Accident Details
On August 18, 2024, at approximately 1600 Hawaii-Aleutian standard time, a Robinson R44 helicopter, registration N740RH, sustained substantial damage during a pre-takeoff accident near Honolulu, Hawaii. The pilot was uninjured. The helicopter was being operated as a Title 14 Code of Federal Regulations Part 135 sightseeing flight.
Pilot Actions
The pilot had completed four previous flights without incident and was starting the helicopter for the fifth flight when the accident occurred. She reported noticing an unusual vibration in the pedals during startup, 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. After exiting, she observed the tail rotor push-pull tube was bent and severed aft of the tail rotor gear box near the bell crank assembly.
Postaccident Examination
A postaccident examination revealed that one of two NAS 6604-4 bolts connecting the flex plate to the yoke flange of the tail rotor gearbox was missing. The missing bolt and associated washers were found 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; however, their threads were intact. The flex plate assembly showed deformation where the NAS 6604-4 bolts were attached, while the NAS 6604-7 bolts and their attachment points displayed no wear or deformation. Torque stripe was observed on the NAS 6604-7 bolts and evidence of torque stripe was noted on the NAS 6604-4 bolts.
Maintenance History
The accident helicopter had undergone a major overhaul in May 2024 and had accumulated about 338 flight hours since that overhaul. A 100-hour inspection was completed on August 1, 2024, which requires inspection of the tail rotor flex plate assembly. The mechanic who signed the logbook for the overhaul reported that he inspected the bolts connecting the tail rotor flex plate and yoke flange assembly by hand. He stated 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.
Manufacturer Procedures
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.”