Event Description
On November 29, 2013, during a routine ground inspection of a Boeing 737-400 at Warsaw Chopin Airport (EPWA), a ramp maintenance mechanic noticed an unusual alignment of main landing gear wheel No. 4. After lifting the landing gear and loosening the nut, removal of the wheel proved impossible. An inspection revealed a cracked wheel drum and a damaged inner bearing. With the wheel blocked and unable to be removed normally, operator LOT Polish Airlines authorized cutting a portion of the drum. Subsequent verification uncovered damage and destruction of the bearing and axle. The complete damaged landing gear assembly was removed and replaced with a serviceable unit from another aircraft.
Examination Findings
The operator's maintenance organization, LOTAMS Maintenance Services, reported the incident using the TORF form on December 2, 2013. Further investigation was taken over by LOT Polish Airlines under the supervision of the State Commission on Aircraft Accident Investigation (PKBWL). A team of engineers from LOT's Technical Office, collaborating with external research entities, examined the components to determine whether the destruction occurred due to sudden bearing seizure or gradual wear. After consultation with the bearing manufacturer, the responsible engineer ruled out the need to ground the B-737 fleet. The brake assembly from the damaged strut was evaluated and qualified for repair.
Laboratory analysis of the damaged components was conducted by the Institute of Materials Science, Faculty of Materials Engineering and Metallurgy, Silesian University of Technology. The analysis concluded:
- The main landing gear wheel No. 4 bearing was damaged in a manner causing multiple cracks in the outer ring and local surface abrasion of the roller working parts.
- The bearing was made of carburizing steel.
- The outer ring material contained numerous metallurgical defects in the form of manganese sulfide (MnS) inclusions.
- The carburized surface exhibited cracks that in early stages propagated within the layer; the combination of such damage with metallurgical defects produced micro-cracks under operational stresses.
- Cracks on the outer ring surface had a mixed nature: both fatigue and overload fractures were present.
- Corrosion on one of the fracture surfaces indicated the bearing had been operated for some time with a cracked outer ring, implying the destruction was not sudden.
Cause
The cause of the incident was identified as metallurgical defects of the outer ring of the main landing gear wheel No. 4 bearing in the form of presence of manganese sulfides.
Preventive Actions by LOTAMS
- The operator was informed of the incident.
- Damaged components and metal shavings were preserved for further investigation.
- Information about the event was forwarded to LOTAMS Technical Personnel Training Section for use in recurrent training on human factors and aircraft type.
- Relevant maintenance personnel were notified of the occurrence.
Safety Recommendations
The commission issued no safety recommendations.