Incident Details
During a landing event, one of two left main wheels separated from the aircraft. The subsequent landing was uneventful, and the tire and wheel remained intact. The axle nut was found with the locking retainer ring installed in the groove on the nut.
Investigation Findings
Evidence collected by the manufacturer indicated that the retainer nut spun off of the axle with the rotation of the wheel. According to the manufacturer, "this condition could only occur if the locking capability of the retaining ring was lost due to the retaining ring not fully seated."
The manufacturer measured the thread dimensions of the inner cylinder axles and retainer nuts. Threads on both axles and retainer nuts were beyond the manufacturer's wear limits. On the incident axle, thread engagement measurement (excluding the damaged area) for the axle was at the manufacturer's wear limit. According to the manufacturer, wear did not compromise the capability of the nut and axle. Their cross section of the nut "did not exhibit enough deformation to support migration of the nut off the axle."
Manufacturer's Analysis
The manufacturer concluded that the retaining ring was not fully seated, leading to loss of locking capability and subsequent nut loosening. The thread wear, while beyond limits, was not deemed to have contributed to the nut's migration.