What happened
During the landing phase of flight, one of the two left main wheels separated from the aircraft. Despite the loss of the wheel, the subsequent landing was performed without further incident. The tire and the wheel assembly remained intact following the separation.
The investigation
Investigators examined the axle nut and found the locking retainer ring installed in its proper groove on the nut. However, evidence provided by the manufacturer indicated that the retainer nut had spun off of the axle due to the rotation of the wheel. According to the manufacturer's analysis, this specific condition could only occur if the locking capability of the retaining ring was lost because the ring was not fully seated.
Measurements taken of the thread dimensions on both the inner cylinder axles and the retainer nuts revealed that the threads were beyond the manufacturer's specified wear limits. On the specific axle involved in the incident, the thread engagement measurement (excluding the damaged portion) was found to be at the manufacturer's wear limit. The manufacturer concluded that while the threads were worn, this wear did not compromise the fundamental capability of the nut and axle, as a cross-section of the nut showed insufficient deformation to support the migration of the nut off the axle.