What happened
On 26 May 2022, an amateur-built AS-06 microlight, identified as 23DI, was performing a local flight at the Guéret-Saint-Laurent aerodrome. The pilot, accompanied by a passenger, had been performing a series of maneuvers, including steep bank turns and low-altitude passes over the runway.
Following a second low pass at high speed, the pilot initiated a smooth pull-up maneuver combined with a left turn at a bank angle of approximately 20 degrees. During this maneuver, a loud clacking noise was heard, and the left wing failed. The aircraft lost altitude abruptly and collided with a field bordering the aerodrome. The accident resulted in one fatality (the pilot) and one severe injury (the passenger), and the aircraft was destroyed.
The investigation
The BEA examined the wreckage of the AS-06 at its premises, where the left wing was partially reconstructed. Investigators analyzed the metal structure of the wing spar, which consisted of square steel tubes, rectangular tubes, and plywood webs connected via a brass soldering (brazing) technique.
Technical examination of the left spar revealed multiple fractures at the square tubes and several failures at the brazed joints. The investigation also reviewed the aircraft's regulatory status, noting that as a microlight, its wing structure had not been subject to mandatory structural examination by the authorities. Furthermore, the technical file for this specific aircraft did not include any structural calculations to validate its design.
Findings
The investigation established that the primary cause of the accident was the failure of the left wing due to structural fatigue and overload. Specifically, the investigators identified several critical factors:
- The brazed joints on the left spar contained numerous defects, including misalignments, improper amounts of filler material, and the presence of blisters.
- Evidence of progressive fatigue cracking was found on the connection between a square tube and a metal angle.
- The structural integrity of the spar was compromised by these defects, making it unable to withstand the stresses of flight.
- The pilot's repeated execution of high-stress maneuvers, such as steep turns and high-speed low passes followed by pull-ups, placed significant load on the unvalidated structure.
- The lack of engineering calculations to validate the primary structure's strength contributed to the wing's inability to endure these maneuvers.