What happened
On April 28, 2012, at approximately 13:55 local time, a towing operation involving two privately owned gliders ended in a ditching in the Yoshii River near the Oku Gliding Field in Setouchi City, Japan. The operation involved a Scheibe SF25C motor glider, registered JA21KA, acting as the tug, and a Schempp-Hirth Discus B glider, registered JA2376, being towed.
During the initial climb following takeoff, the towing aircraft experienced two sudden, unexpected climbs of approximately five meters each. These disturbances occurred while the aircraft was still accelerating toward its required towing speed. Because the JA21KA was unable to maintain the necessary speed to effectively pull the heavier requirements of the JA2376, the pilot faced a critical decision regarding the towing line. To avoid a landing attempt on the airfield that would have exceeded the remaining runway distance, the pilot attempted to steer the formation toward the river to release the line safely. However, the towed glider began to descend and slide to the left, eventually ditching into the river from its left wing. The impact and subsequent tension on the tow line pulled the towing aircraft into the water as well.
The investigation
The Japan Transport Safety Board (JTSB) examined the flight history, aircraft specifications, and meteorological conditions. Investigators analyzed the speed differentials between the two aircraft, noting that the Schempp-Hirth Discus B had a higher minimum operational speed than the Scheibe SF2 .5C. The investigation also reviewed the impact of local topography, specifically a 50-meter hill located east of the airfield, which could contribute to air disturbances. The investigation confirmed that while there were no fatalities, both aircraft sustained significant damage, and the trainee pilot briefly lost consciousness due to the impact of the ditching.