What happened
On October 17, 2021, a training flight involving a flight instructor and a student pilot was underway at the Dierdorf airfield in Germany. The aircraft, an SF 25C «Falke» with registration D-KDEU, was performing a routine instructional flight under Visual Flight Rules (VFR). During the takeoff roll, the aircraft began to veer to the right, deviating from the runway centerline.
As the aircraft accelerated, it experienced a loss of control. The aircraft lifted off approximately 320 meters from the start of the runway but maintained a nose-low attitude. The aircraft struck the ground twice in the adjacent meadow before the pilot attempted an evasive maneuver to avoid a nearby tree. This final impact was severe, causing the propeller tips to break and the right wing to strike a tree, which subsequently sheared the wing off. The aircraft spun 180 degrees and came to a halt on a field path. Despite the significant damage to the airframe, both occupants escaped without injury.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the wreckage and the flight controls. The investigation focused on the structural integrity of the control linkage. Investigators discovered that the right control stick had fractured just above the weld at the transmission joint.
Upon inspecting the broken steel tube, investigators found significant internal corrosion. The material used, a carbon steel, was susceptible to such degradation. The investigation also looked into the maintenance history and the lack of specific inspection protocols for internal corrosion in these specific control components.