What happened
On September 6, 1992, a Moravan Zlin 142 was conducting a training flight for aerobatic instruction near Neustadt am Kulm. The flight, which included maneuvers such as steep turns and spins, was being operated by a flight instructor and a student pilot. While performing maneuvers in the approved aerobatic area over the Speinsharter Forst, the aircraft experienced a catastrophic structural failure in flight.
The aircraft disintegrated, causing the propeller, the left wing, and the horizontal stabilizer to detach from the fuselage. The main wreckage entered a steep descent followed by a flat spin, eventually impacting the forest. The left wing and tail section struck the ground several hundred meters away from the main wreckage. The impact and subsequent post-crash fire destroyed the aircraft. The student pilot was killed, while the flight instructor sustained serious injuries but survived.
The investigation
The investigation focused on the sudden separation of the aircraft's primary structural components. Investigators examined the wreckage and analyzed primary fractures at the Institute for Materials at the Technical University of Braunschweig. The examination revealed that the failure originated at the wing-to-fuselage connection. Specifically, the investigation looked into the condition of the steel fittings on the lower spar of the left wing, the manufacturing processes of these components, and the maintenance history of the aircraft.
Findings
Technical analysis determined that the structural failure was caused by fatigue cracks in the steel fittings of the left wing's main spar lower flange. Several contributing factors were identified:
- Design: The fittings were subject to unfavorable stress distribution, and the material used possessed low fracture toughness. Furthermore, the design of the connection made it difficult to inspect the critical areas.
- Manufacturing: The fittings featured coarse machining burrs within the pilot holes, which acted as stress concentrators. The components were made of a high-alloy tempered steel with high hardness but low toughness.
- Certification and Testing: There was a lack of fatigue testing on actual component samples, and the manufacturer's assessment of the operational load spectrum was deemed unrealistic. Additionally, the inspection protocols only required a visual check of the wing-to-fuselage connection, which could not detect cracks hidden by shim plates.