What happened
On March 18, 1965, an Agusta-Bell 47-J-3B-1 helicopter, registration HB-XBR, was performing a series of cargo transport flights in the Val d’Entremont region of Switzerland. The mission, conducted for the Valais Section of the A.C.S., involved moving materials from Liddes to a construction site in the Fratset Forest.
After completing ten successful trips, the pilot noted increasingly strong and irregular winds descending the valley. Although the pilot initially intended to cease operations due to these conditions, the site foreman requested one final transport to complete the day's work. The pilot agreed to perform a final lift, carrying a 170 kg load.
During the approach to the unloading station, the aircraft experienced sudden, intense vibrations and a loss of lift. The pilot immediately released the external load and attempted an emergency maneuver to clear the terrain by turning left and applying full power. However, the aircraft descended approximately 50 meters toward the slope, striking the treetops. The impact triggered a fire, resulting in the destruction of the helicopter and grievous injuries to the pilot.
The investigation
The investigation examined the pilot's experience, the aircraft's mechanical state, and the prevailing meteorological conditions. The pilot was highly experienced in alpine flight, with significant hours in both fixed-wing and rotary-wing aircraft. While the pilot's license was technically due for renewal on the day of the accident, investigators determined this was not a contributing factor to the crash.
Technical examination of the wreckage was limited as the state of the debris did not allow for detailed mechanical inspections. The investigation focused heavily on the atmospheric conditions at the time of the flight, specifically the presence of wind gusts in the valley.
Findings
- The primary cause of the accident was a rotor stall triggered by sudden turbulence.
- The meteorological conditions involved moderate turbulence and strong, irregular wind gusts of 25 to 30 knots, characteristic of a foehn wind event.
- The sudden loss of lift and accompanying vibrations were consistent with airflow separation on the main rotor blades caused by these gusts.
- While the pilot's emergency responses were judged to be prompt and appropriate, the proximity of the terrain and the severity of the turbulence made recovering the aircraft impossible.