What happened
During the landing phase of flight, the flightcrew attempted to extend the landing gear but observed that the nose gear failed to provide a "down and locked" indication. The crew made several additional attempts to lower the nose gear, but these efforts were unsuccessful. As a result, the aircraft was landed with the nose gear not down and locked.
The investigation
Post-flight inspections of the aircraft revealed that the nosewheel steering system had been incorrectly rigged. Specifically, the nose gear failed to properly center itself, and the tensions in the nosewheel steering cables were found to be below the required limits. This mechanical condition caused the nose gear to strike the side wall of the gear well, which physically prevented the gear from extending fully.
The investigation also noted that approximately two weeks before the incident, the nosewheel steering control valve had been replaced. This maintenance procedure would have necessitated a subsequent check of the nose gear rigging.