DEN05LA030 · Gulfstream Aerospace G-IV

According to the captain, the airplane touched down on runway 25 at 1,000 feet. Shortly after the nose wheel touched down, the aircraft experienced an uncommanded divergence to the right. The captain was unable to correct for the divergence. Approximately 2,800 feet down the runway, skid/scuff marks from the airplane's nose landing gear tires appear as the airplane begins to veer to the right. Approximately 3,900 feet, the airplane departed the right side of the runway. The nose landing gear folded aft, and the airplane's nose and right wing tip struck the terrain. The airplane continued to slide, coming to a stop on the edge of the runway at 4,500 feet. The airplane's forward fuselage and outboard right wing were substantially damaged. EGE AWOS reported calm wind. Several tests of the nose wheel steering servo valve unit revealed that it was commanding a full flow output, resulting in a "hard over," condition. During the disassembly of the servo valve, a small piece of Teflon lacing tape was located under the torque motor cover seal. Moisture, rust and a white powdery substance, was observed under the cover, on the torque motor frame, magnets, and in the armature air gaps. The unit was cleaned, reassembled and installed back on the test stand. A room temperature test and cold soak test was completed. No further "hard over" conditions were detected. According to Parker, the hard over condition is consistent with the foreign object contamination. The contamination would permit accumulation of moisture, which could freeze and exert torque on the torque motor armature. This condition would offset the torque balance and cause the first stage jet to move, commanding the second stage spool into a hard over (P to C2) condition.

Verbatim NTSB analysis and probable cause from the NTSB dataset