What happened
On May 25, 2024, an Alexander Schleicher ASK13 glider, registration JA2189, was participating in its first solo aerotow flight training at the Aso Temporary Operation Site in Kumamoto Prefecture. The glider was being towed by an Avions Pierre Robin DR40/180R.
During the ascent, the aircraft's speed increased beyond the operational limit of 140 km/h. As the tow plane executed a 90-degree turn, the speed reached 170 km/h, causing severe slack in the towline. Fearing the line might entangle the glider's wing, the pilot released the towline. Because the aircraft was at an altitude of only 320 meters—insufficient to return to the departure site—the pilot attempted a forced landing in a nearby grassland. However, the chosen field featured a 6-degree downslope. The pilot was unable to arrest the descent sufficiently, resulting in an overshoot where the glider's wing struck trees, causing substantial damage to the airframe, including the nose, wings, and stabilizer.
The investigation
The JTSB investigation focused on the flight parameters, the pre-flight coordination, and the suitability of the landing site. Investigators analyzed GPS data, which revealed that the climb rate became negative once the speed exceeded 140 km/h. The investigation also examined the pre-flight briefing, noting that the manager of the flying club had coordinated the flight plan with the tow pilot rather than the glider pilot directly. This briefing failed to communicate critical constraints, such as the specific aerotow speed limit or the minimum altitude required for an emergency return to the airfield.
Findings
- The tow pilot climbed at a rate that did not account for the altitude necessary to reach the departure site in the event of a towline break.
- The tow pilot was unaware of the 140 km/h speed limit for the glider, leading to an increase in speed that caused the towline to slacken.
- The glider pilot, being on their first solo aerotow, lacked the experience to select an appropriate landing site, specifically choosing a downslope field that made a safe touchdown impossible.
- The pre-flight briefing was inadequate, as it failed to establish mutual understanding regarding speed limits and emergency procedures.