What happened
On June 29, 1969, at approximately 14:34, a Kranich III glider, registration HB-538, departed from the Bex aerodrome accompanied by one passenger. During the takeoff sequence, the pilot observed that the tow aircraft was unable to gain sufficient altitude. In response to this loss of climb performance, the pilot released the tow rope and performed an emergency landing. The impact of the landing resulted in severe damage to the glider. Both the pilot and the passenger remained uninjured.
The investigation
The investigation focused on the mechanical circumstances that prevented the tow aircraft from maintaining a safe climb gradient. Investigators examined the configuration of the glider at the moment of takeoff and the performance of the tow operation.
Findings
The investigation established that the primary cause of the incident was that the glider's air brakes deployed spontaneously during the takeoff roll without the pilot's knowledge. This unintended deployment increased aerodynamic drag significantly, which prevented the tow aircraft from achieving the necessary altitude to continue the flight.