What Happened
On 22 December 2020, a Robinson R44 II helicopter (registration VH-HOB) was conducting aerial agricultural spray operations near Clare Valley, South Australia. After completing several spraying runs, the pilot initiated a gentle descending turn toward the landing site to replenish chemical product. A loud bang was heard from the rear of the helicopter, followed by a rapid descent and jammed flight controls. The helicopter collided heavily with the loading vehicle and came to rest on its side. The pilot and ground crewman were uninjured, but the helicopter sustained substantial damage.
ATSB Investigation Findings
The Australian Transport Safety Bureau (ATSB) determined that the forward yoke of the helicopter’s clutch shaft failed due to a fatigue crack that originated from an indent in one of the yoke arms. This crack was not detected during inspections. The failure resulted in a loss of drive to both the main and tail rotor systems. The unrestrained clutch shaft displaced the hydraulic reservoir, causing loss of hydraulic fluid and removing hydraulic power assistance from the flight control servos. The pilot faced increased control stick forces and executed an emergency descent from low altitude without tail rotor control. The ATSB noted that the pilot had no training or prior experience for this compound emergency.
Maintenance and Inspection Issues
The yoke was not assigned a service life, so its continued airworthiness relied on inspections. The manufacturer’s maintenance instructions required verification that no cracks, corrosion, or fretting were present, but lacked specific methods for such verification. Visual inspections performed did not detect the crack on the forward face of the yoke because it was obscured by the flex plate after installation. The crack likely existed at the last 2,200-hour inspection, about two months and 188 flight hours before the accident, but was not visible during subsequent routine inspections. The presence of corrosion in the cracked region indicated the crack was present during that overhaul.
Actions Taken
Following the accident, the Robinson Helicopter Company introduced new maintenance requirements for the clutch shaft forward yoke at the 2,200/2,400-hour inspection for the R44. These included replacement of yokes with earlier revision status (A through G) and, for later revisions (H and subsequent), the option of replacement or a more detailed examination including magnetic particle inspection. The manufacturer also changed the yoke paint color from dark grey to white to enhance visibility of fretting dust. The ATSB issued a Safety Advisory Notice advising operators to look for corrosion, fretting, or cracking during inspections. The Civil Aviation Safety Authority, along with the European Union Aviation Safety Agency and the US Federal Aviation Administration, issued bulletins alerting pilots and maintenance personnel to monitor drive train components for signs of deterioration.
Safety Message
The investigation highlighted that non-life-limited components can develop defects and fail in flight. The occurrence underscored the importance of applying inspection criteria from manufacturer publications and contacting the manufacturer for clarification when instructions are unclear. It also reminded pilots and maintenance personnel to remain vigilant for defects in parts with a history of reliability.