Narrative:

We were level at 9000 ft when we received a TCAS 'traffic! Traffic!' alert. I looked at the TCAS display; and the conflict was indicated at our 9-10 O'clock position; same altitude; 2-3 mi away. It was displayed in yellow. As we directed our attention outside to locate the traffic; approximately 3-5 seconds after the initial traffic alert; we received a TCAS RA; 'descend! Descend!' the captain disconnected the autoplt and began an immediate descent per the TCAS instructions. I notified houston approach that we were descending per a TCAS RA. Houston approach instructed us to stop our descent by 8000 ft; as we had traffic below us. Per our manual and training; we continued to follow the TCAS instructions; and I informed houston approach that we would continue our descent per the TCAS RA. The RA topped approximately 10 seconds after the 'descend! Descend!' command; and the captain topped the descent and returned to 9000 ft. We had descended to approximately 8500 ft at the lowest point. I informed houston approach that the RA was resolved and we were returning to 9000 ft. I then looked again at the TCAS display; and noticed 1 target directly below us; indicating 2000 ft below; tracking the same direction and in the expected lateral position of the previous conflicting target. Neither the captain nor I visually saw the other aircraft. I did not see any other targets on our TCAS display that could have potentially been a conflict. I don't have enough knowledge of how the TCAS system works to say exactly why we received the descend command; but it appeared to me that the traffic was displayed to us at the same altitude in error. The traffic that caused the conflict; according to the situation I gathered from houston approach; was below us by at least 2000 ft when the RA occurred. This coincides with the situation we saw on the TCAS display after the RA.

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Original NASA ASRS Text

Title: SF34 RESPONDS TO TCAS RA. FLT CREW FEELS THE ADVISORY WAS ERRONEOUS.

Narrative: WE WERE LEVEL AT 9000 FT WHEN WE RECEIVED A TCAS 'TFC! TFC!' ALERT. I LOOKED AT THE TCAS DISPLAY; AND THE CONFLICT WAS INDICATED AT OUR 9-10 O'CLOCK POS; SAME ALT; 2-3 MI AWAY. IT WAS DISPLAYED IN YELLOW. AS WE DIRECTED OUR ATTN OUTSIDE TO LOCATE THE TFC; APPROX 3-5 SECONDS AFTER THE INITIAL TFC ALERT; WE RECEIVED A TCAS RA; 'DSND! DSND!' THE CAPT DISCONNECTED THE AUTOPLT AND BEGAN AN IMMEDIATE DSCNT PER THE TCAS INSTRUCTIONS. I NOTIFIED HOUSTON APCH THAT WE WERE DSNDING PER A TCAS RA. HOUSTON APCH INSTRUCTED US TO STOP OUR DSCNT BY 8000 FT; AS WE HAD TFC BELOW US. PER OUR MANUAL AND TRAINING; WE CONTINUED TO FOLLOW THE TCAS INSTRUCTIONS; AND I INFORMED HOUSTON APCH THAT WE WOULD CONTINUE OUR DSCNT PER THE TCAS RA. THE RA TOPPED APPROX 10 SECONDS AFTER THE 'DSND! DSND!' COMMAND; AND THE CAPT TOPPED THE DSCNT AND RETURNED TO 9000 FT. WE HAD DSNDED TO APPROX 8500 FT AT THE LOWEST POINT. I INFORMED HOUSTON APCH THAT THE RA WAS RESOLVED AND WE WERE RETURNING TO 9000 FT. I THEN LOOKED AGAIN AT THE TCAS DISPLAY; AND NOTICED 1 TARGET DIRECTLY BELOW US; INDICATING 2000 FT BELOW; TRACKING THE SAME DIRECTION AND IN THE EXPECTED LATERAL POS OF THE PREVIOUS CONFLICTING TARGET. NEITHER THE CAPT NOR I VISUALLY SAW THE OTHER ACFT. I DID NOT SEE ANY OTHER TARGETS ON OUR TCAS DISPLAY THAT COULD HAVE POTENTIALLY BEEN A CONFLICT. I DON'T HAVE ENOUGH KNOWLEDGE OF HOW THE TCAS SYS WORKS TO SAY EXACTLY WHY WE RECEIVED THE DSND COMMAND; BUT IT APPEARED TO ME THAT THE TFC WAS DISPLAYED TO US AT THE SAME ALT IN ERROR. THE TFC THAT CAUSED THE CONFLICT; ACCORDING TO THE SITUATION I GATHERED FROM HOUSTON APCH; WAS BELOW US BY AT LEAST 2000 FT WHEN THE RA OCCURRED. THIS COINCIDES WITH THE SITUATION WE SAW ON THE TCAS DISPLAY AFTER THE RA.

Data retrieved from NASA's ASRS site as of May 2009 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.