What happened
On October 30, 1998, a Gipsable I ultralight, identified as 33-IN, was performing a local flight at La Réole aerodrome. Following takeoff from runway 26, the pilot initiated a shallow left turn. During the maneuver, the pilot felt vibrations through the control stick. As the pilot attempted to reposition the aircraft for the final approach to runway 08, the right wing began to deform and the fabric covering started to shred.
The aircraft subsequently entered a high-bank, steep-diving right turn with a significant nose-down attitude. Although the pilot managed to reduce the pitch and bank angle, the aircraft struck the ground with a high rate of yaw. The accident resulted in two injuries (the pilot and one passenger) and the total destruction of the aircraft.
The investigation
Investigators examined several components recovered from the wreckage, specifically focusing on the wing-to-strut attachment points. The investigation centered on two broken steel bolts and the connection piece between the strut and the wing. A third, intact bolt from the opposite wing was also analyzed for comparison.
Microscopic analysis of the broken bolts revealed significant differences between them. One bolt, which appeared dark, showed heavy signs of corrosion and evidence of fatigue. Scanning electron microscopy (SEM) of this dark bolt revealed macrostriations, indicating that the bolt had undergone progressive damage from cyclic aerodynamic loads. The fracture surface also showed corrosion pitting.
The second broken bolt was lighter in color and showed no signs of corrosion, suggesting it had been installed much more recently. However, this bolt exhibited significant necking, indicating it underwent a massive static deformation before failing. The investigation also noted that the connection piece between the strut and the wing showed corrosion deposits originating from the failure of the dark bolt.
Findings
- The primary cause of the loss of control was the structural failure of the wing.
- The wing failure was triggered by the failure of the wing-to-strut attachment hardware.
- The dark-colored bolt had been weakened by corrosion and cyclic aerodynamic stresses, leading to a progressive failure that likely began well before the flight.
- The second bolt, which was a standard-use bolt without high mechanical properties, was undersized or insufficient to maintain the structural integrity of the attachment once the first bolt failed.