HELI INSTRUCTOR WITH STUDENT AND IN CONTACT WITH ATC CALLED ON CARPET BY POLICE HELI FLC FOR LOW FLYING.

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APCH WAS MADE FROM ANNAPOLIS HELIPORT TO BWI TCA. CLRNC WAS GIVEN BY TWR ON 119.4. I TRANSITIONED THROUGH THE TCA AT 500 FT MSL DIRECT TO THE INNER HARBOR AREA. BWI TURNED ME OVER TO APCH ON 119.7 AND WE WERE APPROVED TO THE INNER HARBOR AREA DIRECT AND CLRED TO ORBIT THE AREA. I HAVE HAD MORE THAN 10 HRS OF NIGHT FLYING IN THIS AREA WITHOUT INCIDENT. THIS FLT WAS FOR TRAINING PURPOSES WITH A STUDENT. I HAD JUST FLOWN UP THE HARBOR ALONG THE SHORELINE AT APPROX 400 FT MSL AND CROSSED INLAND AND ORBITED THE ORIOLES CAMDEN YARDS BASEBALL STADIUM WHICH WAS EMPTY; THEN PROCEEDED IN A COUNTERCLOCKWISE ORBIT OF THE DOWNTOWN WHERE WE MADE VISUAL CONTACT WITH ANOTHER HELI. THE OTHER HELI THEN FLASHED HIS LNDG LIGHTS AND BEGAN TO FOLLOW OUR HELI AND FLEW WITHIN A FEW HUNDRED YARDS OF US. I WAS STILL IN CONTACT; BOTH RADIO AND RADAR; WITH BWI APCH WHEN THE PLT OF THE OTHER HELI MADE CONTACT WITH BWI APCH AND IDENTED HIMSELF AS A POLICE HELI AND WAS FOLLOWING 'A HELI THROUGH DOWNTOWN; FLYING VERY LOW; OBVIOUSLY AVOIDING RADAR CONTACT AND BREAKING EVERY FAR IN THE BOOK.' BWI APCH TOLD HIM THAT THEY (BWI APCH) WERE 'WORKING' US. I WAS THEN INSTRUCTED TO SWITCH TO 123.02 BY BWI APCH AT THE POLICE PLT'S REQUEST. AFTER SWITCHING TO 123.02 AND MAKING CONTACT THE POLICE PLT TOLD ME 'YOU WERE FLYING TOO LOW AND HAD MADE NO RADIO CONTACT WITH ME AND ARE CAUSING HAZARD TO PERSONS ON THE GND AND HIMSELF.' I RESPONDED BY ASKING HIM WHAT I HAD DONE WRONG; AND HE RESPONDED 'I KNOW WHAT YOU DID AND SO DO YOU.' I FINISHED OUR COM BY APOLOGIZING AND EXPLAINING THAT BWI APCH HAD NOT TERMINATED RADIO CONTACT OR ADVISED ME OF ANY OTHER FREQ IN USE. I THEN RECONTACTED BWI APCH AND LEFT THE TCA IMMEDIATELY AND REMAINED 500 FT AND ABOVE. I AM NOT AWARE OF EVER VIOLATING ANY FAR INCLUDING FAR 91.119 OR 91.13. I HAVE SOME EXPERIENCE IN THIS LCL AREA AND OPERATING AT THESE ALTS. ALSO; I HAVE NEVER HAD AN INCIDENT OR ACCIDENT.