What happened
On May 18, 1997, at approximately 16:00, a Cosmos Bidulm ultralight aircraft, identified as 05-BP, was performing a training flight at Gap aerodrome. Shortly after takeoff, while the aircraft was at an altitude of roughly ten meters, it experienced a sudden and violent left-wing pitch-down. The aircraft subsequently struck the runway.
The investigation
The investigation focused on the structural integrity of the aircraft's wing assembly following the impact. Examination of the wreckage revealed that the left strut, which secures the wing to the airframe's central structure, had detached and become entangled in the propeller.
Findings
The investigation established that the failure originated at the wing spar attachment point. The bolt responsible for securing the strut through the wing tube failed at the threaded section. This failure was caused by the improper installation of the bolt, which had been mounted upside down with the head facing upward instead of downward.