3500 FT TFC. I OBSERVED ACR X DSNDING THROUGH 2600 FT TOWARD AN AREA WHERE MY MVA IS 2400 FT, THIS AREA CONTAINS A TALL ANTENNA. I ADVISED ACR X ABOUT THE ANTENNA AND AGAIN TOLD HER TO MAINTAIN 3000 FT. IT IS APPARENTLY COMPANY POLICY TO RESPOND TO ALL TCASII ADVISORIES, EVEN WHEN THE ACFT ARE SEPARATED FROM OTHER ACFT BY LEGAL SEPARATION STANDARDS. ALSO, IN THIS INSTANCE, ACR X'S TCASII PROVIDED AN ERRONEOUS ALT RPT FOR THE VFR ACFT. WHEN ACR X CHOSE THIS COURSE OF ACTION THEY NOT ONLY PUT ANOTHER ACFT IN DANGER (CLBING TOWARD THE SBOUND ACFT), THEY PUT THEIR ACFT AND THEIR PAX IN DANGER BY DSNDING TOWARD THE ANTENNA.

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.