What happened
On November 8, 2017, an Aerospatiale AS332L helicopter, registered JA9672 and operated by Toho Air Service Co., Ltd., was performing a ferry flight from a temporary helipad in Yamanashi Prefecture to Tochigi heliport. During the flight, the crew experienced abnormal vibrations throughout the airframe. In an attempt to manage the instability, the pilot initiated an emergency landing procedure. However, while over Ueno-mura in Gunma Prefecture, the tail rotor separated from the aircraft. This sudden loss of anti-torque control caused the helicopter to become uncontrollable, resulting in a crash at approximately 14:29 JST. The impact destroyed the aircraft and triggered a fire. All four crew members on board—the captain and three mechanics—were killed.
The investigation
The Japan Transport Safety Board (JTSB) examined the wreckage and the maintenance history of the aircraft. Investigators focused on the mechanical state of the tail rotor assembly, specifically the flapping hinge of the white blade. The investigation scrutinized the maintenance records of the company, looking for evidence of unreported malfunctions or failures to follow manufacturer instructions regarding grease usage and environmental storage conditions. The JTSB also analyzed the physical condition of the components, including the spindle bolt and the bearings within the flapping hinge.
Findings
Technical analysis revealed that the tail rotor separation was triggered by an imbalanced rotation of the rotor assembly. This imbalance was caused by the fracture of the spindle bolt located at the flapping hinge of the white blade. The investigation determined that the bolt failed because the bearings within the flapping hinge had become damaged and stuck. This mechanical failure generated the excessive vibrations felt by the crew prior to the separation. Furthermore, the investigation found that the degraded condition of these bearings had not been identified during routine inspections or maintenance tasks. Additionally, there were indications that maintenance information regarding grease usage and the effects of high temperature and humidity on parked aircraft had not been effectively communicated to the mechanics.