What happened
On August 5, 1989, a private pilot operated a Cessna F172 G (HB-CEN) on a cross-country flight within the Swiss Alps. After departing from Porrentruy and stopping in Yverdon to change passengers, the aircraft departed for an alpine excursion at approximately 0930 local time. The pilot flew over several towns including Thoune and Spiez before reaching the Gemmi Pass at 9,200 feet.
The flight path then directed the aircraft toward Goppenstein, which was overflown at 10,000 feet, before entering the Lötschental valley. The pilot consulted a passenger regarding the altitude of the Lôtschenlücke pass, who confirmed it stood at 10,450 feet. Believing he had sufficient performance to clear the obstacle, the pilot maintained a speed of 90 MPH and adjusted the mixture as altitude increased.
However, the aircraft ceased climbing and began to descend. Attempting to compensate, the pilot pitched up, causing airspeed to drop rapidly. Realizing at the midpoint of the valley that a turn was no longer possible due to terrain constraints, the pilot extended flaps by 10 degrees. Upon reaching the base of the pass, it became evident that the aircraft lacked the altitude to clear the ridge. The pilot elected to perform an emergency landing on a snowy slope approximately 25 meters below the pass level. The aircraft impacted the soft snow, slid backward for about 15 meters, turned right, and came to a stop.
The pilot and the front passenger sustained minor injuries, while the rear passengers were uninjured. The aircraft suffered significant damage.
The investigation
The preliminary inquiry was conducted by Remy Henzelin, with the preliminary report submitted on September 27, 1989. The final report was approved unanimously by the Federal Commission for Aviation Accident Investigation on January 26, 1990. The investigation established that the pilot held a valid private pilot license and was medically fit. The aircraft was airworthy, with mass and center of gravity within limits. The engine, a Rolls Royce O-300-D producing 145 horsepower at sea level, was functioning normally. Weather conditions were favorable with good visibility.
Findings
The primary cause of the accident was the pilot's decision to enter the valley with insufficient altitude reserve to cross the pass. Contributing factors included inappropriate mountain flying tactics, a lack of experience in mountain flying, and an overestimation of the aircraft's performance capabilities at high density altitude.
Specifically, at the altitude reached within the valley, the engine produced only about 70% of its rated power (approximately 100 horsepower). Combined with the aircraft's weight of roughly 1,000 kg, there was no margin to counteract even slight downdrafts. The aircraft was too low above Goppenstein to safely enter the valley. By the time the pilot realized the altitude was decreasing, the point of no return had been passed, leaving insufficient space to regain speed for a turn.
Safety action
The report highlights the critical importance of understanding performance limitations in high-altitude mountain environments and the necessity of maintaining adequate altitude reserves when navigating valleys leading to high passes.