Incident Overview
During a primary flight lesson, the student pilot had recorded the session on a tape recorder. After reaching the practice area, the certified flight instructor (CFI) instructed the student to execute a 180-degree left turn. The student complied, performing a shallow bank turn. While cruising at 2,200 ft, the CFI began speaking. Mid-sentence, an undetermined event interrupted his speech, and a wind-like background noise commenced. The student exclaimed "Help."
Radar data corroborated witness reports that the tail boom and main rotor assembly had separated in level flight.
Wreckage Examination
Examination of the wreckage indicated that mast bumping had occurred. A main rotor blade crushed the left side of the inboard tail cone. The entire mast, with attached main rotor blades, broke out from the top of the transmission. The outboard tail boom and rotor assembly have not been located. Evidence of a main rotor blade divergence was found.
Audio Analysis
Spectrum analysis of the tape recording revealed no anomalies until the CFI's voice terminated. Within two seconds, the rotor speed appeared to significantly oscillate and then abruptly end.