What happened
On April 10, 2016, a single-seater S.N. Centrair C 101B glider, registration JA2437, was performing a familiarization flight at the Aso Tourism Ranch in Kumamoto Prefecture, Japan. During a winch launch from runway 26, the aircraft began climbing at a 30-degree pitch when the airspeed started to drop from 100 km/h.
The pilot initially reported the deceleration to the winch operator via radio but did not immediately realize the tow line had lost tension. As the airspeed fell to approximately 70 km/h, the pilot manually released the tow line. Attempting to manage the descent, the pilot initially planned a straight-ahead landing. However, believing the aircraft was at a higher altitude than it actually was, the pilot decided to execute a steep left turn to land elsewhere.
During this maneuver, the pilot retracted the air brakes and maintained a 45-degree bank. Because the pilot had misjudged the actual height above ground level (AGL), the aircraft's left wing struck a grove of trees. The impact caused the aircraft to rotate violently, fracturing the fuselage and breaking the wing tip before the aircraft finally came to rest on the lawn.
The investigation
The Japan Transport Safety Board (JTSB) examined the mechanical state of the winch, the flight records from a cockpit video camera, and the pilot's decision-making process. The investigation focused on the sudden loss of airspeed, the functionality of the automatic release mechanism in the tow hook, and the discrepancy between the pilot's perceived altitude and the actual altitude recorded by the camera.
Investigators also scrutinized the maintenance history of the winch, which was a self-assembled unit constructed from various truck parts and had been in service for over 20 years. The investigation found that no formal design documents, manufacturing specifications, or maintenance logs existed for this custom-built winch.
Findings
- Winch failure: The primary trigger for the accident was the fracture of the winch's propeller shaft. This failure interrupted the towing force, causing the airspeed to decay.
- Altitude miscalculation: The pilot failed to accurately estimate the AGL because the barometric altimeter displayed altitude in feet, while the club's safety criteria were provided in meters. The pilot had not prepared a mental or physical conversion for these units.
- Decision making: The pilot's decision to attempt a turn rather than a straight-ahead landing was based on an overestimated altitude. The investigation concluded that a straight-ahead landing would have been the safest option.
- Maintenance deficiencies: The winch's propeller shaft was a complex, consolidated part that was difficult to inspect visually. The lack of standardized inspection protocols for this self-built equipment likely contributed to the undetected fatigue that led to the fracture.