Fatigue cracking originated within the aircraft nose landing gear (NLG) right axle as the result of surface damage associated with grinding during manufacture, and was probably assisted in its initiation by hydrogen evolved during plating processes.
For the reasons given above. The source of the issue was found and the axles in the fleet were inspected.
As a result of their own internal and ongoing investigations into the axle failure, the aircraft manufacturer was working with the landing gear supplier to determine the extent of the problem. The aircraft manufacturer also released a communication to designated 737 stakeholders informing of three (including the subject) recent NLG inner cylinder fractures and their likely origin being related to grinding operations. To reduce the likelihood of future possible NLG axle failures, the manufacturer recommended that inspections for thermal damage be undertaken during overhaul of the NLG inner cylinder. This was to be achieved by Nital etch inspection after stripping the plating from the bearing journals, or by Barkhausen inspection in lieu of stripping the chrome plate.
The ATSB is satisfied that the action taken by the aircraft manufacturer adequately addresses the safety issue.
Immediately following the occurrence, the aircraft operator conducted a detailed visual and non-destructive inspection of all axles in their 737 fleet with similar service history. Particular attention was paid to the area underneath the bearing spacer. No defect indications were found.
