HELI FLT INSTRUCTOR AND PLT EXPERIENCE DYNAMIC ROLLOVER DURING HOVER AUTOROTATION PRACTICE DURING A TRAINING FLT.

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I WAS GIVING FLT INSTRUCTION TO A STUDENT SEEKING A PVT PLT HELI ADD-ON. WE WERE APPROX 1 HR INTO THE LESSON WHEN WE EXPERIENCED DYNAMIC ROLLOVER. THE SKY WAS CLR; VISIBILITY WAS UNLIMITED; AND THE WINDS WERE LIGHT -- LESS THAN 5 KTS. WE WERE PRACTICING HOVERING AUTO-ROTATIONS. THE PREVIOUS 4 HOVERING AUTOS WERE NORMAL FOR A STUDENT WITH LESS THAN 20 HRS IN HELI. THE STUDENT INITIATED THE 5TH HOVERING AUTO-ROTATION AND EVERYTHING APPEARED TO BE NORMAL. HE WAS DOING EVERYTHING BY THE BOOK. WHEN THE HELI STARTED TO DSND; THE STUDENT INITIATED THE COLLECTIVE PITCH PULL. WHEN THE L SKID CAME IN CONTACT WITH THE GND; THE STUDENT TOOK OUT THE R CYCLIC INPUT AND ADDED A SLIGHT L CYCLIC INPUT. AT THIS POINT I ADDED FULL R CYCLIC; FULL DOWN COLLECTIVE; AND L PEDAL TO TRY TO STOP THE ROLLING MOTION. THE HELI CONTINUED TO ROLL TO THE L WHERE IT CAME TO REST ON THE L SIDE. I WAS ON THE CTLS WITH THE STUDENT THROUGHOUT THE MANEUVER AS I WAS TAUGHT WHEN I WAS OBTAINING MY CFI HELI RATING. BOTH THE STUDENT AND MYSELF WALKED AWAY FROM THE ACFT UNHARMED AND UNASSISTED.