Accident Details
On May 26, 2022, at approximately 1340 eastern daylight time, a Robinson R44 helicopter, registration N132TC, operated by High Tide Helicopters LLC, was substantially damaged in an accident near North Myrtle Beach, South Carolina. The helicopter was conducting a Title 14 Code of Federal Regulations Part 91 instructional flight with a flight instructor and a student pilot aboard; neither occupant was injured.
According to the flight instructor, the student pilot performed a visual approach that terminated in a hover, then landed and executed a takeoff to a hover. Seconds later, the helicopter began to “aggressively shudder.” Loud, rapid reports were heard, and the nose yawed left. The instructor assumed control, closed the throttle, and performed an autorotation from a hover, landing the helicopter upright on its skids. The event caused substantial damage to the airframe.
After engine shutdown and egress, the instructor observed the main rotor still turning while the tail rotor remained stationary, and recorded the turning rotor with his telephone. A cursory examination revealed no visible exterior damage, but “bits” of engine/transmission drive belts were found on the runway surface.
Postaccident Examination
A postaccident examination determined that the clutch shaft yoke had fractured at one of its two bolt lugs where the yoke attached to the flex plate. All other damage appeared to be impact damage from rotating assemblies left unsecured by the fractured lug. The fractured and separated lug piece was recovered and retained for further examination, along with components of the flex plate and yoke assemblies.
Fatigue Findings
An NTSB materials engineer examined the fractured surfaces of the accident bolt lug. The examination revealed signatures consistent with fatigue. According to the engineer’s 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.”
Examination of the nucleation sites did not reveal any crack initiation precursors, such as damage, inclusions, or corrosion. The yoke-to-flex plate clamping surfaces between attachment hardware, yokes, and lugs were clean, with no paint, primer, or inclusion. Actual torque values applied to the attachment hardware could not be determined.
Related Safety Advisories
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 damage described and depicted matched those of the accident helicopter. 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, noting that such an event had not been reported in the United States or by any U.S. operators before that date.