What happened
On August 2, 1974, a two-seat L-13 Blanik D-1160 glider was conducting a training flight near Muottas Muragl, Switzerland. The flight involved two pilots, one of whom was a flight instructor acting in a passenger capacity. After being released from a winch launch at an altitude of approximately 550 meters, the aircraft flew along the slope of the mountain.
During the flight, the aircraft was flying at a speed of approximately 80-85 km/h. As the glider approached the mountain slope, it entered a zone of significant turbulence caused by the Malojawind. Witnesses observed the glider flying at a very low altitude, estimated between 20 and 80 meters. Suddenly, the aircraft pitched forward and entered a vertical descent. The impact occurred at an altitude of 2450 meters, resulting in the death of the front-seat pilot and leaving the rear-seat pilot with severe injuries. The aircraft was destroyed.
The investigation
An investigation conducted by the Swiss authorities examined the flight path, weather conditions, and the mechanical state of the L-13 Blanik D-1160. Investigators reviewed witness accounts from individuals near the crash site, including another glider pilot who observed the aircraft entering a vertical right-hand spin.
Technical inspections of the wreckage revealed that the flight controls, including the ailerons, elevators, and rudder, were in working order and properly connected. There were no signs of mechanical failure or technical defects. The investigation also analyzed the meteorological conditions, noting that the crash site was at the upper limit of the Malojawind, a region characterized by wind shear and increased turbulence.
Findings
- The primary cause of the accident was the undershooting of the minimum flight speed while flying at a low altitude above the terrain.
- Strong atmospheric turbulence in the area likely caused the airflow to separate from the wing, leading to the stall.
- The aircraft was flying with insufficient speed margin, making it highly vulnerable to the turbulent air.
- There was evidence of unclear command authority on board, as the rear-seat pilot had been providing instructions to the front-seat pilot throughout the flight, yet failed to intervene effectively during the critical phase.
- The extremely low altitude prevented the crew from recovering the aircraft once the nose dropped.