What happened
On September 25, 2011, a Dyn’Aéro MCR01 VLA Sportster, registration F-PSLA, departed from Mulhouse Habsheim aerodrome for a local flight. Approximately five minutes into the flight, witnesses observed the aircraft appear to stall, stabilize momentarily, and then stall a second time before impacting the ground near Heimsbrutnn. The pilot, who was the sole occupant, was killed in the crash, and the aircraft was destroyed.
The investigation
The investigation focused on the aircraft's unique flaperon system, which combines aileron and flap functions using a screw-and-nut mechanism (a screw-driven mobile carriage). Investigators examined the wreckage and found that the left flaperon's screw-driven mechanism had become detached from its threaded insert.
Technical analysis revealed significant wear on the threaded inserts of both the left and right flaperon controls. The investigation also scrutinized the aircraft's maintenance records and documentation. It was noted that the maintenance manuals contained contradictory instructions regarding the frequency and method of checking axial play. Furthermore, the investigation found that the maintenance of the aircraft's flaperon system was difficult to perform effectively due to a lack of specific tools and clear inspection procedures.
Findings
- The primary cause of the accident was the failure of the flaperon control mechanism, which led to asymmetric lift and a subsequent loss of roll control.
- Excessive axial play, likely exceeding the 0.3 mm limit, had caused the threads to wear down rapidly.
- The wear pattern (described as "diabolo" shaped) indicated a misalignment between the screw and the insert, possibly due to improper assembly during construction or structural deformation.
- The use of an ISO-profile screw, which is better suited for fastening than for heavy-duty guiding, contributed to the mechanical instability.
- Inadequate maintenance documentation and the lack of a standardized inspection method prevented the detection of the excessive play before the failure occurred.