R22 IN THE PATTERN AT AN UNCONTROLLED AIRPORT HAS A NEAR MISS WITH A LOCAL LAW ENFORCEMENT HELICOPTER.

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ROBINSON R22B; WAS ON R XWIND FOR RWY 10 AT ZZZ; HAVING JUST DEPARTED FOR FLT TRAINING IN THE TFC PATTERN. ANOTHER R22 WAS ON R BASE FOR THE SAME RWY. 3 FIXED WING ACFT WERE IN THE L TFC PATTERN FOR RWY 10. ALL ACFT WERE MONITORING AND PARTICIPATING ON THE CTAF FREQ. DURING THE R TURN TO DOWNWIND AND AT APPROX 1000 FT MSL THE INSTRUCTOR IN THE R22 NOTICED A LARGE SHADOW ON THE GND WHICH APPEARED TO BE CAST BY AN OVERTAKING ACFT. THE INSTRUCTOR IMMEDIATELY INITIATED A DSCNT AND L TURN AWAY FROM THE ARPT. UPON TURNING THE INSTRUCTOR OBSERVED A SIKORSKY S76 OVERFLYING THE ARPT AT NO MORE THAN 1000 FT; OVERTAKING THE R22 FROM THE R REAR; AND PASSING NO MORE THAN 150 FT ABOVE THE R22. UPON CONFERRING WITH OTHER PLTS WHO WERE IN THE PATTERN AT THE TIME; NO ONE RECALLED HEARING THE S76 HELI MAKE A RADIO TFC ADVISORY OR POS RPT. PROB: TFC NEAR MISS CREATED BY S76 HELI OVERFLYING ARPT ENVIRONMENT AT PATTERN ALT WITHOUT THE INTENT TO LAND. FAILURE BY THE S76 HELI TO PARTICIPATE ON THE CTAF. THIS SITUATION IS NOT UNUSUAL. SOLUTION: THE S76 ROUTINELY DOES THIS EXACT TRANSITION. THEY NEED TO BE REMINDED OF PROPER PRACTICE WHEN OVERFLYING ARPTS AND ADVISED THAT IT MIGHT BE A GOOD IDEA TO USE THE RADIO.