Left Outboard Main Wheel Separation During Takeoff (N9090U)

Incident Summary

During takeoff, the left outboard main wheel separated from the aircraft. The flight crew landed the airplane without any further incidents.

Investigation Findings

The investigation revealed that the outer bearing had failed. The heat and mechanical damage observed were consistent with inadequate lubrication. It could not be determined whether pre-existing mechanical damage existed. The wheel bearing had accumulated 349 landing cycles since its initial installation and 219 cycles since the last brake change. It could not be confirmed that the bearing had been repacked with grease after initial installation. An unknown quantity of grease had migrated away from the bearing cavity during operation.

Maintenance Context

The operator's average interval for wheel changes on their 737-200 aircraft was 259 cycles, while Boeing reported a worldwide average of 200 cycles. An optional outer grease seal, which would have helped retain grease within the bearing, was not installed. The bearing and wheel assembly manufacturers stated that the lubrication properties of grease degrade rapidly after 150 cycles.