What happened
On September 14, 2004, an Aero Asahi Corporation McDonnell Douglas MD900, registration JA6757, was performing a return flight to Tokyo Heliport via a refueling stop at Shizuoka. During the flight, the crew experienced a sudden loss of yaw control, which was immediately followed by the failure of the No. 2 engine. The pilot attempted to manage the emergency by holding over the Shinomiya helipad to reduce aircraft weight through fuel consumption before deciding to divert to Atsugi Airfield.
During the approach to the airfield's west apron, the pilot attempted a running landing on a grass area. While attempting to align the aircraft with the runway, the helicopter entered an uncontrollable spin. The aircraft ultimately made a hard landing, resulting in substantial damage to the fuselage, tail boom, skids, and main rotor blades. Both the pilot and the mechanic sustained minor injuries.
The investigation
The investigation focused on the mechanical failure of the NOTAR (No Tail Rotor) system. Investigators examined the aircraft's wreckage and performed a detailed analysis of the NOTAR fan assembly. This included a close examination of the T-T (tension-torsion) strap, a critical component of the fan assembly. Technical modeling was used to analyze the stresses placed on the Kevlar fiber material within the strap. Additionally, the investigation reviewed maintenance logs, crew statements, and the manufacturer's fatigue load test spectrum to determine why the component failed.