A COMMERCIAL INST RATED HELI PLT REACTS TO A TCASI TA.

Narrative

I WAS IN CRUISE CONFIGN IN MY BELL 407; AT ABOUT 127 KTS AND 7500 FT MSL BOUND FROM STP TO SBM ON A GPS TRACK BTWN THE 2 POINTS. MANY ACFT HAD BEEN HEARD ON 118.5 (OSH CTAF); AND MANY ACFT HAD BEEN SEEN VISUALLY AND ON MY BFG TCASI UNIT OVER ITS 6 NM RANGE. WITHOUT ANY OTHER KINDS OF INDICATION; THE TCASI UNIT SOUNDED A 'TFC; TFC;' ADVISORY; AND I LOOKED DOWN INTO THE COCKPIT TO SEE THE LOCATION OF THE BLIP ON THE TCASI SCREEN. THERE WAS CLRLY DEPICTED A TARGET RIGHT ON TOP OF THE CTR 300 FT ABOVE ME. I IMMEDIATELY HIT THE RANGE REDUCTION BUTTON TO EXPAND TO THE 2 MI SCALE; AND A TARGET WAS THEN VERY SLIGHTLY SEPARATED BELOW THE CTR OF THE INST; AND STILL WITH AN ALT DIFFERENCE OF 300 FT DIRECTLY ABOVE AND BEHIND MY HELI; WITH THE TARGET BLIP SLIGHTLY OVERLAPPING THE CTR INDICATOR ON THE INST. I THEN CONFIRMED THAT THE TARGET INDICATED 300 FT ABOVE (+) ME AND DSNDING; AND THEN QUICKLY REDUCED COLLECTIVE AND NOSED OVER SIMULTANEOUSLY WITH A BANKING R ROLL OF 90 DEGS. THE BLIP THEN STARTED INDICATING AN UP ARROW WHICH MEANT IT WAS CLBING AWAY FROM ME; AND SEPARATED VERY RAPIDLY IN WHAT APPEARED TO BE A WESTERLY DIRECTION. AT THE COMPLETION OF MY 180 DEG TURN; THE TARGET WAS WELL SEPARATED AND CLBING VERY RAPIDLY.