Left Outboard Main Wheel Separation During Takeoff Roll (N988UA)

Incident Description

During the takeoff roll phase of flight, the left outboard main wheel detached from the airplane axle. This event occurred without any reported prior anomalies or warning signs.

Component Examination

Examination of the recovered components revealed that six roller bearings had severely damaged rolling surfaces. However, the roller ends were relatively free of scuffing damage, indicating that lubrication and bearing axle preload were adequate. No defects were observed in the keyway. The left hand axle outboard end exhibited severe resultant damage that was consistent with a primary outboard wheel bearing failure.

Failure Determination

The exact mode of the bearing failure could not be determined. The axle nut washer and lock ring were not recovered; however, physical evidence supports that they were installed at the time of the incident.