Incident Description
An instructor pilot and a new company pilot were practicing autorotations with power-recovery. The instructor first demonstrated a straight-in autorotation, noting that the governor lagged but stayed within the green range, which he considered typical for that helicopter. The student then set up for a 180-degree autorotation.
Maneuver Execution
During the 180-degree autorotation, the instructor told the student to roll on the throttle at 200 feet AGL. At 50 feet AGL and 50 to 55 knots, the student flared the helicopter. At 15 feet AGL, the student increased collective pitch, and the instructor also pulled collective. The instructor reported that the torque gauge did not move, the engine sound dragged down, and rotor rpm decayed on the dual tachometer. The instructor took the controls and leveled the aircraft.
Hard Landing and Damage
The helicopter made a hard landing and came to a complete stop. After shutdown, the tail boom was found still attached but hanging to the ground. A Bell Operations Safety Notice advises that tail boom failure can occur in autorotations where touchdown occurs at 70% main rotor speed due to resonance response of the structure to the main rotor.
Postaccident Inspection
A postaccident test of the governor function was performed. Power checks simulating an autorotation revealed that the engine was slow to accelerate and the governor surged with continual fluctuation of Ng readings.