What happened
On July 20, 2008, at approximately 10:23 JST, a privately owned Alexander Schleicher ASK 13 glider, registration JA2130, crashed at the Yomiuri Otone Gliding Field in Saitama Prefecture, Japan. The accident occurred during a training session while the aircraft was being launched via a winch system.
A student pilot was the sole occupant of the aircraft. During the fourth launch of the session, the aircraft took off but failed to maintain sufficient airspeed. The pilot, noticing a lack of acceleration, requested more power via radio. However, the aircraft's speed dropped significantly as the winch operator abruptly halted the launch. The pilot attempted to release the cable and pitch the nose down to regain airspeed, but the aircraft was at too low an altitude to recover, resulting in a ground impact that caused serious injuries to the pilot and substantial damage to the aircraft's fuselage, wings, and empennage.
The investigation
The JTSB investigation focused on the mechanical state of the winch and the sequence of the launch. Investigators examined the winch, its operating manuals, and interviewed the student pilot, the instructor, the winch operator, and piste staff. The investigation revealed that while the operator intended to launch the aircraft using the right drum, the left drum—which was supposed to be disengaged—began winding in its cable simultaneously. This unexpected movement caused the left cable to strike the winch structure, creating loud noises and sparks, which prompted the operator to immediately abort the launch.
Findings
- The primary cause of the crash was the sudden suspension of the winch launch, which left the aircraft with insufficient energy to maintain flight.
- The winch operator failed to fully disengage the left drum, leaving the selector lever in an intermediate position.
- A mechanical deficiency in the winch allowed both drums to be engaged at once because the stopper was too short to prevent the levers from reaching a simultaneous engagement position.
- The winch operator was unaware that the left drum was winding because he did not expect it to move and the indicator light for that drum was non-functional.
- Inadequate management of the winch was noted, as the owner lacked the manufacturer's operating manual, had no maintenance logs, and continued to use the winch despite a malfunctioning selection indicator light.