On October 4, 1997, a Boeing 737-200 (registration N129SW) operated by Southwest Airlines was involved in an aviation accident near Chicago, IL. Investigators recorded the probable cause as: The crack in the axle, which had been repaired, originated at a brake flange hole, and the repair involved excessive heating. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 4 related events involving the same aircraft type or operator are linked below.
Photo: San Diego Air & Space Museum Archives / Public domain, via Wikimedia Commons
After landing, the left main landing gear outboard wheel/brake assembly separated. Inspection revealed a previously repaired crack in the axle, with metallurgical evidence of excessive heating during repair.
Incident Overview
During the landing roll, the left main landing gear outboard wheel/brake assembly separated from the airplane. The captain reported no difficulties in controlling the aircraft following the separation.
Inspection Findings
Post-incident inspection of the failed axle revealed a crack that originated at the surface of one of the brake flange holes. This crack had been the subject of a prior repair. Detailed metallurgical examination of the crack indicated properties consistent with the surface having been heated to an excessive temperature during the repair process.
Conclusion
The investigation determined that the wheel/brake assembly separated due to a crack in the axle that had been previously repaired. The repair process involved excessive heating, which compromised the integrity of the axle.
Probable cause
The crack in the axle, which had been repaired, originated at a brake flange hole, and the repair involved excessive heating.
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