What happened
On June 29, 2020, at approximately 09:30 JST, a Fuji-Bell 204B-2 helicopter, registration JA9383, was performing a ferry flight from Nara Prefecture Heliport to the Takaoka operation site in Hyogo Prefecture. The aircraft was carrying four occupants, including the pilot and three mechanics.
As the helicopter approached the landing site at an altitude of roughly 40 feet, the crew began to hear unusual rhythmic noises emanating from the rear of the aircraft. This was quickly followed by a continuous banging sound. The crew identified the phenomenon as engine surging, noting significant fluctuations in the torque meter. In response to the power instability, the pilot attempted an immediate vertical landing. During the descent, the engine power dropped abruptly, causing the aircraft's descent rate to increase significantly. The helicopter struck the ground at a vertical speed of approximately 11.4 ft/s, exceeding the design limit. The impact caused substantial structural damage to the lower fuselage and landing gear.
The investigation
The Japan Transport Safety Board (JTSB) conducted a detailed teardown of the Textron Lycoming T53-L-13B engine. The investigation involved experts from the United States and Canada, representing the state of design and the state responsible for engine maintenance.
Investigators examined the axial compressor, where they discovered that one first-stage blade had fractured. The fracture surface showed evidence of high-cycle fatigue originating from subsurface damage. The release of this blade caused significant downstream damage to the compressor, stator vanes, and the gas producer turbine, characterized by thermal distress and material loss. The investigation also noted damage to the No. 1 bearing, which exhibited features consistent with skidding and thermal distress. Analysis of the landing impact confirmed that the descent rate exceeded the maximum design load of the landing gear cross tubes, leading to the buckling of the fuselage components.