Accident Summary
On November 8, 2017, at approximately 14:29 JST, an Aerospatiale AS332L helicopter, registered JA9672 and operated by Toho Air Service Co., Ltd., crashed in Ueno-mura, Tano County, Gunma Prefecture, Japan. The helicopter was on a ferry flight from Arakura temporary helipad in Hayakawa Town, Yamanashi Prefecture, to Tochigi heliport. All four crew members (a captain, a mechanic in charge, and two mechanics) were killed. The aircraft was destroyed and a fire broke out.
Flight History
According to flight records and radar track data, the helicopter departed Arakura temporary helipad and was en route when abnormal vibrations occurred. The crew attempted an emergency landing. During this attempt, the tail rotor separated from the airframe, leading to loss of control and the crash. Witnesses reported unusual sounds and erratic flight behavior before impact.
Damage to the Aircraft
The helicopter was completely destroyed by impact and subsequent fire. Examination of the wreckage revealed that the tail rotor had separated in flight. The fracture of the spindle bolt on the flapping hinge of the White Blade was identified as the initiating failure. The bearings of the flapping hinge were found damaged and stuck, indicating pre-existing corrosion and wear that had not been detected during maintenance.
Probable Causes
The Japan Transport Safety Board determined that the accident was highly probable due to the following sequence: The helicopter experienced abnormal vibrations in flight, prompting an emergency landing. During this maneuver, the tail rotor separated, causing a loss of control and crash. The separation resulted from imbalanced rotation caused by the fracture of the spindle bolt of the flapping hinge of the White Blade. This fracture was caused by damaged and stuck bearings of the flapping hinge. The damaged condition of the bearings was not identified during inspections and maintenance, leading to failure to take appropriate corrective actions.
Maintenance Issues
Investigation revealed that the damaged bearings were not reported or addressed in prior maintenance. The company had not fully disseminated technical information from Airbus Helicopters regarding grease usage and procedures for parking in high-temperature, high-humidity environments. These factors likely contributed to the bearing deterioration but are not stated as direct causes in the source.
Recommendations
The Japan Transport Safety Board recommended that Toho Air Service improve malfunction reporting to designers and manufacturers, and ensure that maintenance information from manufacturers is promptly reviewed and shared with mechanics.