What happened
On July 28, 2024, a privately owned Agusta A109E, registration JA02KG, was conducting a routine radio communication check near Yuki City, Ibaraki Prefecture. The flight, operated by a captain and a mechanic, proceeded normally until the aircraft was descending over Yachiyo Town at approximately 2,000 feet.
During the descent, a loud banging sound was heard, followed by a sudden drop in engine torque values to 1%. The pilot found that the collective pitch lever (CPL) no longer responded to inputs, making it impossible to maintain altitude. Despite the loss of power control, the pilot managed to avoid high-voltage power lines by performing a left descending turn and successfully executed a forced landing in an unused playground known as Higashi Fukist Sports Park.
During the landing, the helicopter struck the park's outer fence, causing minor damage to the airframe and the destruction of a section of the fence. There were no injuries to the two occupants, and no fire occurred.
The investigation
The Japan Transport Safety Board (JTSB) examined the wreckage and the maintenance history of the aircraft. Investigators discovered that the lower link of the rotorcraft's scissors had become completely detached from the upper swashplate stud bolts. The associated cotter pin, nut, and washers were missing from the site.
Review of maintenance records showed that the scissors linkage had been replaced during periodic checks conducted between April and June 2024. Upon inspecting the remaining hardware, investigators found that the upper cotter pin had been improperly installed; it was not sufficiently bent to secure the components and lacked the required anti-corrosive coating. The investigation also noted that the maintenance tasks for these components were performed by a single mechanic.
Findings
- The sudden loss of control was caused by the detachment of the scissors lower link from the upper swashplate.
- This detachment caused the engine torque to drop significantly because the movement of the upper swashplate could no longer be effectively transmitted to the main rotor blades.
- The improper installation of the self-locking nut and cotter pin is the primary reason the assembly failed during flight.
- The failure was likely due to insufficient tightening torque, inadequate bending of the cotter pin, or the omission of anti-corrosive compounds as required by the maintenance manual.
- Inadequate verification of the installation work following the recent maintenance of the scissors assembly contributed to the incident.