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Summary

Summary

After the Boeing 747-400 aircraft, registered N109UA, landed on runway 27 at Melbourne Airport, the crew vacated the runway via taxiway M at the western threshold. They then entered taxiway E, parallel to runway 27. The crew reported that while taxiing eastward along taxiway E, the aircraft veered left of the taxiway centreline. The co-pilot, who was handling the aircraft, applied right tiller and pedal to correct the veer. When the aircraft began to move right of the centreline, the captain took control because he felt that the co-pilot's correction was not arresting the divergence. The captain applied a left correction and reported that he felt that the aircraft was not responding. When he applied additional left control input, the aircraft responded rapidly and he was unable to stop the aircraft oversteering the centreline. The aircraft failed to respond to the captain's corrective actions and he applied brakes. However, he was unable to stop the aircraft before the nose wheel and the left wing and body gear left the taxiway and became partially bogged in the grassed area beside the taxiway.

The operator reported that a post maintenance inspection of the nosewheel steering system found low cable tensions on the nose gear steering cables. Subsequent removal of the hydraulic nosewheel steering metering valve and laboratory examination by the component manufacturer found some anomalies but the valve was capable of normal steering operation. Although the low tension of the steering cables was considered a possible factor in the development of the occurrence, the reason for the loss of steering control was not positively determined.

 
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