Narrative:

This report submitted to document case of TCASII direction evasive maneuver toward threat aircraft! We were on approach to tjsj, level at 1500 ft, waiting to intercept the GS. I had earlier noted a call from approach cautioning another aircraft of an ATR descending out of 3000 ft. I checked our altitude and confirmed it was probably us, and so there was a light civil near and below us. There was no TCASII target. After leveling at 1500 ft, TCASII announced 'traffic.' the yellow TA symbol indicated '00' (co-altitude), with no significant lateral separation on the 6 NM scale. I started to look out, but the TA immediately became an RA commanding a 1500 FPM descent. I kicked off the autoplt and complied. After a few moments, the captain motioned for me to stop the descent, saying 'it's ok.' I leveled off and the TCASII went clear of conflict. The captain told me he saw the aircraft (a cessna centurion, I think) below us. Apparently, the TCASII acquired late, read threat altitude wrong and gave an erroneous RA directing descent toward the threat (and the ground). I suspect that the cessna may have turned on his transponder in our immediate vicinity (given lack of target initially, cessna's low altitude, and erroneous altitude reading). In my experience, TCASII is better than nothing, but TA/RA usually comes after cpa. TCASII ain't all it's cracked up to be.

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Original NASA ASRS Text

Title: AN ATR42 FO COMPLAINS OF THE TCASII RA THAT CALLED FOR DSCNT INTO A C210 BELOW THEM 12 MI W OF TJSJ, PR.

Narrative: THIS RPT SUBMITTED TO DOCUMENT CASE OF TCASII DIRECTION EVASIVE MANEUVER TOWARD THREAT ACFT! WE WERE ON APCH TO TJSJ, LEVEL AT 1500 FT, WAITING TO INTERCEPT THE GS. I HAD EARLIER NOTED A CALL FROM APCH CAUTIONING ANOTHER ACFT OF AN ATR DSNDING OUT OF 3000 FT. I CHKED OUR ALT AND CONFIRMED IT WAS PROBABLY US, AND SO THERE WAS A LIGHT CIVIL NEAR AND BELOW US. THERE WAS NO TCASII TARGET. AFTER LEVELING AT 1500 FT, TCASII ANNOUNCED 'TFC.' THE YELLOW TA SYMBOL INDICATED '00' (CO-ALT), WITH NO SIGNIFICANT LATERAL SEPARATION ON THE 6 NM SCALE. I STARTED TO LOOK OUT, BUT THE TA IMMEDIATELY BECAME AN RA COMMANDING A 1500 FPM DSCNT. I KICKED OFF THE AUTOPLT AND COMPLIED. AFTER A FEW MOMENTS, THE CAPT MOTIONED FOR ME TO STOP THE DSCNT, SAYING 'IT'S OK.' I LEVELED OFF AND THE TCASII WENT CLR OF CONFLICT. THE CAPT TOLD ME HE SAW THE ACFT (A CESSNA CENTURION, I THINK) BELOW US. APPARENTLY, THE TCASII ACQUIRED LATE, READ THREAT ALT WRONG AND GAVE AN ERRONEOUS RA DIRECTING DSCNT TOWARD THE THREAT (AND THE GND). I SUSPECT THAT THE CESSNA MAY HAVE TURNED ON HIS XPONDER IN OUR IMMEDIATE VICINITY (GIVEN LACK OF TARGET INITIALLY, CESSNA'S LOW ALT, AND ERRONEOUS ALT READING). IN MY EXPERIENCE, TCASII IS BETTER THAN NOTHING, BUT TA/RA USUALLY COMES AFTER CPA. TCASII AIN'T ALL IT'S CRACKED UP TO BE.

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.