NMAC BTWN AN ENSTROM HELI ON A SIGHTSEEING FLT; AND A LOW WING PIPER DOING TRAINING MANEUVERS. THE HELI PLT FIRST OBSERVED THE PIPER ACFT GO UNDERNEATH HIS ACFT FROM R TO L AT A DISTANCE OF 150 FT.

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AT XX15 LCL 5 NM E OF RAINTREE AIRPARK ON OCT/XA/98; SIGHTSEEING; 2 PAX. 15-20 MINS PRIOR TO INCIDENT; A PIPER WAS TRAINING OUT TO THE N APPROX 5 NM. I WAS ON A SIGHTSEEING TRIP WITH 2 PAX; AND AS ALWAYS THEY ARE TAKING PICTURES AND MOVING ABOUT. THE ENSTROM HELI HAS A BENCH SEAT; SO ALL 3 OF US SAT NEXT TO EACH OTHER. I REPEATEDLY ASKED THE 2 OF THEM TO SIT BACK AS MUCH AS POSSIBLE SO I COULD SEE OTHER TFC. I KNEW THE PIPER WAS IN THE IMMEDIATE AREA AND I WAS ACTIVELY LOOKING FOR IT. AGAIN; I SAW THE 2 PAX LEANING VERY FAR FORWARD AND TO THE R; ONCE AGAIN BLOCKING MY FIELD OF VISION. I ASKED THEM AGAIN TO LEAN BACK ONCE AGAIN. JUST AS THEY LEANED BACK THE PIPER FLEW UNDERNEATH ME; AT ABOUT 150 FT BELOW ME. IT THEN WENT OUT TO MY 8 O'CLOCK POS PARALLEL TO MY COURSE. DISTANCE WAS APPROX 1/4 MI FROM ME AND IT MAINTAINED THAT POS FOR 5-10 MINS AND THEN ROLLED L AND FLEW OFF. I CONTINUED ON MY PRESENT HDG. I'M NOT SURE IF BECAUSE OF MY 2 PAX BLOCKING MY FIELD OF VIEW THAT I NEVER SAW THE PLANE COMING AT ME FROM THE R; OR AS I STATED EARLIER; I OBSERVED ACFT #2 DOING MANEUVERS AND TRAINING; THAT IT POSSIBLY CAME OUT OF A MANEUVER NOT SEEING ME AND TOOK EVASIVE ACTION.