What happened
On May 4, 1965, a Piper PA-24 Comanche 250, registration HB-OTL, departed Geneva Airport for a flight toward Beromünster. Prior to departure, the pilot consulted with a weather advisor at Geneva, who issued a strong warning against flying due to an approaching cold front and imminent thunderstorms. Despite this advice and the fact that the aircraft was not certified for instrument flight, the pilot proceeded with the takeoff at 14:16.
Shortly after departure, the aircraft encountered deteriorating weather. By 14:30, the pilot reported being in clouds near the Thonon region. Radio communications became increasingly difficult due to heavy precipitation and turbulence. The final transmission from the aircraft occurred around 14:35, noting the presence of heavy rain and clouds. An eyewitness near Rivaz reported hearing a loud engine noise and seeing a brief flash of light followed by a dull thud over the lake. Subsequent searches by authorities discovered an oil slick and fragments of the aircraft, including a seat, in Lake Geneva, but the wreckage and the two fatalities were never recovered.
The investigation
SUST examined the flight's progression, the meteorological conditions, and the pilot's qualifications. The investigation established that a cold front was moving through the region, bringing heavy rain, high winds, and significant turbulence. Investigators also reviewed the pilot's previous flight history, noting a pattern of non-compliance with aviation regulations, including unauthorized landings and flight route deviations. The investigation confirmed that the aircraft was equipped with several navigational instruments but lacked the necessary certification for instrument flight rules (IFR).