What happened
On July 25, 2020, an ASK 23 B glider, registration HB-1796, was performing a training flight during a gliding camp at Zweisimmen Airfield (LSTZ). The aircraft was being towed by a Robin DR 400. During the takeoff roll on the 587-meter paved runway, the glider's nose wheel initially remained on the ground before the aircraft suddenly rotated and lifted off the runway. This initial rotation triggered a series of oscillations around the lateral axis, known as porpoising, where the glider repeatedly struck the runway.
As the towplane continued its climb, the glider descended and struck the runway again. This second impact initiated a second, more violent cycle of oscillations. The impacts became increasingly severe, causing visible deformation of the wings and significant damage to the runway surface. The pilot eventually decided to abort the takeoff by releasing the towline. Following the release, the glider entered a steep climb that the pilot could not arrest due to damaged flight controls. The aircraft eventually pitched nose-down and landed in the meadow adjacent to the runway. The pilot was uninjured, but the glider sustained heavy damage to the fuselage, landing gear, and control systems.
The investigation
Investigators examined the aircraft, GPS data, video recordings, and runway markings. They found that the impact of the nose gear had deformed the fuselage structure, which restricted the movement of the elevator. Specifically, the elevator could only move between neutral and full upward positions and could no't be moved forward. The investigation also reviewed the pilot's training history, noting significant experience in winch launches but limited recent experience in aerotow operations. Furthermore, the investigation analyzed conflicting manufacturer and regulatory guidance regarding takeoff techniques and trim settings.
Findings
- The primary cause of the accident was a late decision to abort the takeoff once the oscillations began.
- A nose-heavy elevator trim setting contributed to the onset of the oscillations.
- The pilot's focus on stabilizing the aircraft during the first oscillation likely led to a failure to notice the glider's descent during the second phase of the takeoff.
- Conflicting operational guidance regarding the timing of liftoff and the use of nose-wheel pressure may have contributed to the instability.