NTSB Identification: ENG08IA042
Scheduled 14 CFR Part 121: Air Carrier operation of Southwest Airlines, Co. (D.B.A. Southwest Airlines)
Incident occurred Wednesday, October 01, 2008 in Chicago, IL
Probable Cause Approval Date: 03/23/2010
Aircraft: BOEING 737, registration: N551WN
Injuries: 133 Uninjured.

NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft incident report.

The airplane experienced separation of the right nose landing gear (NLG)wheel assembly as it exited runway after landing. Metallurgical examination revealed that the NLG axle fractured in a transverse plane, and that the failure of the axle was due to intergranular cracking that acted as stress concentrators from which fatigue cracks emanated. The fracture of the right side inboard journal on the NLG inner cylinder assembly was most likely due to the failure of a prior tapered roller bearing. Eventually the size of the fatigue crack and the forces generated during landing were sufficient to drive the crack in overstress, leading to fracture of the inboard journal. The change in ownership of the plane and the monthly swapping of bearings and wheels make it difficult to determine when the bearing failure occurred or why.

The National Transportation Safety Board determines the probable cause(s) of this incident to be:

The right nose landing gear axle's failure from intergranular and fatigue cracking due to an earlier bearing failure.

Full narrative available

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