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

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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.