What happened
During the takeoff roll, the captain deactivated the electric nose wheel steering system. Shortly thereafter, the first officer, who was controlling the aircraft, announced that the plane was yawing to the right without any input from the flight controls. The captain immediately assumed control of the aircraft in an attempt to correct the deviation. Despite these efforts, the crew was unable to stop the aircraft before it left the paved surface of the runway.
The investigation
Following the incident, post-mishap tests were conducted on the affected system. These examinations revealed a malfunction within the nose wheel steering mechanism. The root cause was traced to improper replacement of valves located in the hydraulic steering actuator unit.
Findings
The deviation from the runway was directly linked to the uncommanded yaw during the critical takeoff phase. The mechanical failure prevented effective directional control via the nose wheel steering system at high speeds.