What happened
On May 11, 1968, an Alouette Astazou helicopter, registration HB-XCN, was conducting a flight in the Bernese Alps. The mission, operated by the Federal Office of Civil Aviation, involved transporting a Brazilian delegation and a Swissair official for mountain landing demonstrations.
While approaching the Blümlistalp region, the pilot modified the flight plan to attempt a landing near the Blümlisalp hut. After performing reconnaissance maneuvers to assess wind conditions, the pilot identified a light northeast wind and prepared for a landing. However, as the aircraft approached the intended landing site, it was struck by a sudden downdraft (rabattant) coming from behind. Despite the pilot immediately increasing the collective pitch to its maximum allowable limit, the aircraft could not arrest its descent. The helicopter crossed a ridge and the tail rotor made contact with the snow on the opposite slope. The pilot subsequently lowered the collective to settle the aircraft firmly into the snow to prevent a loss of control.
The investigation
The investigation examined the aircraft's mechanical state, the pilot's qualifications, and the meteorological conditions at the time of the incident. The Alouette Astazou was found to be in excellent mechanical condition, with only 33 hours of service time. The pilot was fully qualified for mountain operations and held a valid professional license.
Investigators also reviewed the weather reports, which indicated a foehn regime with significant wind speeds and gusts expected in the area. While the aircraft's documentation was noted to be in a transitional state regarding its registration for specific flight types, this was determined to have no impact on the accident sequence.
Findings
- The primary cause of the incident was a sudden wind gust acting as a downdraft during low-altitude maneuvering near rugged terrain.
- The pilot was caught by the downward force at a height too low to allow for an effective recovery.
- The tail rotor and drive shaft sustained damage due to contact with the snow, though the four occupants escaped the incident uninjured.