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.