ERA22LA256

Seconds after liftoff to a hover, the helicopter began to “aggressively shudder.” Loud, rapid reports were heard, and the nose of the helicopter yawed to the left. The instructor assumed control of the helicopter, closed the throttle, and performed an autorotation from a hover, landing the helicopter upright on its skids; however, the airframe sustained substantial damage. The instructor completed an engine shutdown, egressed the helicopter, and noticed that the main rotor was still turning while the tail rotor remained stationary. A cursory examination of the helicopter revealed that the clutch shaft yoke fractured at one of its two bolt lugs where the yoke attached to the flex plate. All other damage depicted appeared to be impact damage from the rotating assemblies left unsecured by the fractured lug. Examination of the damaged yoke and flexplate in the NTSB materials laboratory revealed a fractured attachment lug on the forward clutch shaft yoke due to fatigue failure with multiple origins.

On May 26, 2022, about 1340 eastern daylight time, a Robinson R44 helicopter, N132TC, operated by High Tide Helicopters LLC, was substantially damaged when it was involved in an accident near North Myrtle Beach, South Carolina. The flight instructor and the student pilot were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. According to the flight instructor, the student pilot performed a visual approach, which he terminated at a hover. The student landed the helicopter and then performed a takeoff to a hover. Seconds later, the helicopter began to “aggressively shudder.” Loud, rapid reports were heard, and the nose of the helicopter yawed to the left. The instructor assumed control of the helicopter, closed the throttle, and performed an autorotation from a hover, landing the helicopter upright on its skids. The event resulted in substantial damage to the airframe. The instructor completed an engine shutdown, egressed the helicopter, and noticed that the main rotor was still turning while the tail rotor remained stationary, and recorded the turning rotor with his telephone. A cursory examination of the helicopter revealed no visible exterior damage and “bits” of engine/transmission drive belts on the runway surface. A postaccident examination revealed that the clutch shaft yoke fractured at one of its two bolt lugs where the yoke attached to the flex plate. All other damage depicted appeared to be impact damage from the rotating assemblies left unsecured by the fractured lug. The fractured and separated lug piece from the yoke was recovered and retained for further examination, along with the components of the flex plate and yoke assemblies. Examination of the fractured surfaces of the accident bolt lug by an NTSB materials engineer revealed signatures consistent with fatigue. According to his report, “The fracture faces exhibited contact damage and gross mechanical distortion; however, two fatigue cracks were observed. One fatigue crack nucleated at the bolt hole bore chamfer at the outer periphery on the flex plate side. The fracture faces exhibited contact damage and progressed towards the shaft side of the lug. Fatigue crack progression marks were observed over about 80% of the fracture surface. A second fatigue crack nucleated under the washer of the mounting bolt on the flex plate side of the lug. The crack progressed through the lug thickness towards the shaft side of the lug and outwards towards the free surface of the lug. Fatigue crack progression marks were present over about 90% of the fracture surface.” Figure 1 Image of the as-received evidence with fractured lug in top center of frame. (NTSB) Examination of the nucleation sites did not reveal any crack initiation precursors, such as damage, inclusions (foreign object damage), or corrosion. The yoke-to-flex plate clamping surfaces between the attachment hardware, the yokes, and the lugs were clean and presented nopaint, primer, or inclusion. The actual torque values applied to the attachment hardware could not be determined. On June 21, 2021, the Australian Transport Safety Bureau (ATSB) had published a Safety Advisor Notice titled, R44 helicopter drive train failure, in which the circumstances and the damage described and depicted matched that of the accident helicopter. Figure 2 Fractured Clutch Shaft Yoke (Source: ATSB) On April 11, 2022, the Federal Aviation Administration published Special Airworthiness Information Bulletin (SAIB), AIR-22-08, Main Rotor Drive System, Tail Rotor Drive System to echo the Australians’ safety concerns and safety message, as such an event had not been reported in the United States or by any U.S. operators before that date.