Narrative:

Air carrier X on base leg about 1 NM from final approach course TCAS avoidance alert occurred for conflicting air carrier Y 200 ft above and about 1.5 NM horizontal. Command was 1000 FPM down (heading 235 degree and about 2 mi outside marker). Descent to 3500 MSL removed conflict and ATC advised. ATC said air carrier Y was no factor. After intercepting final approach course we were told to slow to final approach speed. Air carrier Y was now 1.5 NM ahead on TCAS scope. Tower asked us to land on runway 28L. Runway change executed at 1000 ft or less. Under positive control and being vectored 'in close', it is difficult to force oneself to leave altitude because 'TCAS' says so. There are seemingly so many parameters under which TCAS gives a false call I am very skeptical about it. I do use it for 'radar advisory' and think it is a good tool. I did follow its command (with hesitation) and descended 500 to stop TCAS call. Our sep from previous traffic was obviously too close, as our final separation on final TCAS showed less than 2 mi. If we had not had TCAS, we would have been completely oblivious to any problem until in final.

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Original NASA ASRS Text

Title: ACR X HAD LTSS FROM ACR Y. SYS ERROR. TCASII RA EVASIVE ACTION TAKEN. NONADHERENCE TO ATC CLRNC. UNAUTH DSCNT FROM ASSIGNED ALT.

Narrative: ACR X ON BASE LEG ABOUT 1 NM FROM FINAL APCH COURSE TCAS AVOIDANCE ALERT OCCURRED FOR CONFLICTING ACR Y 200 FT ABOVE AND ABOUT 1.5 NM HORIZ. COMMAND WAS 1000 FPM DOWN (HDG 235 DEG AND ABOUT 2 MI OUTSIDE MARKER). DSCNT TO 3500 MSL REMOVED CONFLICT AND ATC ADVISED. ATC SAID ACR Y WAS NO FACTOR. AFTER INTERCEPTING FINAL APCH COURSE WE WERE TOLD TO SLOW TO FINAL APCH SPD. ACR Y WAS NOW 1.5 NM AHEAD ON TCAS SCOPE. TWR ASKED US TO LAND ON RWY 28L. RWY CHANGE EXECUTED AT 1000 FT OR LESS. UNDER POSITIVE CTL AND BEING VECTORED 'IN CLOSE', IT IS DIFFICULT TO FORCE ONESELF TO LEAVE ALT BECAUSE 'TCAS' SAYS SO. THERE ARE SEEMINGLY SO MANY PARAMETERS UNDER WHICH TCAS GIVES A FALSE CALL I AM VERY SKEPTICAL ABOUT IT. I DO USE IT FOR 'RADAR ADVISORY' AND THINK IT IS A GOOD TOOL. I DID FOLLOW ITS COMMAND (WITH HESITATION) AND DSNDED 500 TO STOP TCAS CALL. OUR SEP FROM PREVIOUS TFC WAS OBVIOUSLY TOO CLOSE, AS OUR FINAL SEPARATION ON FINAL TCAS SHOWED LESS THAN 2 MI. IF WE HAD NOT HAD TCAS, WE WOULD HAVE BEEN COMPLETELY OBLIVIOUS TO ANY PROBLEM UNTIL IN FINAL.

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.