Incident Overview
During a practice autorotation with a power recovery, the flight instructor heard a noise from the rear of the helicopter. When the power recovery was attempted, the engine responded to the throttle application but the low RPM warning horn activated. The helicopter subsequently struck the ground in a level attitude, causing the main rotor blades to sever the tailboom.
Examination Findings
Post-accident examination of the high speed output driveshaft coupling at the transmission end revealed that the four bolts were installed, but the four nuts and four washers could not be found in the assembly. Two nuts were later located in the scupper drain of the transmission deck.
Conclusion
The missing nuts and washers from the driveshaft coupling assembly are noted as a finding, but the report does not explicitly state a probable cause for the accident.