What happened
On December 9, 2007, a Eurocopter EC135T2 helicopter, registered JA31NH, departed Tokyo Heliport on a ferry flight destined for Shizuoka Heliport. The aircraft was operated by All Nippon Helicopter Co., Ltd. and was carrying a captain and a mechanic.
During the flight, while passing near Lake Ashinaka, the aircraft experienced a sudden and severe swing. The captain discovered that the rudder was non-functional, despite applying full left pedal input. The mechanic on board reported that the aircraft's attitude worsened when attempting to troubleshoot by returning the pedals to neutral. Despite the crew's efforts to manage the emergency and a decision to attempt a landing at the destination heliport, the aircraft crashed into a marsh in Shizuoka City. The accident resulted in one fatality (the captain) and one serious injury (the mechanic). The aircraft was destroyed in the impact.
The investigation
The Japan Transport Safety Board (JTSB) conducted an extensive investigation involving the examination of the wreckage, flight control characteristics, and the physical condition of the tail rotor components. Investigators analyzed the tail rotor control rod and the ball pivot assembly to determine why the control system failed. The investigation included metallurgical analysis of the fractured rod and reviews of maintenance records and manufacturer service bulletins.
Findings
Technical analysis revealed that the tail rotor control rod ruptured due to fatigue. The investigation established that the ball pivot assembly had become stiff, which likely contributed to the mechanical stress on the rod. Specifically, the investigation found that the threaded area of the rod had become loose, leading to the development of a crack.
Furthermore, the investigation identified that the failure was linked to the specific design of the rod's threaded area and the ball pivot. While the manufacturer had previously issued service bulletins regarding the replacement of these components with newer versions, the replacement was not mandatory at the time of the accident. The mechanical failure of the rod prevented the pilot from maintaining directional control, ultimately leading to the loss of the aircraft.