What happened
On November 3, 2021, an Alexander Schleicher ASK13 glider, registration JA100K, was undergoing a training flight at the Shinshinotsu Gliding Field in Hokkaido, Japan. The flight was being conducted via a winch launch with a student pilot in the front seat and an instructor in the rear.
Prior to the launch, the crew experienced significant difficulty communicating with the winch operator due to poor radio signal quality. To bypass this, the launch director at the command post acted as a relay between the glider and the winch. During the takeoff roll, the launch director noticed the acceleration seemed slower than usual and repeated the launch command multiple times.
As the glider climbed, the winch operator—believing the repeated signals and the lack of visible movement indicated a severed towline—decided to stop the winch operation. The operator did not communicate this stoppage to the crew. Without the forward tension of the winch, the towline eventually disengaged from the glider's hook. The crew heard a distinct "clack" sound and realized the connection was lost. The instructor attempted to recover the aircraft by lowering the nose, but the glider's airspeed dropped well below stall speed. The aircraft subsequently performed a hard landing in a grass area, resulting in two serious injuries and damage to the aircraft's wing rib and control surfaces.
The investigation
The Japan Transport Safety Board (JTSB) examined the radio communication logs, the winch operation procedures, and video footage recorded by a head-mounted camera worn by the trainee. The investigation focused on the sequence of events leading to the towline disengagement and the decision-making process of the ground crew. Investigators also reviewed the Association's established winch launching protocols and the physical condition of the aircraft and winch equipment following the impact.