What happened
On May 12, 2007, at approximately 15:24 JST, a privately operated Alexander Schleicher ASK23B glider, registration JA2409, was performing a pleasure flight near the Nagano City gliding field in Nagano Prefecture, Japan. The aircraft had been launched via winch and was engaged in ridge soaring between Mount Tenno and Mount Taro.
While executing a figure-eight pattern, the pilot attempted a left turn to avoid a mountain ridge. During this maneuver, the pilot temporarily lost visual contact with overhead power lines. Upon regaining sight of the wires, the pilot attempted immediate evasive action by increasing the bank angle and lowering the nose. However, the aircraft was already too close to the infrastructure, and the right wing struck the lowest power line on the valley side. The impact caused the aircraft to plummet into a forested area near the summit of Mount Kotaro, where the airframe was destroyed upon impact with the terrain and trees.
The investigation
The investigation examined the flight path using portable GPS data, which tracked the aircraft's altitude and position until the moment of the crash. Investigators also reviewed the pilot's statements, witness accounts from a local volunteer, and the aircraft's maintenance records. The investigation focused on the visibility of the power lines against the mountain backdrop and the effectiveness of the pilot's evasive maneuvers. The physical damage to the JA2409 was analyzed, specifically the destruction of the right wing and the breakage of the fuselage at the wing root. Additionally, the investigation reviewed the "Basic Rules for Slope Soaring" established by the local gliding association regarding altitude and safety margins near power lines.