What happened
On July 28, 1975, an L-13 Blanik glider, registration HB-995, was conducting a flight near Samedan, Switzerland. The pilot, accompanied by his daughter as a passenger, was attempting to utilize thermal lift near the Schafberg mountain. After an initial unsuccessful attempt to find lift at the Schafberg, the crew returned to the Muottas Muragl area before trying the Schafberg again.
During the second attempt to transition to the Schafberg, the aircraft was flying at approximately 80 km/h with the Fowler flaps partially extended. While flying close to the terrain, the aircraft suddenly banked to the right. In an attempt to recover, the pilot retracted the Fowler flaps. The aircraft entered a series of turns, losing altitude rapidly, and ultimately struck the western slope of the Unteren Schafberg at a steep angle. The impact resulted in the death of the pilot and serious injuries to the passenger. The aircraft was destroyed.
The investigation
An investigation conducted by the Swiss Transportation Safety Investigation Board (SUST) examined the aircraft wreckage, flight logs, and witness accounts. Investigators noted that the aircraft's weight and center of gravity were within permissible limits and that the aircraft was airworthy with no technical defects found.
Witnesses on the ground and in the air observed the glider performing tight circles near the slope, noting a significant loss of altitude and a steep impact angle of approximately 50 degrees. Testing conducted during the investigation revealed that retracting Fowler flaps after a stall or bank in this specific model often leads to a spin. Furthermore, discrepancies were noted in the aircraft's flight manual regarding the clarity of stall warnings.
Findings
- The primary cause of the accident was the underspeeding of the aircraft while flying in close proximity to the terrain.
- Contributing factors included inappropriate flight tactics and the improper handling of the Fowler flaps during the recovery attempt.
- Thermal lift conditions at the site were noted to be weak at the time of the accident.
- The pilot was highly experienced in alpine soaring, having completed numerous flights in the Engadin region.