Narrative:

We were descending to an assigned altitude of 6000'. At 6200' TCAS issued a resolution advisory to descend at 1500 FPM. We were at 1300 FPM already so we continued the descent below 6000' assigned altitude. The intruder aircraft that TCAS issued the resolution advisory for was not in sight TCA's was tracking 2 aircraft at the time, one at 1 O'clock and one at 11 O'clock. As we continued the descent, at 5800' TCA's went invalid, so we climbed back to 6000'. Dtw approach collision alert went off due to the traffic below us. The traffic below us was the 11 O'clock traffic TCA's was looking at. We are taught to follow a TCAS resolution advisory if the traffic is not in sight, which we did. TCAS, I believe, issued a resolution advisory for the 1 O'clock traffic and descended us into the 11 O'clock traffic. Both traffic aircraft were mode C equipped. Bad logic by TCAS. Supplemental information from acn 104603: dtw approach advised that their information showed a C/a and we were descending into conflicting traffic--just opposite of what the TCAS had issued. Appropriate TCAS evaluation reports were submitted to the company. My experience with TCAS is limited, but I felt it contributed considerable to cockpit workload in this particular instance.

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION OVERSHOT WHILE COMPLYING WITH TCAS RESOLUTION ADVISORY.

Narrative: WE WERE DSNDING TO AN ASSIGNED ALT OF 6000'. AT 6200' TCAS ISSUED A RESOLUTION ADVISORY TO DSND AT 1500 FPM. WE WERE AT 1300 FPM ALREADY SO WE CONTINUED THE DSCNT BELOW 6000' ASSIGNED ALT. THE INTRUDER ACFT THAT TCAS ISSUED THE RESOLUTION ADVISORY FOR WAS NOT IN SIGHT TCA'S WAS TRACKING 2 ACFT AT THE TIME, ONE AT 1 O'CLOCK AND ONE AT 11 O'CLOCK. AS WE CONTINUED THE DSCNT, AT 5800' TCA'S WENT INVALID, SO WE CLBED BACK TO 6000'. DTW APCH COLLISION ALERT WENT OFF DUE TO THE TFC BELOW US. THE TFC BELOW US WAS THE 11 O'CLOCK TFC TCA'S WAS LOOKING AT. WE ARE TAUGHT TO FOLLOW A TCAS RESOLUTION ADVISORY IF THE TFC IS NOT IN SIGHT, WHICH WE DID. TCAS, I BELIEVE, ISSUED A RESOLUTION ADVISORY FOR THE 1 O'CLOCK TFC AND DSNDED US INTO THE 11 O'CLOCK TFC. BOTH TFC ACFT WERE MODE C EQUIPPED. BAD LOGIC BY TCAS. SUPPLEMENTAL INFO FROM ACN 104603: DTW APCH ADVISED THAT THEIR INFO SHOWED A C/A AND WE WERE DSNDING INTO CONFLICTING TFC--JUST OPPOSITE OF WHAT THE TCAS HAD ISSUED. APPROPRIATE TCAS EVALUATION RPTS WERE SUBMITTED TO THE COMPANY. MY EXPERIENCE WITH TCAS IS LIMITED, BUT I FELT IT CONTRIBUTED CONSIDERABLE TO COCKPIT WORKLOAD IN THIS PARTICULAR INSTANCE.

Data retrieved from NASA's ASRS site as of August 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.