STUDENT PLT IN A HELI DEPARTS HIO; OR; AND CROSSES AN ACTIVE RWY CLOSE BY A DEPARTING ACFT.

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I GAVE A STUDENT FLT INSTRUCTION FOR A PVT HELI RATING IN A ROBINSON R22 HELI. ON ALL OUR TRAINING FLTS; BEFORE HIS SOLO; I HAD NEVER THE FEELING THAT HE HAD PROBS WITH THE RADIO COMS. AFTER HIS FIRST SOLO HE CONTINUED WITH HIS SOLO FLTS; PARALLEL TO HIS INSTRUCTED FLTS. DURING SEP/98 HE CROSSED THE ACTIVE RWY WITHOUT A CLRNC FROM THE ATC ON ONE OF HIS SOLO FLTS. IN THIS CASE; HE CROSSED THE RWY 2 OF HILLSBORO ARPT; OR APPROX 500 FT IN FRONT OF A DEPARTING CESSNA; BECAUSE HE THOUGHT HE WAS CLRED TO CROSS BOTH RWYS. ALSO; THE CESSNA PLT DID A MISTAKE -- HE WAS JUST CLRED TO LAND; BUT HE DID A TOUCH-AND-GO. APPARENTLY; HILLSBORO TWR CLRED HIM FOR A 'N DEP;' BUT TOLD HIM TO REMAIN CLR OF RWY 2. HILLSBORO TWR FILED A RPT ABOUT THE HELI AND CESSNA INCIDENT TO THE FAA. SUPPLEMENTAL INFO FROM ACN 415395: I UNDERSTOOD NOW THAT A 'N DEP' WAS APPROVED; AND I WAS CLRED TO CROSS RWY 30. I REPEATED 'XING RWY 30; DEPARTING N; HELI XX.' I THOUGHT THAT I WAS CLRED TO CROSS THE ACTIVE RWY. THE TWR INFORMED AFTER THIS THAT I CROSSED THE RWY 2 WITHOUT CLRNC AND IN FRONT OF A DEPARTING CESSNA; APPROX 500 FT AWAY.