What happened
On July 7, 2022, during a developmental flight test, a WT 9 600 RG Dynamic aircraft, registration D-MLXL, experienced a sudden loss of longitudinal control. The flight was part of a certification process involving testing various airspeeds during descent. While accelerating to 350 km/h at approximately 10,816 feet, the pilot heard a loud impact from the rear of the aircraft. The aircraft immediately entered a steep climb and became unresponsive to control inputs.
The pilot responded by shutting down the engine and activating the integrated ballistic parachute system at 9,600 feet. The aircraft descended under the parachute and impacted a field near Lehota pod Vtáčnikom. Upon impact, the aircraft overturned, resulting in one light injury to the pilot. The aircraft was destroyed.
The investigation
The investigation focused on the mechanical failure of the elevator control system. Investigators examined the wreckage and identified that a ball joint head from the elevator control linkage had broken off the central connecting rod.
Technical analysis by forensic and welding institutes revealed that the broken component was made of high-strength, zinc-plated steel. Microscopic examination of the fracture surface showed a brittle, cleavage-type failure without signs of fatigue or prior deformation. Further investigation into the bearing end caps used in the control system identified significant internal cracks within the material.
Findings
- The primary cause of the accident was the loss of longitudinal control due to the destruction of the elevator control linkage ball joint head.
- This failure was caused by hydrogen embrittlement of the high-strength steel component.
- The mechanism of failure involved the interaction between the high-hardness steel, diffused hydrogen, and the preload stress caused by the locking nut.
- The zinc plating process likely contributed to the migration of hydrogen into the material, leading to intercrystalline and quasi-cleavage fractures.
- The pilot performed all emergency procedures in accordance with the aircraft's operational manual.