What happened
On August 7, 1992, at approximately 0830 local time, a private round flight began from Wangen-Lachen aerodrome involving a Cessna F152, HB-CIX, carrying one pilot and one passenger. The aircraft was operated by the Ausserschwyzerische Fluggemeinschaft. The planned route traversed the Glärnischfirn glacier toward Linthal and aimed to cross the Claridenpass (Claridenfirn), which sits at an elevation of 2,945 meters (9,718 feet AMSL).
During the climb over Schindellegi, the pilot pushed the mixture control forward for full power but did not adjust it again during the subsequent ascent. By the time the aircraft passed Mollis, it had reached 8,000 feet, and later climbed to approximately 9,000 feet near Rotstock. As the pilot approached the Claridenpass directly, he initially believed the crossing was feasible under normal conditions. However, the aircraft began to lose altitude progressively. Recognizing that a turn-back would be too risky at that stage, the pilot continued straight toward the pass.
As the gap between the aircraft and the terrain closed, the pilot pulled back sharply on the controls in an attempt to gain height. The Cessna F152 struck the softened snow of the glacier approximately 20 meters below the actual pass crest at around 0920 local time. The main gear contacted first, followed by the nose wheel, leaving a 22-meter track in the snow before the aircraft flipped over and came to rest on its back. Emergency locator transmitter (ELT) signals were detected, prompting a joint military and civilian rescue operation.
The investigation
The Swiss Transportation Safety Investigation Board (SUST/STSB) conducted an inquiry into the accident. The pilot held a valid private pilot license with 49 hours of total flight time, including 29 hours in the Cessna F152 model within the preceding 90 days. The aircraft was airworthy, and maintenance records were compliant. No technical failures or mechanical defects were found during the post-accident inspection.
Weather data from the Swiss Meteorological Institute indicated exceptionally high temperatures for early August, with a temperature of 8°C recorded at the accident site—approximately 12°C above the standard value for that altitude. This resulted in a density altitude of roughly 10,750 feet. The investigation noted that the pilot had relied on daily press and radio reports for weather information but did not adequately calculate the impact of this extreme density altitude on aircraft performance.
Findings
The accident was caused by a combination of operational errors and environmental factors:
- Incorrect flight tactic: The pilot attempted a direct approach to the Claridenpass with insufficient altitude reserve. The steep rise of the glacier just before the pass likely created a visual illusion that sufficient height was maintained.
- Failure to account for density altitude: The extreme heat significantly degraded aircraft performance. The investigation calculated that the indicated altitude required to clear the pass should have been 9,990 feet when accounting for temperature and pressure corrections, yet the aircraft struggled to maintain level flight.
- Neglect of mixture adjustment: Although the Pilot’s Operating Handbook specifies leaning the mixture for maximum RPM above 3,000 feet, the pilot failed to do so. The cockpit check revealed the mixture was still set to "RICH" at the time of impact, further reducing engine power output in thin air.
- Optimistic risk assessment: The pilot’s limited experience with this specific aircraft model and under extreme thermal conditions contributed to an overly optimistic evaluation of the situation.
The pilot remained uninjured, while the passenger sustained minor injuries. The aircraft suffered substantial damage. No third-party property damage occurred.
Safety action
The investigation highlighted the critical importance of calculating density altitude and adhering to engine management procedures (specifically mixture leaning) during high-altitude climbs in hot conditions. It emphasized that visual cues on glaciated terrain can be deceptive, and pilots must rely on precise performance calculations rather than intuition when approaching mountain passes.