S76 FLT CREW MISSES HOLD SHORT CLRNC AND HOVER TAXIES ONTO ACTIVE RWY.

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IFR TRAINING FLT; SINGLE PLT. PLT AND INSTRUCTOR ON BOARD. REQUESTED REPOSITION TO RWY 27L FOR TKOF. PLT INCORRECTLY THOUGHT RADIO WAS ON TWR FREQ. INSTRUCTOR HAD RADIO ON GND FREQ; OBSERVING. ACFT ORIENTED N BTWN 2 PARKED ACFT. NO TURN COULD HAVE BEEN MADE SAFELY ON THE GND. ATC RESPONDED; CLRING THE ACFT TO RWY 27L; 'GIVE WAY TO AIRPLANE TAXIING W ON TXWY R TO RAMP.' WE WERE W OF AIRPLANE TAXIING; AND IT APPEARED HE WOULD CONTINUE W. WE WERE HOVER-TAXIING. THE INSTRUCTOR IN THE L SEAT UNDERSTOOD THE SAME INSTRUCTIONS AND DIRECTED ME TO CROSS TXWY R TO RWY 27L; ADJACENT TO OUR POS. DURING THE SECOND ATC CALL FROM GND; WE HAD LIFTED AND WERE XING TXWY R; BECAUSE I ASKED FOR REPOSITION AND THOUGHT WE WERE ON TWR FREQ. THEY STATED; 'GIVE WAY AND HOLD SHORT.' WE HEARD THE FIRST PART OF THE SECOND CALL AND THEN WERE ADVISED TO MOVE OFF THE RWY FOR LNDG TFC AND THEN CALL TWR SUPVR AFTER OUR FLT. THERE WAS AN ACFT TAXIING ON TXWY R. WE REQUESTED HOVER TAXI FOR REPOSITION ONTO RWY 27L; NOT TAXI TO RWY 27L. WE COULD NOT BOTH GIVE WAY TO THE TAXIING AIRPLANE AND REPOSITION TO THE HOLD SHORT LINE; AS THE HELI WOULD STILL BE ON TXWY R; IN THE PATH OF THE AIRPLANE. SINGLE PLT OPS IN A HOVER MAKE IT DIFFICULT TO SWITCH FREQS ONCE A CLRNC HAS BEEN ISSUED. THE PLT/CTLR GLOSSARY SUGGESTS CHANGING TO THE APPROPRIATE FREQ BEFORE HOVERING. SUGGEST A SECTION SHOULD BE ADDED IN AIM/REG; ETC; AND ALSO ATC INSTRUCTIONS THAT WHEN A HELI IS INFLT; WHEELS/SKIDS OFF THE GND; HE/SHE SHOULD BE CTLED BY TWR. THEY ARE AFTER ALL INFLT; REGARDLESS OF POS ON THE ARPT.