What happened
On July 5, 2008, at approximately 15:45, an Aviakit Vega 912 T ultralight, registration 89-KO, was conducting a local flight departing from Auxerre. While cruising at 2,500 feet, the pilot heard a loud noise, after which the aircraft began a right-hand bank and a descent. Initially suspecting an engine or propeller issue, the pilot reduced power, but soon observed the right aileron flapping violently.
Suspecting a control linkage failure, the pilot attempted to compensate by holding the control stick fully to the left. Despite the aircraft's continued descent and banking, the pilot elected not to deploy the onboard emergency parachute. The pilot shut down the engine and performed an emergency landing in a wheat field. The aircraft sustained slight damage, and the pilot was able to evacuate the aircraft and alert authorities.
The investigation
Investigators examined the wreckage of the 89-KO and found that the right aileron was damaged, specifically noting that the horn attachment and the control linkage were broken. The underside of the wing skin was also perforated near the linkage area.
Metallurgical analysis of the control linkages and aileron attachments revealed significant structural issues. The right-side control linkage had failed due to fatigue cracking, which originated at the weld bead on the lower face. This crack had progressed to occupy approximately 90% of the plate section. Furthermore, the right aileron attachment failed due to fatigue under alternating bilateral bending. The investigation also found that the left-side control linkage exhibited fatigue cracking originating from the upper weld bead.
Detailed inspection of the welds showed poor preparation and execution. The metal plates were spaced too far apart, and the welds exhibited irregular widths, excessive thickness, and improper transitions, along with significant lack of fusion and penetration.
Findings
- The primary cause of the failure was defective welding and inadequate design of the aileron control linkages.
- The right-side control linkage failed due to a fatigue crack originating from a poorly executed weld.
- The right aileron attachment failed due to fatigue caused by alternating bending stresses.
- The structural failures on the left side of the aircraft were secondary consequences of the initial failure of the right-side components.