What happened
On October 17, 2004, at approximately 16:00, a group of three microlights, including the 34-OB, departed from Montpellier Candillargues for a local flight north of the city. The aircraft involved was a single-seat, multi-axis amateur-built microlight, a replica of the "La Moto du Ciel" design. While flying near the Pic Saint-Loup area at an altitude of 1,500 feet, the pilot of the 34-OB notified the other aircraft in the group that the elevator controls had become ineffective. Moments later, the pilot reported that the aircraft was entering a dive.
The aircraft subsequently struck the ground in a garrigue area at La Baume. The impact was severe enough to cause structural disintegration; the right wing was found 400 meters from the fuselage, and a portion of the left wing was located 185 meters away. Analysis of the wing spars indicated they had been subjected to loads acting from front to back with a slight upward component, confirming that the wing failures occurred due to overspeed during the descent.
The investigation
The investigation focused on the elevator control linkage, which was found to have become disconnected. Specifically, the assembly connecting the main control rod to the rear end ball joint had separated. Investigators examined the threads of the retaining screw and found no evidence of abnormal wear or deformation. This suggested that the screw had likely rotated slowly within the main control rod during previous flights until the locking nut eventually detached.
Because the assembly is located inside the rear cone of the fuselage, underneath the vertical stabilizer, it is not easily accessible and requires the removal of the fabric skin to inspect. The investigation also reviewed the construction history of the aircraft. The pilot had purchased the bare airframe from the manufacturer three years prior and completed the assembly of the engine and accessories. However, it appeared the pilot did not personally install the control linkages and had not performed a verification of those specific components.
Findings
- The primary cause of the accident was the insufficient tightening of the locking nut during the aircraft's construction.
- The loose nut allowed the retaining screw to migrate within the control rod, eventually leading to the disconnection of the elevator linkage.
- The loss of pitch control led to an uncontrolled dive and subsequent high-speed impact with the ground.
- The structural failure of the wings was a result of aerodynamic loads exceeding design limits during the high-speed descent.