What happened
On May 14, 2017, at approximately 09:40 local time, a pilot took off from the Villefranche-de-Rouergue aerodrome in a RANS S-10 SAKOTA ultralight, identified as 12CT, to perform aerobatic maneuvers. Following takeoff, the pilot executed a roll and returned to horizontal flight. While performing a loop at an altitude between 2,500 and 3,000 feet, the aircraft entered a high-energy descent. A witness observed the aircraft striking the ground at a 45-degree angle under high engine power. During the final stage of the loop, the witness noted a distinct vibrating or "flapping" sound coming from the wings.
The investigation
The investigation focused on the structural configuration of the 12CT. Although the aircraft was built based on RANS S-10 SAKOTA plans, investigators discovered that the pilot had installed two wing extensions, approximately 60 cm each, equipped with winglets. These extensions were attached to the wingtips and secured to the trailing edge ribs, but they were not part of the original construction plans and were not documented in the aircraft's identification file.
Examination of the wreckage revealed that the aircraft struck the ground with significant energy. While the primary wreckage was concentrated, the two wing extensions had detached and were found roughly ten meters upstream from the main debris field. The aileron hinges were also torn away. The investigation confirmed that the control linkages for pitch and roll remained continuous.
Findings
- The installation of the wing extensions increased the total wing surface area by approximately 20%.
- This modification altered the aspect ratio and shifted the center of lift toward the wingtips, disrupting the aircraft's aerodynamic equilibrium.
- The addition of the extensions made the wing susceptible to aeroelastic flutter, particularly during high-speed, high-load aerobatic maneuvers.
- It is concluded that divergent oscillations developed in both wing halves, transmitting vibrations through the rigid flight controls and rendering the aircraft uncontrollable. As a result, the pilot was unable to recover from the loop to avoid the fatal impact.