What happened
On November 8, 2014, an Alexander Schleicher ASK18 glider, registration JA2320, was participating in a leisure flight at the Kirigamine Gliding Field in Nagano Prefecture, Japan. The flight, operated by the Suwa City Glider Association, was initiated via a winch launch.
During the ascent, the pilot encountered a crosswind that caused the aircraft to yaw left due to the weathercock effect. Simultaneously, the winch winding speed decreased. The pilot, distracted by attempts to correct the heading and expecting the winch speed to recover, did not immediately notify the operator of the slow tow or release the towline. Eventually, at an altitude of only three to four meters, the launch director signaled a stop to the winch, prompting the pilot to release the towline.
Believing the aircraft could clear an area of rough, wooded terrain ahead, the pilot delayed landing. However, the glider lost altitude and speed, eventually touching down on the sloping, uneven ground. The impact caused the aircraft to shake violently to the right, resulting in structural damage.
The investigation
The JTSB investigation focused on the sequence of events following the towline release and the physical evidence at the site. Investigators examined the aircraft's damage, which included a bent fuselage, a damaged tail wheel, and a dent in the leading edge of the right wing.
At the accident site, investigators located a broken tree that matched the damage found on the glider's wing. The investigation also reviewed the winch operator's actions, noting that the slow winding speed was not due to mechanical failure but was a result of the operator being overly cautious due to the lighter weight of the single-seater aircraft compared to larger models. Furthermore, the topography of the field prevented the operator from seeing the launch point, making the pilot's lack of communication even more critical.
Findings
- The primary cause of the accident was the pilot's failure to land in a predetermined safe area immediately after the towline was released during a slow-tow emergency.
- The pilot's decision to attempt to fly over the rough terrain was influenced by a desire to extend the flight duration and a fear of damaging the tail wheel through an abrupt landing using dive brakes.
- The pilot was distracted by the need to correct the aircraft's heading in the crosswind, which prevented the timely reporting of the slow winch speed.
- The winch operator's cautious approach to the launch contributed to the low winding speed, but the lack of communication from the cockpit meant the operator was unaware of the developing situation.