What happened
On March 26, 1971, a Piper Apache HB-LCA was conducting a non-scheduled commercial flight from Geneva to Saanen with three passengers on board. After departing from runway 23, the pilot climbed to 2,500 feet. During the cruise phase, the left engine began to vibrate and exhibit irregular power output. In an attempt to stabilize the aircraft, the pilot increased power to the right engine, but the right engine soon began exhibiting similar irregularities.
As the aircraft flew near Nernier, the pilot noted that the engine exhaust was turning black, suggesting an overly rich fuel mixture. Despite attempts to lean the mixture, the engines continued to lose power, and the aircraft began a forced descent at a rate of approximately 100 to 200 feet per minute. Realizing that the destination airfield at La Côte was unreachable, the pilot identified a suitable field between a main road and a lake for an emergency landing. During the approach, the aircraft brushed against tall trees lining the lakeshore. Upon touchdown, the aircraft traveled 112 meters across very soft ground, which caused the nose gear to dig into the earth and eventually collapse. All occupants escaped the incident without injury, though the aircraft sustained heavy damage.
The investigation
Investigators examined several potential causes for the engine malfunctions, including mechanical failure, fuel contamination, and carburetor icing. Technical inspections of the fuel system and engines revealed no mechanical defects. Furthermore, fuel samples showed no evidence of water contamination, and the pilot had performed fuel tank sumping prior to departure.
Meteorological data indicated that the aircraft was flying at an altitude where the temperature was near the condensation level, creating conditions highly conducive to ice formation. The investigation also reviewed the pilot's actions regarding the use of the carburetor heat system and the decision-making process during the emergency descent.