Background
During a training flight, a private pilot attempted a 180-degree autorotation maneuver. After the attempt was unsuccessful, the certificated flight instructor (CFI) directed the pilot to perform the maneuver again.
Accident Sequence
The private pilot initiated the autorotation and used the collective to maintain rotor rpm within the normal range. As the helicopter descended through the turn, the instructor observed the helicopter gradually assuming a nose-low attitude. The instructor advised the student to level the helicopter and continue the turn.
Subsequently, the instructor assumed control of the flight controls. The helicopter continued descending through 100 feet above ground level (agl) at an airspeed of 40 knots. The helicopter was vibrating, and the low rotor warning horn was sounding. The instructor leveled the helicopter as it simultaneously contacted the ground. The helicopter touched down hard and slid on its belly to the edge of the runway surface.
Instructor's Statement
The instructor reported no mechanical malfunctions or failures with the airframe or engine prior to the accident. He stated that the accident may have been prevented if he had assumed authority of the controls earlier in the maneuver.