Accident Overview
On November 26, 2015, about 1145 central standard time, a Robinson R44II, registration N7512N, was substantially damaged during a separation of the tail rotor drive shaft while conducting a power check at Destin Executive Airport in Destin, Florida. The commercial pilot and three passengers were not injured. The helicopter was being operated under 14 Code of Federal Regulations Part 91 as a local air tour flight. Visual meteorological conditions prevailed, and no flight plan was filed. The flight was originating at the time of the accident.
Pilot Account
The pilot reported that he performed a preflight inspection, and the passengers then boarded. The engine was started, and ground run-up checks were satisfactory. While performing a power check involving applying up collective, he heard a loud "pop." He immediately lowered the collective and secured the helicopter.
Examination Findings
Examination of the helicopter revealed that the steel tail rotor drive shaft (part number C196-1) was circumferentially fractured at the weld of the forward flange. A portion of the intermediate flex plate (part number A947-2) remained bolted to the flange, while the remaining portion remained bolted to the clutch shaft yoke (part number C195). The tailcone attachment frame (part number C237-1) and the damper assembly mount angle were fractured. No damage was reported to any frame or interior of the tailcone aft of the damper. The helicopter's total time at the time of the accident was 1,770 hours.
NTSB Materials Laboratory Analysis
According to the NTSB Materials Laboratory factual report, the fracture between the tail rotor drive shaft and the flange was consistent with torsional overstress. A portion of the flex plate attached to the flange also exhibited features consistent with overstress. Inspection of the bolted connection between the clutch shaft yoke and the flex plate revealed that all hardware was in the correct sequence except for the omission of a washer between the flex plate and the clutch shaft yoke. This washer omission was consistent on both connections between the clutch shaft yoke and the flex plate. The hardware stack-up for the connection between the flex plate and the drive shaft flange was correct.
Maintenance History
Review of maintenance records revealed an entry dated November 12, 2015, at a helicopter total time of about 1,759 hours, indicating that the sprag clutch was removed and replaced. The maintenance manual includes a caution regarding the C-195 yoke removal and installation: "There must be one AN960-416L or one AN960-416 washer between each arm of C195 yoke and A947-2 flex plate…." The manual also references Section 7.330, which pertains to intermediate flex plate installation and shimming and contains measurements for proper shimming.
Mechanic Statement
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 exactly the same. He also indicated that when new hardware was installed, he obtained the same dimensions.
Additional Notes
According to the maintenance record entry returning the helicopter to service after the accident, there was no record that the governor was removed, replaced, or repaired. A representative of the helicopter manufacturer stated that if misalignment of the clutch shaft, sheave, and intermediate flex coupling was not corrected during installation, there could be significant preload or tension in the entire length of the driveshaft between the main rotor gearbox and tail rotor gearbox. The representative also indicated that the effect of a missing washer would have caused a fatigue-type failure of the tail rotor driveline, and they had not previously seen a failure of the tail rotor driveshaft at the flange as occurred in the accident helicopter.