Introduction
On Saturday, 11 May 1968, at 0830 local time, a Sud Aviation Alouette "Astazou" SA.318C helicopter registered HB-XCN was involved in an incident near the Blümlisalp hut in the Bernese Alps. The aircraft, owned by the Swiss Confederation and operated by the Federal Air Office, was on a service flight carrying three passengers. All four occupants escaped without injury, and the helicopter sustained light damage to its tail rotor and transmission shaft.
Aircraft and Pilot
The helicopter was a single-rotor, three-blade tail-rotor type powered by an Astazou turboshaft engine. It had been imported new from France on 18 April 1968 and had accumulated 33 hours and 18 minutes of service by the day of the incident. On that day, it carried a restricted navigation permit and provisional certificate of airworthiness valid until 31 May 1968 for test flights in France, overflight from Marignane to Bern, and instruction flights in Switzerland, provided a valid Bureau Veritas certificate was on board. The investigation noted that the aircraft was not authorized for the type of flight performed, but considered this a formal infraction with no influence on the accident.
The pilot, born in 1916, held a professional helicopter license since 13 November 1962, with Alouette qualification from 12 August 1963 and a mountain landing endorsement from 16 June 1964. He had logged 5,158 total flight hours, including 795 hours on helicopters, more than half of which were on Alouette types.
Flight Sequence
Several days before the flight, the pilot was tasked by the Federal Air Office to conduct mountain landings for a visiting Brazilian delegation. On the morning of 11 May, the pilot departed Bern Airport at 0745 local in HB-XCN with two Brazilian delegation members and the secretary general of Swissair. The intended destination was the Kanderfirn glacier at 2900 m. En route, at a passenger's request, the pilot decided to first land near the Blümlisalp hut on the northern side of the Blümlisalp massif.
Arriving above the hut, the pilot performed a reconnaissance circle at about 100 m above ground. He then overflew the intended landing site—a snowy ridge above the hut—multiple times at low speed and different headings to determine wind direction and strength. Detecting a light wind from the northeast, he turned to land into the wind but found the site too steep and aborted the landing. Continuing his turn, the pilot was surprised by a downdraft gust from behind as he approached the ridge slightly downstream of the original spot. He immediately increased collective pitch to the maximum but could not stop the descent. The helicopter barely crossed the ridge and struck the snow on the far side with the tail rotor. Feeling strong vibrations, the pilot lowered the collective, forcing the helicopter onto the slope.
Weather Conditions
The aeronautical forecast valid from 0600 to 1200 predicted wind at 3000 m from 220° at 30 knots. At the accident site, the actual weather featured foehn winds on ridges between 2000 and 3000 m. Around 0700, the average wind speed was 20 knots with gusts to 30-35 knots, increasing to 20-25 knots with gusts to 35-40 knots by 1000. Sky was partly cloudy (5-7/8 altocumulus and altostratus, base 4000 m), visibility over 50 km, temperature 0°C at 2800 m, and pressure 3.5 mb above standard. The pilot was aware that such conditions could produce strong downdrafts on lee slopes, particularly near terrain during landing or takeoff.
Damage and Evacuation
The helicopter sustained damage to its tail rotor and transmission shaft, requiring replacement of these parts. Repair costs were approximately 3% of the aircraft's value. No ground damage occurred. The four occupants left the aircraft uninjured and were evacuated by air later that morning.
Investigation Findings
The investigation concluded that the pilot held the necessary licenses and qualifications for the intended flight. The aircraft, although not formally authorized for the type of flight, was in airworthy condition. The incident was attributed to the pilot being surprised by a strong downdraft while maneuvering near the ridge.