CEN21LA389

While attempting to land at a staging location for a planned aerial application flight, the helicopter began to rotate to the right. The pilot was unable to regain control. The helicopter skids impacted a dirt field, and the helicopter rolled over onto its right side. The helicopter sustained substantial damage to its main rotor blades and fuselage. Disassembly of the helicopter for recovery revealed the tail rotor output shaft was fractured into two pieces. Examination of the output shaft revealed the fracture occurred between the mating tapered roller bearing and the bevel gear. The bevel gear side surface displayed progressive crack arrest marks consistent with fatigue cracking. Due to smear damage on the other fracture, it could not be determined if the fatigue cracking was the result of unidirectional bending fatigue (one origin) or reverse bending fatigue (two origins). The tail rotor output shaft function is to drive the tail rotor blades. A failed tail rotor output shaft would result in the loss of drive to the tail rotor blades, which in turn would cause a loss of tail rotor thrust to counter main rotor torque and loss of yaw control.

On August 27, 2021, about 0800 central daylight time, a Robinson R22 Beta helicopter, N827SH, sustained substantial damage when it was involved in an accident near Arnaudville, Louisiana. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 positioning flight. According to the pilot, while attempting to land at his staging location for a planned aerial application flight, the helicopter began to rotate to the right. The pilot was unable to regain control; the helicopter skids impact a dirt field, and the helicopter rolled over onto its right side. The helicopter sustained substantial damage to its main rotor blades and fuselage. While disassembling the helicopter for recovery, the tail rotor output shaft was found fractured into two pieces. The shaft was retained for further examination by the National Transportation Safety Board (NTSB) Materials Laboratory. According to Robinson Helicopters, the tail rotor gearbox contains a splash-lubricated spiral-bevel gear set. The function of the tail rotor output shaft is to drive the tail rotor blades. A failed tail rotor output shaft results in the loss of drive to the tail rotor blades, which in turn causes a loss of tail rotor thrust to counter main rotor torque and loss of yaw control. A review of the helicopter records revealed a serviceable tail rotor gearbox (serial number 3242) was installed on April 18, 2020, at a total helicopter time of 4,255.6 hours. The helicopter total time at the time of the accident was not reported. NTSB Material Laboratory examination of the output shaft revealed the fracture occurred between the mating tapered roller bearing and the bevel gear. Heavy smearing damage was observed on both fracture surfaces. The damage was likely secondary and due to contact of the fracture surfaces post-separation. The bevel gear side surface displayed progressive crack arrest marks consistent with fatigue cracking. Due to smear damage on the other fracture, it could not be determined if the fatigue cracking was the result of unidirectional bending fatigue (one origin) or reverse bending fatigue (two origins).