What happened
On August 28, 2009, an Antonov AN-12 BK cargo aircraft was parked at the North side of the East apron at Münster-Osnabrück Airport, prepared for a flight to Tripoli, Libya. The aircraft was operated by a Ukrainian company and carried a crew of seven.
At approximately 20:45 local time, during the engine start-up sequence, the strut of the right main landing gear failed. The failure caused the right side of the aircraft to sink, leaving the plane tilted to the right. The crew immediately shut down the engines, and the aircraft remained at its parking position. There were no injuries to the crew, though the aircraft sustained minor damage.
The investigation
The BFU examined the aircraft's landing gear components, specifically the tubular strut of the right main gear. Metallurgical analysis was conducted by the Institute for Materials (IfW) at the Technical University of Braunschweig. Using scanning electron microscopy, investigators analyzed the fracture surfaces and the internal structure of the strut. The investigation also included an evaluation of the flight data recorder and cockpit voice recorder, though no usable data could be retrieved from these devices.
Findings
The investigation established that the landing gear strut failed due to hydrogen-induced stress corrosion cracking.
Investigators found that moisture, likely containing traces of de-icing salts, had penetrated the interior of the tubular strut. The internal walls of the strut showed heavy corrosion, particularly at the bottom (6 o'clock position). This corrosion process generated atomic hydrogen, which migrated into the high-strength, low-alloy steel.
The structural integrity of the strut was compromised by a thumb-sized crack that had developed over time. The additional mechanical load applied by the activation of the third engine during the start-up sequence reached a critical point, triggering the final unstable crack propagation and the subsequent failure of the strut.