What happened
On April 18, 1971, a pilot departed from Lausanne-La Blécherette airfield in a Standard-Cirrus glider, registration HB-987, intending to complete a flight between Lausanne and Courtelary. After reaching Courtelary and turning back, the pilot encountered deteriorating flying conditions. While navigating the region near Colombier, the pilot noted that thermal activity was weak over the plateau but observed flags moving on the south slope of the Montagne de Boudry, suggesting potential ridge lift.
Seeking to utilize this dynamic lift, the pilot began performing figure-eight turns along the slope. During the second turn, the aircraft experienced a sudden drop of the right wing. The glider struck the treetops and plummeted vertically through the forest canopy before coming to rest on the ground. The pilot sustained grievous injuries, and the aircraft was severely damaged.
The investigation
An investigation conducted by the commission, in collaboration with local authorities, examined the aircraft and the flight conditions. The investigation confirmed that the pilot held all necessary licenses and medical qualifications for the flight. The aircraft, which had been recently imported, was found to be in a proper weight and balance configuration, with the center of gravity within authorized limits.
Physical examination of the Standard-Cirrus revealed no evidence of mechanical failure or technical defects prior to the impact. Investigators noted that the landing gear was in the retracted position, and the locking mechanism had been disengaged only by the force of the impact. Evidence from broken trees and the nose of the fuselage confirmed that the aircraft struck a tree head-on before descending into the woods.
Findings
- The pilot was attempting to utilize ridge lift along the mountain slope.
- The aircraft encountered turbulence while performing turns near the terrain.
- The accident was caused by a loss of airspeed during a turn while flying at an altitude too low to recover from a stall.