A B-757 FLT CREW RECEIVES A TCASII TA RA ON A HELI THAT TWR THOUGHT WOULD BE NO FACTOR.

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APCHING 1300 FT AGL; LINED UP FOR RWY 9L; LNDG CHKLIST COMPLETE AND CLRED TO LAND; ON LOC AND GS; SAW TCASII VSI DISPLAY TURN YELLOW AND TFC SHOWN AHEAD AND BELOW -700 FT. THE MAINT; DISPLAY WAS IN LARGE SCALE AND I TURNED IT TO THE LOWEST SCALE TO GET A BETTER IDEA WHERE THE TFC WAS RELATIVE TO US. SHORTLY THEREAFTER THE ALERT TURNED TO RED AND TCASII ANNOUNCED 'MONITOR VERT SPD' ONCE. RATE OF DSCNT AT THE TIME WAS ABOUT 700-800 FT WITH A SLIGHTLY ROUGH RIDE FROM AFTERNOON THERMALS. GREEN ARC ON THE VSI WAS IN THE 0-300 FPM RANGE. I FLEW INTO THE GREEN ARC FOR ABOUT 5 SECONDS AT WHICH TIME THE DISPLAY RETURNED TO NORMAL. MEANWHILE THE PNF NOTIFIED THE TWR; WHICH RESPONDED THEY KNEW OF HELI TFC; BUT THOUGHT IT WOULD NOT BE A FACTOR. ALMOST SIMULTANEOUSLY THE PNF REFED TCASII THEN VISUALLY SPOTTED THE TFC. HE ANNOUNCED; 'A HELI; NO FACTOR.' TCASII RETURNED TO NORMAL AND A CORRECTION WAS MADE TO RETURN TO THE GS. A NORMAL LNDG WAS MADE WITHIN THE COMPANY PUBLISHED LIMITATIONS FOR A STABILIZED APCH AND WITHIN THE DEFINED LNDG AREA.