Eagle, CO, USA
N420QS
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.
On November 29, 2004, at 1236 mountain standard time, a Gulfstream Aerospace G-IV, N420QS, owned by Consolidated International Services LLC., and operated by NJI, Inc., was substantially damaged when it departed the right side of the runway and impacted terrain during landing roll at Eagle County Regional Airport (EGE), Eagle, Colorado. Visual meteorological conditions prevailed at the time of the accident. The airline transport certificated captain, the airline transport certificated first officer, and one flight attendant reported no injuries. An instrument flight rules (IFR) flight plan was filed for the positioning flight being conducted under Title 14 CFR Part 91. The flight originated at San Francisco, California, at 1040. According to the captain, the airplane touched down on runway 25 at 1,000 feet down the runway. Shortly after the nose wheel touched down, the aircraft experienced an uncommanded divergence to the right. The captain was unable to correct the divergence through rudder input. Approximately 2,800 feet down the runway, skid/scuff marks from the airplane's nose landing gear tires appeared as the airplane began to veer to the right. At 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 was substantially damaged and the outboard right wing leading edge sustained minor damage. According to data provided by Gulfstream, a slight differential braking pressure of 200 psi was noted on the flight data recorder. The recorded brake pressures were not sufficient to provide any appreciable total or differential braking action. Gulfstream engineering analysis indicated that the aircraft would have remained on the runway had maximum differential braking been applied at the time the flight data recorder captured the brake pressure increase as indicated. Shortly after the incident, Gulfstream issued a letter to operators indicating the use of differential braking as the preferred method to counteract any uncommanded divergence of the nose wheel steering system. On March 17, 2005, the airplane's nose wheel steering servo valve (p/n 74133-01, s/n 409), was examined on a test stand at Parker to determine if the valve was failing electrically or hydraulically. The valve had previously been removed from another G-IV for reports of a hard right turn upon landing. The valve was initially tested under room temperature conditions and it operated normally. The valve was then cold soaked to minus 70 degrees F., and tested with and without electrical input. The servo valve failed in both configurations. After the valve was warmed up to room temperature, the valve operated normally. This testing confirmed, that at minus 70 degrees F., the unit commanded a full flow output, or "hard over," from the "P" to "C2" port. During the disassembly of the servo valve, it was noted that a small piece of Teflon lacing tape used to secure the motor lead wires was located under the o-ring seal for the torque motor cover. The cover and seal protects the torque motor cavity from the external environment. Moisture was observed under the cover and on the torque motor. Rust was evident on the torque motor frame and magnets. A white powdery substance, consistent to an aluminum corrosion product, was also found at the base of the torque motor, on the magnets and in the armature air gaps. The white powder was removed from the air gaps. The air gaps were cleaned with cleaning alcohol and dried with filtered shop air. The torque motor cover was re-installed. A second room temperature test, and cold soak tests were completed. No further "hard over" conditions were produced. According to Parker, the hard over condition is consistent with the foreign object (white powder) contaminating the air gaps. The contamination would permit accumulation of moisture, which would freeze when subjected to minus 70 degrees F. The expansion of the ice and contaminant in the air gaps would exert a torque on the torque motor armature, which 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. No other airplane system or component anomalies were noted. At 1235, the reported weather conditions at Eagle County Regional Airport was, wind, 000 degrees at 0 knots; visibility, 10 statute miles; sky condition, clear; temperature, -8 degrees C.; dew point, -16 degrees C.; altimeter setting, 30.13. Runway conditions were clear and dry along the center of the runway, with standing water and slush along the edges.
nose wheel steering system failure due to foreign material substance contamination.
Source: NTSB Aviation Accident Database
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