What happened
On October 28, 1967, a student pilot was performing a training flight at the Thun airfield in Switzerland. The flight involved a powered tow using a Stinson aircraft to pull a Grunau Baby II glider, registration HB-1abl, to an altitude of approximately 300 meters.
As the tow aircraft signaled for release by rocking its wings, the student pilot attempted to trigger the release mechanism. However, the glider failed to detach immediately from the tow rope. As the aircraft continued to be pulled, its speed increased significantly, exceeding the maximum allowable towing speed of 120 km/h by at least 30 km/h. In an attempt to reduce this speed, the pilot pulled back on the control stick. This action, combined with the tension of the rope, caused the glider to pitch up sharply and enter a spin.
Surprised by the sudden loss of control, the pilot decided to abandon the aircraft. After removing the canopy and unbuckling the harness, the pilot exited the glider and deployed a parachute. While the pilot's descent was initially stable, the unpiloted glider continued to spin in a shallow spiral near the descending pilot before impacting the ground. The pilot sustained a fractured tibia upon landing.
The investigation
Investigators examined the mechanical functionality of the release mechanism and the tow rope. Testing confirmed that the release hook and the rope itself were in good working order. The investigation also noted that the aircraft had recently undergone an inspection where several deficiencies were noted, including a lack of clear labeling on the control handles.
Regarding the pilot's actions, the investigation considered whether the student had pulled the incorrect lever, as the release handle on this model was located on the instrument panel, unlike the student's previous training aircraft. However, the student maintained he used the correct handle. The investigation also looked into the student's flight experience, noting he had only completed a few solo flights prior to the accident.
Findings
- The primary cause of the accident was the improper execution of the release maneuver by the student pilot, which left the glider attached to the tow plane at excessive speeds.
- The student's subsequent reaction to the resulting spin—pulling the control stick back—directly contributed to the aircraft's sudden pitch-up and loss of control.
- The pilot's decision to parachute was deemed an inappropriate response to the flight condition, as neutralizing the controls could have recovered the aircraft from the spin.
- The pilot's focus on the descending aircraft during the parachute descent led to a distracted landing, resulting in a leg injury.