ACR X TCASII TA HAD NMAC WITH VFR HELI ON FINAL APCH. EVASIVE ACTION TAKEN.

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DFW JAN/TUE/94; ACR X HAD NEAR MISS OF 400 FT VERT AND 0 FT HORIZ. WE TOOK EVASIVE ACTION. THE WX WAS ABOUT 2000 FT OVCST WITH BETTER THAN 5 MI VISIBILITY. TCASII WAS PLACED IN TFC ONLY (NO RA) MODE PER GUIDANCE FROM COMPANY WHEN IN THE TFC PATTERN. AS WE CROSSED THE 36L OM; I TWICE CHKED ON DFW TWR FREQ BUT RECEIVED NO ANSWER. I MENTIONED OUR POS BY CALLING OUT THE NAME OF THE 36L OM ON ONE OF THE CALLS. NEITHER CALL WAS ACKNOWLEDGED ALTHOUGH 2 CTLR VOICES WERE CONSTANTLY TALKING ON THE FREQ; MOSTLY TO ACFT LINED UP TO TAKE OFF ON THE INSIDE RWY. THERE WERE MANY TARGETS ON THE TCASII AND ONE OF THEM TURNED YELLOW ACCOMPANIED BY THE AURAL WARNING OF 'TFC; TFC.' THE INTRUDER WAS AT 11 O'CLOCK AND ABOUT 500 FT BELOW US. I ORDERED THE FO TO ARREST THE DSCNT; AND SO WE WENT ABOVE THE GS IN AN EFFORT TO AVOID THE INTRUDER Y. WE WERE ABOVE THE GS ABOUT 2 1/2 MI FROM THE END OF THE RWY AND ABOUT 900 FT AGL WHEN THE TCASII INDICATED THE INTRUDER GOING UNDER US AT 400 FT BELOW US. THE INTRUDER WOULD HAVE BEEN CLOSER IF WE HAD BEEN ON OR SLIGHTLY BELOW (NORMAL BRACKETING MANEUVERS) THE GS. WE NEVER VISUALLY ACQUIRED THE INTRUDER. WHILE WE WERE BUSY WITH TCASII AND EVASIVE ACTION; I HEARD THE CTLR VOICE I TOOK TO BE A TRAINER OR SUPVR SPEAK OUR CALL SIGN. HE SAID SOMETHING ABOUT A HELI; BUT IT WAS NOT AT ALL CLR TO ME THAT THE INTRUDER WE OBSERVED WAS THE ONE HE WAS MENTIONING. I WAS SIMPLY TOO BUSY TO ANSWER HIM UNTIL AFTER THE INTRUDER ENCOUNTER WAS SUCCESSFULLY RESOLVED. I'M RETHINKING THE WISDOM OF SELECTING AWAY FROM THE TCASII RA MODE. I BELIEVE WE WOULD HAVE GOTTEN BETTER PROTECTION AND EVASIVE GUIDANCE IF WE HAD HAD THE TCASII FULLY ARMED. AFTER LNDG I ASKED THE GND CTLR FOR A PHONE NUMBER. I CALLED TWR SUPVR AND HE STATED THE TFC WAS A HELI WHO WAS BEING HANDLED BY ANOTHER CTLR ON A DIFFERENT FREQ AND THAT THE HELI WAS BEING OPERATED IN ACCORDANCE WITH A TWR LOA. HE FURTHER STATED THAT THE HELI SAW US. I ASKED THE SEPARATION STANDARD; AND HE REPLIED THAT ALL THE HELI HAD TO DO WAS NOT HIT US! SUPVR STATED THAT THEY (DFW TWR) HADN'T HAD THESE PROBS UNTIL TCASII WAS INSTALLED.