What happened
On September 25, 2016, at approximately 14:13 local time, a Fauvel AV 36 CR glider was participating in an aviation event at the Haiterbach-Nagold airfield. The aircraft was being aero-towed and, upon reaching an altitude of approximately 1,000 meters, the pilot unhooked to perform a series of aerobatic maneuvers, including loops.
After completing eight to nine figures, the aircraft was flying nearly horizontally toward the airfield from a northerly direction. During this phase, the right wing of the flying wing structure failed at the wing-fuselage junction. The pilot sustained serious injuries during the breakup. The pilot's automatic rescue parachute deployed successfully, allowing for a landing in a wooded area north of the airfield, where the aircraft wreckage also fell.
The investigation
The BFU examined the wreckage and conducted material analyses through the Fraunhofer Institute. The investigation focused on the structural integrity of the wooden components, specifically the main spar and the elevator auxiliary spar.
Investigators analyzed the breakage patterns of the main spar and the elevator auxiliary spar. While the main spar showed some minor fungal decay and a slight density deficiency in its upper laminates, these were not determined to be the primary cause of the failure. However, the investigation of the elevator auxiliary spar revealed significant structural degradation. The analysis found that the plywood skin had poorly adhered to the wooden ribs, creating gaps that allowed moisture to penetrate. This moisture led to the growth of wood-destroying fungi and the leaching of metallic salts from a corroded screw, which further weakened the wood. Additionally, the investigation noted that the German version of this aircraft utilized a hinge mounting near the control linkage through-hole, a modification from the original design that introduced structural weaknesses and facilitated moisture ingress.
Findings
- The primary cause of the accident was the structural failure of the elevator auxiliary spar, which preceded the failure of the main spar.
- The elevator auxiliary spar was significantly weakened by several pre-existing conditions, including poor glue adhesion, moisture-induced fungal decay, and material degradation from metallic salts.
- The German design modification, which used a hinge for the elevator attachment near the control linkage, weakened the component and promoted moisture penetration.
- The use of smoke canisters on the wingtips increased aerodynamic drag by approximately 5%, contributing additional longitudinal forces to the auxiliary spar.
- The location of the damage was difficult to inspect due to poor accessibility for maintenance and control procedures.