What happened
On November 29, 2013, during a routine inspection of a Boeing 737-400 operated by PLL LOT SA at Warsaw Chopin Airport (EPWA), a ramp maintenance mechanic noticed an unusual orientation of the fourth main landing gear wheel. Upon lifting the landing gear and removing the axle nut, technicians found that the wheel could not be removed due to a damaged internal bearing. Further inspection revealed a crack in the wheel rim of the fourth wheel. Because the wheel was jammed and could not be disassembled through standard procedures, a portion of the rim had to be cut away with the operator's permission. The inspection further identified damage to both the bearing and the axle. The damaged landing gear assembly was subsequently removed and replaced with a functional unit from another aircraft.
The investigation
The investigation, conducted by the operator under the supervision of the PKBWL, focused on determining whether the damage resulted from a sudden, violent seizure of the bearing or a gradual process of wear. Engineering teams from the operator, working alongside external specialists, analyzed the damaged components. Metallurgical analysis was performed by the Institute of Materials Science at the Silesian University of Technology. This analysis examined the structural integrity of the bearing, specifically looking for evidence of fatigue, corrosion, or manufacturing defects.
Findings
Laboratory analysis of the fourth main landing gear wheel bearing revealed that the outer ring had sustained numerous cracks and localized surface wear on the roller contact area. The investigation established that the bearing was manufactured from case-hardening steel, but the outer ring material contained metallurgical defects in the form of manganese sulfides (MnS). These defects, combined with cracks in the case-hardened layer, allowed operational stresses to create micro-cracks. The cracks on the outer ring surface were found to be of a mixed nature, consisting of both fatigue and sudden-onset cracks. Additionally, corrosion found on one of the fracture surfaces indicated that the bearing had been in operation with a cracked outer ring for some time, proving that the failure was not a sudden event.
Safety action
Following the incident, the operator was notified, and all damaged components and metal shavings were preserved for further study. The incident details were shared with the LOTAMS technical training section to be used in periodic training regarding human factors and aircraft type-specific maintenance.