Embraer EMB-135LR N843RP

A regional jet airplane experienced an uncommanded swerve during landing roll. The captain stated that after the nose wheel touched down, the airplane began to veer to the right of the centerline. The first officer noticed that the airplane required more left rudder than usual, and that inputs were not bringing the nose straight, and adding more left rudder did not correct the veer to the right. Lastly, the captain utilized the till in an attempt to keep the airplane on the landing surface. Just as the till was moved to the left, the airplane veered sharply off the right side of the runway. With the assistance of the operator's maintenance personnel, the NTSB IIC activated the hydraulic system, while the airplane was under APU power. The tiller was then used to move the nose wheel. When the till was moved to the left, the nose wheel turned to the right. When the till was centered, the nose wheel remained turned to the right. When the till was moved to the right, the nose wheel turned further to the right. Further electrical and hydraulic testing of the nose wheel steering system revealed that the steering handle potentiometer checked normally. The rudder potentiometer was difficult to center with slightly erratic readings around the center. The feedback potentiometer centered normally but had a small open reading at approximately the left 45-degree position. All potentiometers were electronically centered and the airplane was jacked. When hydraulic power was applied, the nose wheel turned slowly to the right reaching the seven-degree safety switch in approximately 4 to 5 seconds. The seven-degree switch functioned normally, cutting off hydraulic power to the system. The steering manifold was removed and replaced with a serviceable unit. The same check was then performed with normal results. Detailed examination of the hydraulic manifold revealed that the only one valve responded when pressure testing the Electrohydraulic Servovalve (EHSV).The EHSV, serial number 748, was removed and replaced with a new EHSV, serial number 075A. When pressure was applied to the unit, both pressure vent valves registered the same pressure. The removed EHSV was then tested again at another facility with the same results. The ESHV tear down revealed damage to an o-ring and a small piece of contamination was found blocking the C1 valve orifice . Embraer published an Operational Bulletin on April 13, 2004, "Nose Wheel Steering System Malfunction and Uncommanded Swerving Events." The intent of the bulletin was to provide operators of the EMB-145, ERJ-140, and EMB-135 airplanes with procedures to be followed in the event of uncommanded swerving during high speed taxi, takeoff and landing. The bulletin stated that the steering handwheel should not be used to correct uncommanded swerving on the ground. Additionally, Embraer called for the installation of a cockpit decal stating: "WARNING: DO NOT ACTUATE THE STEER HANDLE IN CASE OF UNCOMMANDED SWERVING OR INADVERTENT STEER INOP MSG." The accident airplane did not have the decal installed at the time of the accident. However, the time for accomplishment of the bulletin was "within the next 150 flight hours or 4 months, whichever occurs first." The bulletin was issued in April 6, 2004, and the incident occurred on June 11, 2004.

Quoted verbatim from the NTSB record.