ERA21LA386

During cruise flight, the pilot heard a “pop” sound followed by the engine accelerating to a high rpm. According to the pilot, the helicopter began spinning counterclockwise at a high rate; the cyclic stick would not move; and collective and pedal control inputs had no effect. The pilot continued his attempts to counteract the helicopter’s rotation as it descended to ground impact. Postaccident examination of the wreckage revealed that airframe damage was consistent with ground impact and that main rotor blade deformation and damage were consistent with powered impact with terrain. Damage to the tail rotor blades was consistent with the tail rotor not being under power at impact, and the tail rotor gearbox input pinion bevel gear was found fractured from its gear shaft. Metallurgical examination of the input pinion gear shaft revealed that it had fractured from fatigue. The fatigue cracking initiated at gouge marks on the outer surface of the cylindrical shaft and propagated through nearly the entirety of the shaft cross-section. The length of the fatigue cracks and the fine spacing of the striations were indicative of high cycle fatigue crack propagation. Review of maintenance records revealed that the tail rotor gearbox was disassembled, visually inspected, and reassembled about 330.3 hours before the accident. Based on the high cycle fatigue crack propagation and the presence of fatigue cracking on the majority of the fracture cross-section, it is likely the gouge marks on the input pinion gear shaft were created during reassembly of the tail rotor gearbox during this maintenance action. A loss of drive to the tail rotor would result in a clockwise, nose-right yaw of the helicopter due to the torque effect on the fuselage of the counterclockwise rotation of the main rotor. The sudden loss of tail rotor drive due to the fracture of the input pinion bevel gear would unload the powertrain of the power being used by the tail rotor, leading to a temporary increase of rotor rpm as well as engine rpm, consistent with the pilot’s report. Although the pilot reported that the helicopter began to spin counterclockwise, it is likely he misidentified the direction of yaw.

On September 29, 2021, about 1030 eastern daylight time, a Hiller UH-12E helicopter, N430NR, was substantially damaged when it was involved in an accident near Millen, Georgia. The pilot sustained minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot reported that he was enroute to his next jobsite at 500 ft above ground level and 70 knots airspeed when he heard a “pop” sound. The engine accelerated to a high rpm, and the helicopter began to spin counterclockwise at a high rate. The pilot reported that the cyclic stick would not move and that pedal and collective control inputs had no effect. The pilot continued his attempts to counteract the helicopter’s rotation as it descended to ground impact. The helicopter came to rest on its left side in a field. Examination by a Federal Aviation Administration inspector revealed that the impact sheared the right skid and that the outer sections of the two main rotor blades separated. The separated sections of the two main rotor blades were found about 225 ft west-northwest and 180 feet north-northwest of the main wreckage site, respectively. The ‘yellow’ control rotor had separated and was found about 270 ft southwest of the main wreckage site. The main rotor head, containing the main rotor hub, gimbal ring assembly, collective ballast assembly, the upper portion of the main rotor mast, the ‘blue’ and ‘yellow’ main rotor blade inboard ends, and the ‘blue’ control rotor, had separated and was found about 135 ft south of the main wreckage site. Postaccident examination of the wreckage revealed that airframe damage was consistent with ground impact and that main rotor blade deformation and damage were consistent with powered impact with terrain. Examination of the main rotor system and the flight control system revealed no evidence of preimpact anomalies that would have prevented normal flight. Damage to the tail rotor blades was consistent with the tail rotor not being under power at impact, and the tail rotor gearbox input pinion bevel gear was found fractured from its gear shaft. The National Transportation Safety Board Office of Research and Engineering, Materials Laboratory Division, performed metallurgical examinations of the main rotor hub, the ‘yellow’ control rotor and trunnion, and pieces of the tail rotor gearbox input pinion. Examination revealed that the fractures and separations of the ‘yellow’ control rotor and trunnion studs occurred due to overload and that the tail rotor gearbox input pinion bevel gear shaft had fractured from fatigue. Examination revealed that the fatigue cracking initiated at gouge marks on the outer surface of the cylindrical shaft and propagated through nearly the entirety of the shaft cross-section. Review of maintenance records revealed that the tail rotor gearbox was disassembled, visually inspected, and reassembled on August 28, 2018, about 330.3 hours before the accident. Attempts tocontact the mechanic who performed this maintenance action were unsuccessful.