ANC24LA077

A failure of the flex plate yoke flange assembly due to incorrectly torqued bolts. Contributing was the maintenance personnel’s inadequate inspection of the tail rotor flex plate assembly.

The pilot reported that, before boarding passengers, she started the engine, engaged the clutch and, as the main rotor system began to engage, she felt and saw the left pedal deflect full forward without any control input. The pilot immediately shut down the engine, stopped the rotors, and exited the helicopter to inspect the tail rotor assembly. The tail rotor push-pull tube was found bent and severed aft of the tail rotor gear box, near the bell crank assembly. A postaccident examination revealed a bolt missing from the flex plate assembly, just forward of the tail rotor gearbox. The bolt was found in the tailboom just below the flex plate assembly and exhibited significant circumferential wear marks. A second bolt from the flex plate assembly exhibited similar wear; the two other bolts in the assembly exhibited no wear and were firmly torqued to the flex plate assembly. Black dust was observed on the flex plate and associated hardware, indicative of fretting. The helicopter went through a major overhaul in May of 2024, at which time the tail rotor drive shaft, aft flex plate assembly, and tail rotor push-pull tube were installed. Since May of 2024 the accident helicopter had flown about 338 hours. The most recent 100-hour inspection was completed on August 1, 2024, by the operator. The 100-hour inspection requires the operator to inspect the tail rotor flex plate assembly. A review of the airframe maintenance logbooks showed evidence of compliance with the helicopter manufacturer’s 100-hour inspection to include the flex plate assembly. The circumferential wear and black dust present during the postaccident examination indicate that the bolts were likely loose during the 100-hour inspection; however, no evidence of fretting or looseness was documented. It is likely that, over time, two of the bolts in the flex plate assembly became loose during operation because they were not properly torqued during installation.

On August 18, 2024, about 1600 Hawaii-Aleutian standard time, a Robinson R44 helicopter, N740RH, sustained substantial damage when it was involved in a pre-takeoff accident near Honolulu, Hawaii. The pilot was uninjured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 135 sightseeing flight. The pilot had completed four flights without incident and was starting the helicopter for the fifth flight when the accident occurred. The pilot reported she noticed an unusual vibration in the pedals as she was starting the helicopter, and the left pedal deflected forward to the stop and could not be recentered. The pilot immediately cut off fuel to the engine and disengaged the clutch without further incident. The pilot exited the helicopter and noticed the tail rotor push-pull tube was bent and severed aft of the tail rotor gear box near the bell crank assembly. According to the Robinson Illustrated Parts Manual, the attachment bolts holding the flex plate and yoke flange assembly together consisted of (2) NAS 6604-4 bolts, (2) NAS 6604-7 bolts, (8) NAS 1149F0432P (AN960-416L) washers, (4) D210-4 nuts, and (4) B330-13 palnuts. A postaccident examination of the helicopter revealed that 1 of the NAS 6604-4 bolts connecting the flex plate to the yoke flange of the tail rotor gearbox was missing. The bolt and associated washers were located at the bottom of the tailboom below the flex plate coupling assembly. The NAS 6604-4 bolt matched witness marks found on the fractured tail rotor push-pull tube. Both NAS 6604-4 bolts exhibited circumferential wear marks on the bolt shaft; the bolt threads were intact. The flex plate assembly exhibited deformation where the NAS 6604-4 bolts were attached; however, the NAS 6604-7 bolts and their attachment points on the flex plate exhibited no wear or deformation. Torque stripe was observed on the NAS 6604-7 bolts and evidence of torque stripe was noted on the 6604-4 bolts. The accident helicopter had gone through a major overhaul in May of 2024 and had flown about 338 hours since overhaul. A 100-hour inspection was completed on August 1, 2024. The 100-hour inspection requires the operator to inspect the tail rotor flex plate assembly. The mechanic who signed the logbook entry for the overhaul reported that he inspected the bolts connecting the tail rotor flex plate and yoke flange assembly by hand. He reported that all four bolts, their respective washers and palnuts were torqued correctly and had torque stripe. A review of the airframe maintenance logbooks showed evidence of compliance with the helicopter manufacturer’s 100-hour inspection; however, no evidence of fretting or looseness was documented. The Robinson R44 Maintenance Manual 100-hour procedure for the flex plate assemblies requires maintenance personnel to: “Inspect condition. Verify no obvious damage. If fretting is detected, replace the flex plate. Verify bonded washers are installed on both sides of flex plate arm. Verify security and operating clearance.”