What happened
On September 11, 2014, a D4 Fascination ultralight aircraft departed from Juist airfield, bound for Aalen-Heidenheim/Elchingen. While flying in a south-southeasterly direction, the aircraft entered a spiral dive and crashed near the village of Nausis in Hesse, Germany. Radar tracking recorded the aircraft at an altitude of 3,200 ft AMSL traveling at 120 knots shortly before the impact. The crash resulted in two fatalities and the total destruction of the aircraft.
The investigation
The BFU examined the wreckage and recovered components, including the main fuselage, engine, and various pieces of the right wing. Investigators found that the outer portion of the right wing, including the aileron, had separated from the main structure and been found at a distance from the primary impact site.
Laboratory analysis of the aileron's control tube revealed that the weld connecting the mass balance holder to the steel tube had failed. Metallographic examinations identified irregular welding, including lack of fusion, weld beads, and micro-pores. The presence of fatigue striations indicated that the failure had been progressing over a period of time. Furthermore, the investigation found that the aircraft was significantly overweight; the combined empty weight and mass of the two occupants totaled 493 kg, exceeding the maximum takeoff mass of 450 kg.
Findings
- A manufacturing defect in the weld of the aileron mass balance caused the connection to fail.
- The loss of the mass balance allowed the aileron's "spade" to deflect downward, creating an aerodynamic force that pushed the aileron to its upper limit and caused the right wing to break.
- The aircraft was operating in an overweight condition, as the combined weight of the airframe and occupants exceeded the 450 kg limit.
- The aircraft was flying at approximately 200 km/h, which was within the yellow maneuver speed range, making the sudden aerodynamic shift from the aileron failure particularly violent.