What happened
The incident occurred during the landing roll of an aircraft. While the plane was rolling out on the runway, a critical failure took place in the landing gear assembly. Specifically, the swivel link pin located on the left main gear failed structurally. This mechanical breakdown allowed the rear set of tires to become completely detached from the main strut. Once separated, these tires dragged along the surface of the runway, creating significant friction and damage to the airframe components involved.
The investigation
A thorough examination of the aircraft's maintenance history revealed that the left main gear bogie had been last overhauled in 1981. At the moment the failure occurred, this specific assembly had accumulated 4459 hours since its previous overhaul. This extended service interval without intermediate inspection or replacement was a key factor in the subsequent metallurgical analysis.
Experts conducted a detailed metallurgical study of the failed swivel pin to determine the root cause of the fracture. The examination uncovered several critical findings regarding the condition of the metal:
- Intergranular cracking was observed, penetrating through approximately 40% of the pin's cross-section. This type of cracking suggests internal material degradation rather than surface wear.
- There was clear evidence of stress corrosion, which originated from overtorquing the nut on the threaded end of the pin. This improper installation technique introduced excessive residual stress into the component.
- Traces of copper were found within the crack. Copper is sometimes used as a preparatory layer before chromium plating during overhaul procedures, indicating that residues from previous maintenance work remained in the stress-concentrated area.
- The pre-existing crack showed no signs of overload at the time it formed, confirming that the initial damage was due to corrosion and stress rather than immediate mechanical failure.
- The final fracture of the pin was determined to be caused by overload, which occurred only after the structural integrity had been compromised by the earlier corrosion and cracking.
Findings
The investigation concluded that the failure was not a result of sudden mechanical shock but rather a progressive degradation process. The combination of stress corrosion from improper nut installation and the presence of copper residues created a vulnerable environment for intergranular cracking. Over time, this crack grew until the remaining cross-section could no longer support the loads experienced during landing, leading to catastrophic failure.