Incident Description
According to the captain, the aircraft touched down on runway 25 at the 1,000-foot mark. Shortly after the nose wheel touched down, the aircraft began an uncommanded divergence to the right. The captain was unable to correct the deviation. Skid marks from the nose landing gear tires appeared at approximately 2,800 feet down the runway, coinciding with the onset of the veer. At about 3,900 feet, the aircraft departed the right side of the runway. The nose landing gear then folded aft, causing the aircraft's nose and right wing tip to strike the terrain. The aircraft continued sliding and came to a stop on the edge of the runway at approximately 4,500 feet.
Damage and Conditions
The forward fuselage and outboard right wing sustained substantial damage. The Automated Weather Observing System at the airport reported calm wind conditions at the time of the incident.
Investigation Findings
Examination of the nose wheel steering servo valve unit revealed that it was commanding a full flow output, resulting in a "hard over" condition. During disassembly of the servo valve, a small piece of Teflon lacing tape was found under the torque motor cover seal. Additionally, moisture, rust, and a white powdery substance were observed under the cover, on the torque motor frame, magnets, and in the armature air gaps.
The unit was subsequently cleaned, reassembled, and reinstalled on the test stand. After undergoing a room temperature test and a cold soak test, no further "hard over" conditions were detected. According to Parker (the component manufacturer), the hard over condition is consistent with foreign object contamination. The contamination would allow moisture accumulation, which could freeze and exert torque on the torque motor armature. This condition would offset the torque balance, causing the first stage jet to move and commanding the second stage spool into a hard over (P to C2) condition.