What happened
On September 27, 2014, an ASK 21 glider, registration HB-1620, was participating in a training course at the Olten airfield. The flight was a winch launch intended to practice emergency procedures. During the ascent, the instructor initiated an unannounced simulation of a cable break by releasing the winch cable at a low altitude.
Following the release, the pilot attempted to execute a left-hand turn to perform a landing on the opposite runway. However, the aircraft's airspeed decreased significantly during the maneuver. As the glider entered a precarious state, the instructor took control to prevent a nose-first impact. Despite these efforts, the aircraft struck the ground tail-first, causing the horizontal stabilizer to break off and the fuselage to fracture behind the wings. The aircraft slid into a hedge where it eventually came to a stop. Both occupants escaped the wreckage without injury.
The investigation
SUST examined the flight path using FLARM data, which confirmed the aircraft's altitude and trajectory. The investigation compared the accident flight to a successful simulation performed earlier that same day. Investigators analyzed the flight instructor's experience levels, noting that while the instructor was highly qualified in general instruction, they had not performed a winch launch in over a year and had no prior experience instructing winch launch procedures. The investigation also reviewed the pilot's reaction to the sudden release and the instructor's intervention timing.
Findings
- The primary cause of the accident was the loss of control during a low-altitude turn following a simulated cable break.
- The direct causes were a misjudgment of the altitude at the moment of cable release and the subsequent selection of an unsuitable flight path.
- Contributing factors included the instructor's lack of recent experience with winch launches and a lack of specific experience in instructing such maneuvers.
- The instructor's intervention to regain control was delayed due to distractions during the verbal exchange with the pilot regarding the landing plan.