What happened
A student was conducting a primary flight lesson, which was being recorded on a tape recorder. Upon reaching the practice area, the flight instructor directed the student to perform a 180-degree left turn. The student executed a shallow bank turn as instructed.
While the aircraft was cruising at 2,200 feet, the instructor began speaking. During this time, an undetermined event occurred that interrupted the instructor's speech. A wind-like noise was recorded on the tape, followed by the student calling out "help." Radar data and witness reports confirmed that the tail boom and main rotor had separated from the aircraft during level flight.
The investigation
An examination of the wreckage revealed that mast bumping had occurred. One main rotor blade crushed the left side of the inboard tail cone, and the entire mast, including the attached main rotor blades, broke out of the top of the transmission. Investigators were unable to locate the outboard tail boom or the rotor assembly. Additionally, evidence was found indicating main rotor blade divergence.