What happened
On Saturday, June 18, 1983, at approximately 09:30 local time, a Cessna F 172 N bearing registration HB-CWO departed from Buttwil airfield in Switzerland. The aircraft was carrying the pilot and three passengers for a private recreational flight over the Alps. No flight plan was filed, and it remains unclear whether the pilot consulted the official aviation weather forecast (GAFOR) prior to departure.
The intended route likely involved climbing toward Hausen, then proceeding through Amsteg, Andermatt, and Airolo before crossing the Nufenen Pass. However, the aircraft collided with a snow-covered mountain ridge on the southern slope of the Chilchhorn (elevation 2,784 m), located just south of the Nufenen Pass (elevation 2,478 m) in the canton of Ticino. The impact occurred at an altitude of 2,560 m. The plane struck the terrain in a nose-down attitude, flipped over, and came to rest on its back. All four occupants were killed instantly, and the aircraft was destroyed.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the wreckage, pilot credentials, and weather data. The pilot, born in 1960, held a valid private pilot license issued in June 1979. He had logged 76 hours of total flight time, with only 16 hours on the Cessna F172 type. His mountain flying experience was noted as limited. The aircraft was airworthy, with weight and balance within limits, and no mechanical failures were found in the engine or control systems.
Weather reports indicated overcast skies (8/8 coverage) with cloud bases between 1,700 m and 2,500 m. Snowfall was occurring in the southern valleys, and visibility outside clouds was good, but inside clouds it was poor. The GAFOR forecast for the Gotthard-Nufenen route explicitly closed mountain passes to visual flight due to low cloud bases and weather hazards.
Findings
The investigation concluded that the primary cause of the accident was the pilot’s probable continuation of a Visual Flight Rules (VFR) flight into Instrument Meteorological Conditions (IMC). This led to spatial disorientation or loss of terrain awareness, resulting in controlled flight into terrain (CFIT).
Contributing factors included:
- The pilot’s lack of experience in mountain flying.
- Poor decision-making regarding the deteriorating weather conditions; specifically, a failure to abort the flight when meteorological minima were exceeded.
- The absence of a filed flight plan or pre-flight weather briefing verification.
The emergency locator transmitter (ELT) was armed and activated by impact but failed to transmit signals, likely because the antenna was buried in snow due to the aircraft resting on its back.
Safety action
This report highlights the critical importance of adhering to VFR weather minima in mountainous terrain. It underscores the necessity for pilots to review official aviation weather forecasts (such as GAFOR) and flight restrictions before departing, particularly when planning routes over high-altitude passes that are frequently closed to visual flight due to low clouds and turbulence.