What happened
On August 1, 1973, a private flight departed from the Planura Glacier in Switzerland. The pilot performed a standard pre-flight inspection and engine run-up using the Champion 7GCB, registration HB-UAU, which revealed no initial anomalies. Prior to departure, the pilot adjusted the mixture lever to a setting optimized for the specific takeoff altitude based on previous experience.
During the takeoff roll toward the west, the aircraft traveled approximately 60 meters before the engine experienced an irregular running condition and a sudden drop in RPM to 1600/min. In response to this power loss, the pilot attempted to abort the takeoff by initiating a left turn to bring the aircraft to a stop on the glacier plateau. During this maneuver, the right ski of the aircraft sank into a patch of softer snow, causing the aircraft to capsize. The aircraft came to rest on its back after traveling roughly 130 meters. There were no injuries to the pilot, and the aircraft sustained only minor damage with no fire reported.
The investigation
The investigation focused on the pilot's experience, the mechanical state of the aircraft, and the environmental conditions. The pilot was a highly experienced professional with over 2,900 flight hours, including significant experience with mountain landings. No health issues or prior incidents were noted in the pilot's records.
Technical inspections of the Lycoming 0-360-A2A engine, including post-accident testing, showed no evidence of mechanical failure. Weather conditions at the time of the accident were favorable, characterized by good visibility, light southwest winds, and temperatures of +2°C.
Findings
The investigation identified two potential triggers for the engine's RPM drop:
- A loose mixture control cable sheath at the carburetor side, which may have shifted due to takeoff vibrations, altering the fuel-air mixture.
- An incorrect manual adjustment of the mixture lever by the pilot, resulting in an overly lean mixture.
Ultimately, the investigation concluded that the engine malfunction—caused either by a technical issue or pilot error—led to the aborted takeoff, which subsequently caused the aircraft to sink into soft snow and overturn.