EMBRAER EMB145EXPRESSJET AIRLINES INC

Shortly after the nose wheel touched down during the landing, the airplane turned hard left and departed the runway.  Review of the Flight Data Recorder data revealed that the crew attempted to counter the initial left turn using rudder and differential breaking, and that their inputs were not the cause of the left turn.  The nose wheel steering, brakes, spoilers, thrust reversers, and rudder were inspected at the aircraft level with no anomalies found.  In addition, many components from the nose wheel steering system were removed and tested at the component level. Two of these components, the Steering Manifold Assembly and the EHSV, were found to have defects that could potentially affect nose wheel steering operation. The Steering Manifold Assembly failed flow tests related to bypass valve operation and a piece of plastic was discovered on the C2 filter screen of the bypass valve.  The bypass valve operates when the nose wheel steering is disengaged.  It connects the two steering actuating cylinder hydraulic chambers which allows the nose wheel to free caster.  The observed low flow condition caused by the plastic debris could result in the reduced effectiveness of the bypass function when the nose wheel steering is disengaged. In addition, the plastic debris was in a location where it could have potentially reduced or blocked fluid flow through the C2 hydraulic port during normal nose wheel steering operation.  This could have resulted in the inability or reduced capability of the nose wheel to turn in the commanded direction.  Any blockage caused by this plastic debris would result in reduced performance only and could not have initiated or caused the nose wheel to turn uncommanded. The EHSV passed the manufacturer's acceptance test; however, disassembly revealed that the C1 orifice filter seal was damaged and the C2 orifice filter seal was missing material. The missing material was not located, but the void that remained on the seal from the missing fragment was larger than the C2 nozzle orifice.  Depending on how the missing material separated from the C2 orifice filter seal, the size of the material was potentially large enough to block the C2 nozzle. A blocked C2 nozzle would allow hydraulic fluid pressure to build within the C2 fluid chamber, causing the control spool to move in a direction that would command a left nose wheel turn.  The undersized condition of the C1 and C2 filter plug outer diameters likely contributed to the premature deterioration of the C1 and C2 orifice filter seals.  The plug diameters were required to be checked when the EHSV was subjected to Service Bulletin (SB) 145-32-0099; however, the out of tolerance condition remained.