What happened
On February 26, 2005, at approximately 15:50, a Jodel D 113 amateur-built aircraft, registered F-PLJC, was conducting a training flight involving circuit patterns at the Uzès aerodrome. The flight originated from Nîmes-Courbessac. After performing several touch-and-go landings on unpaved runway 36, the crew attempted a final landing. During this touchdown, the aircraft veered sharply to the right. After rolling approximately 40 meters, the aircraft tipped forward onto its nose.
There were no fatalities or injuries among the two occupants, which included an instructor and a student pilot. The aircraft sustained minor damage.
The investigation
The investigation focused on the structural failure of the main landing gear components. Post-accident inspections revealed that the axle retaining the two linkage arms for the right wheel was missing, leaving the right wheel loose on the landing gear axis. Additionally, the self-locking nut for the left gear linkage axis was also absent.
Investigators examined the maintenance history and technical documentation of the aircraft, which had been purchased by the current owner approximately six months prior. While the aircraft's maintenance program did not mandate specific inspections of the landing gear, the previous owner had reportedly replaced these specific nuts during every inspection. However, no formal technical interventions were recorded in the aircraft's logs. Furthermore, the technical drawings provided with the aircraft contained no specific information regarding the characteristics of these axles and nuts.
Findings
The investigation established that while the diameters and thread pitches of the installed axles and nuts were compatible for assembly, the actual thread profiles were mismatched. This discrepancy prevented proper contact between the flanks of the screw threads and the nut threads, rendering the assembly incapable of maintaining effective clamping force.
It was determined that inappropriate maintenance practices—specifically the repeated tightening of these components—led to the progressive deterioration of the thread ends and the nylon locking ring. These self-locking nuts are designed for single-use only. Under the influence of operational vibrations and mechanical stresses, the nuts eventually loosened and fell off the assembly.