What happened
On July 9, 1972, a Slingsby Skylark 3 F, registration HB-665, departed from Bex aerodrome for a training flight in the Morgins region. During the tow climb over Vionnaz and Troistorrents, the aircraft encountered significant turbulence. A gust of wind likely caused the automatic release mechanism to trigger, resulting in the glider becoming unattached from the tow plane at an altitude of approximately 1,925 meters, northwest of Troistorrents.
Following the release, the pilot continued the flight toward Morgins, maintaining an average descent rate of 3.5 m/s. The flight path took the glider over the southern slope of the valley toward the Val d’Illiez, where the altitude reached 1,520 meters. The pilot then began a ridge soaring maneuver along the southern crest, heading back into the Val de Morgins. An observer on the northern side of the valley noted the glider flying southwest just below the ridge line. As the aircraft attempted to clear a forested area, it entered a right-hand turn, lost altitude, and struck a group of fir trees. The Slingsby Skylark 3 F ultimately crashed into a pasture at an altitude of 1,805 meters. The pilot sustained grievous injuries, and the aircraft was destroyed.
The investigation
The investigation examined the pilot's credentials, the mechanical state of the aircraft, and the prevailing meteorological conditions. The pilot held a valid glider license, though total flight experience was limited to 23 hours and 28 minutes, with only 3 hours and 13 minutes specifically on this aircraft type. There was no evidence of health issues or prior regulatory infractions.
Technically, the aircraft was airworthy and properly maintained. Investigators determined that the accidental release of the tow hook was not due to any mechanical failure of the release mechanism itself. Meteorological data indicated a weak foehn wind on the northern side of the Alps, with an approaching thunderstorm from the southwest and local thermal turbulence.
Findings
While the accidental release from the tow plane occurred early in the flight, investigators concluded this was not the primary cause of the accident, as the pilot maintained sufficient altitude to return to the departure airfield. The investigation established that the loss of airspeed was the critical factor. It is believed that the pilot's flying speed was insufficient to maintain control while navigating the heavy turbulence present in the area, leading to the descent into the trees.