What happened
During a flight approximately 13 km south-east of Clare Valley Aerodrome, South Australia, a Robinson R44 II, registration VH-HOB, experienced a critical mechanical failure. A fracture in the clutch shaft forward yoke caused the engine to lose drive to both the main and tail rotor systems. This failure forced the pilot into an emergency descent from an altitude of approximately 100 feet. During the descent, the pilot encountered significant control difficulties and was unable to avoid a collision with a ground vehicle, resulting in a collision with terrain.
The investigation
The investigation focused on the structural failure of the yoke and the effectiveness of existing maintenance protocols. Investigators determined that the yoke failed due to fatigue crack propagation that began on the forward face of the yoke arm near a bolt hole. One side of the crack originated from a mechanical surface defect. The presence of corrosion and crack progression bands indicated that the fatigue process had been occurring for a significant period, likely spanning across an overhaul cycle.
Upon the fracture of the yoke, the clutch shaft disconnected from the main gearbox. This caused the upper and lower sheaves to misalign and the vee-belts to displace, which directly caused the loss of power to the rotor systems. While the aircraft's drive train had been inspected on the morning of the accident and during the previous scheduled interval, the specific location of the crack remained hidden.
Findings
- The fatigue crack was located on the forward face of the yoke arm, a position that is obscured by the forward flex plate once the component is installed.
- Because the crack was hidden by the flex plate, it was not detectable during routine daily or 100-hour inspections.
- The only opportunity for detection was during a 2,200-hour inspection or unscheduled disassembly, yet the crack likely existed during the most recent 2,200-hour inspection held 188 flight hours prior.
- The manufacturer's maintenance instructions lacked a specific, standardized method for verifying the absence of cracks, leaving the inspection subject to interpretation.
- While the maintenance organization used visual inspections, the manufacturer utilized more rigorous magnetic particle inspections when components were returned for assembly, highlighting a discrepancy in inspection depth.
Safety message
Visual inspections may be insufficient for detecting cracks in critical components where the defect is obscured by hardware. The lack of specific, mandatory inspection methods for verifying the integrity of the yoke increased the risk that a developing crack would remain undetected.