AN MLG ACR ACFT HAD A TCASII RA NEAR THE TFC PATTERN; THEN THE COPLT HAD AN ALTDEV IN THE TFC PATTERN.

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JUST A FEW MINS PRIOR TO OUR ALTDEV; WE HAD RECEIVED A TCASII CLB RA WHICH WAS VALID AND THIS CONTRIBUTED TO WHAT HAPPENED. THE FO WAS FLYING THE ACFT ONLY 10 MONTHS AS COPLT; AND WE WERE CLRED TO 3000 FT ON A DOWNWIND. I WAS TALKING TO APCH AND TRYING TO LOCATE THE ARPT AND KEEP A TFC WATCH. I CHKED THAT THE FO WAS LEVELING AT 3000 FT AND LOOKED BACK OUT. FOR SOME REASON THE FO THOUGHT WE WERE CLRED FOR THE VISUAL AND FOR SOME UNEXPLAINED REASON 1000 FT WAS SET IN THE ALT ALERT WINDOW. I KNOW THAT I DIDN'T SET IT AND HE SAID THAT HE HADN'T. AS WE WERE TURNING TOWARD THE ARPT; I SENSED WE WERE DSNDING AND TURNED MY ATTN BACK IN THE COCKPIT AND SAW WE WERE DSNDING. I TOLD HIM TO STOP THE DSCNT AT 2500 FT; AND TOLD APCH WE HAD THE ARPT. APCH HAD NO COMMENT EXCEPT TO CLR US FOR THE APCH. THEY DID ASK ME TO CALL THE SUPVR ON THE GND BUT THE ONLY THING HE WANTED WAS INFO ON OUR TCASII RA. I BELIEVE THE MAIN FACTOR IN THIS WAS THAT BOTH OF US HAD A BREAK IN CONCENTRATION DUE TO THE TCASII RA AS IT WOULD HAVE BEEN A VERY NEAR MISS IF NOT FOR TCASII. ANOTHER FACTOR WHICH MAY HAVE CONTRIBUTED WAS THE RELATIVE INEXPERIENCE OF THE FO; ABOUT 150-200 HRS IN 10 MONTHS. I LET MY ATTN FOCUS MORE ON LOOKING FOR TFC AND THE ARPT AND I SHOULD HAVE BEEN PAYING MORE ATTN TO THE FLYING.