What happened
On May 10, 1979, a Bell 206 B helicopter, registration HB-XDP, was engaged in mountain landing training near the Col des Mosses in Switzerland. After performing several training landings in the Bernese Oberland and Valais regions, the crew landed at the Col des Mosses landing site at approximately 12:15 local time.
During the takeoff attempt shortly before 13:00, the student pilot initiated the turbine start. As the engine power increased, the student noticed the aircraft tilting to the right. In an attempt to correct this, the student moved the cyclic control to the left before the engine had reached full power. At this moment, the left landing skid released from the snow while the right skid remained embedded. This caused the aircraft to tilt sharply to the right. Despite the instructor's attempt to intervene by lowering the collective, the main rotor struck the snow-covered ground, causing the helicopter to capsize.
The investigation
The investigation examined the mechanical condition of the aircraft, the environmental conditions, and the actions of the crew. The Bell 206 B was found to be in good technical condition, with no pre-existing mechanical defects noted. The aircraft's weight and center of gravity were within limits, and the last 100-hour inspection had been completed earlier that year.
Investigators noted that while the aircraft was equipped with a high-skid landing gear, it was not fitted with the snow skis typically used for landings on snow. The ground surface consisted of a soft, 40-to-50-centimeter layer of old snow with a slight downward slope to the right. The investigation also reviewed the flight profiles and the specific maneuvers performed by the student and instructor during the training session.
Findings
- The landing skids had sunk into the soft snow and may have been partially frozen to the surface.
- The student pilot performed an inappropriate control maneuver by applying left cyclic before the aircraft had stabilized.
- The instructor failed to provide sufficient supervision during the critical phase of the takeoff.
- The lack of snow skis on the landing gear contributed to the skids sinking into the snow surface.
- The instructor's late attempt to lower the collective may have inadvertently pressed the right skid deeper into the snow, contributing to the rotor strike.