A Lat‚coŠre 631 flying boat was destroyed when it crashed off Cap Ferret, France. All twelve on board were killed. The flying boat was being operated by SEMAF, an organisation that was founded in March 1949 to operated the remaining Lat‚coŠre 631’s that were no longer used by Air France following the fatal accident of F-BDRC in 1948. SEMAF began using F-WANU to conduct test flights in order to establish the cause of the accident involving F-BDRC. The accident happened when the aileron control couplings failed as a result of severe vibration of one of the engines’ gear boxes during one of these test flights. Probable Cause: The inquiry came to the conclusion that the probable cause of the accident was fatigue failure of the aileron control couplings resulting from the simultaneous occurrence of several vibratory phenomena: The cruising speed of the propeller with a 7/16 reducing gear in resonance with the critical vibratory frequencies of the wing and the aileron (excitation amplified by the propeller cuffs); The occurrence of extreme aileron flutter, aerodynamically induced as a result of failure of the linkage between the aileron and the slat. It was impossible for the crew to detect these phenomena before their results became irreparable.