Robinson Helicopter Company R44 II N7512N accident near Destin, Florida

Probable cause

The overstress fracture between the tail rotor driveshaft and intermediate flex plate for reasons that could not be determined based on the available evidence.

NTSB analysis

The commercial pilot was performing a power check before an air tour flight with three passengers onboard. While raising the collective, he heard a loud "pop." He then immediately lowered the collective and secured the helicopter.

Postaccident examination of the helicopter revealed that an overstress fracture existed between the tail rotor driveshaft and intermediate flex plate. Examination revealed that all the hardware was in place except for a washer, which is required at each connection between the clutch shaft yoke and flex plate per a caution in the helicopter's maintenance manual. The maintenance manual also contained measurements to determine proper shimming required at the intermediate flex plate. A review of maintenance records revealed that, about 11 flight hours before the accident, the sprag clutch had been removed and replaced. The mechanic who performed the clutch removal and installation stated that the measurements he took at the intermediate flex plate during and after his repairs were the same. He also indicated that, when they installed new hardware, he obtained the same dimensions. Although a misalignment of the intermediate flex coupling would place significant tension along the entire length of the driveshaft between the main rotor gearbox and tail rotor gearbox and a missing washer both could have caused a fatigue-type failure of the tail rotor drive line, the parts fractured due to overstress, not fatigue; therefore, it is unlikely that the missing hardware contributed to the failure. The reason for the overstress fracture between the tail rotor driveshaft and intermediate flex plate could not be determined based on the available evidence.

Text from the NTSB. Open the NTSB docket for ERA16LA054 for the full record.