What happened
On January 10, 1976, a Cessna 177 B, registration HB-CWU, departed Ascona, Switzerland, for a planned VFR flight to Grenchen. The aircraft was carrying the pilot and two passengers. During the flight, the pilot reported being "on top" at approximately 13,500 feet, navigating via the Willisau VOR. However, at approximately 15:50, the aircraft entered an uncontrolled descent, striking a steep, snow-covered slope of the Hinter-Tierberg near the Göscheneralp at an angle of roughly 80 degrees. All three fatalities were confirmed at the scene, and the aircraft was destroyed.
The investigation
The investigation, conducted in collaboration with the Uri Cantonal Police, examined the aircraft wreckage, weather reports, and radio communications. Investigators recovered an ICAO map from the debris containing flight route markings. Radio transcripts from the Grenchen Tower and a recording from Bern-Belp confirmed the pilot's last communications, where he requested positioning information. The investigation also reviewed meteorological data from the Swiss Meteorological Central Institute, which indicated a significant weather front moving through the area, bringing heavy clouds and high winds.
Findings
Several critical factors contributed to the accident. The investigation established that the pilot was flying just above the cloud tops and near the aircraft's service ceiling of 14,600 feet. This left a very narrow margin for maneuvering, especially given the presence of moderate to strong turbulence and wind shear in the area. The investigation highlighted that the pilot failed to check the current weather conditions for the destination airport, Grenchen, or the alternate, Bern, prior to or during the flight.
Crucially, the aircraft unintentionally entered the clouds while operating under VFR conditions. This entry led to a subsequent loss of control over the aircraft. While the exact reason for the descent into the clouds could not be definitively determined, investigators noted that the aircraft's limited climbing performance at high altitudes and the potential for carburetor icing—given the high humidity and low temperatures—were significant risks during the flight.