What happened
During a flight intended to evaluate flight controls following significant engine vibrations, a six-engine seaplane prototype was conducting a test mission. While maintaining an altitude of approximately 1,200 meters, the aircraft experienced a structural failure on its right wing when a component detached. This loss of structural integrity caused the aircraft to become uncontrollable, leading to a crash into the ocean roughly one kilometer northwest of Le Cap Ferret.
Following the impact, personnel from a French Marine vessel and a local trawler discovered debris floating on the surface of the water. Despite the recovery efforts, there were no survivors among the 12 fatalities recorded from the crew, which included pilots, engineers, mechanics, and technical staff.
Findings
An investigation into the accident determined that the crash was caused by a fatigue failure of the aileron control couplings. This failure was triggered by several simultaneous vibratory issues. Specifically, the propeller's cruising speed, utilizing a 7/16 reducing gear, entered resonance with the critical vibration frequencies of both the wing and the aileron, an effect that was further amplified by the propeller cuffs.
Additionally, the investigation identified extreme aileron flutter caused by an aerodynamic failure in the linkage between the slat and the aileron. The inquiry concluded that these complex mechanical phenomena occurred too rapidly for the crew to identify or rectify the situation before the aircraft became irreparably damaged.