What happened
On August 29, 2011, at approximately 12:36 CDT, an Embraer 145 XR, registration N27152, operated by ExpressJet Airlines as United Express flight 5821, departed the left side of runway 10 during the landing roll at Quad City International Airport in Moline, Illinois. The flight had arrived from Denver International Airport with 50 passengers and 3 crew members on board. There were no injuries reported.
Weather conditions were visual meteorological conditions with calm winds. The crew reported that the approach, flare, and touchdown were normal. However, once the nose wheel touched down, the aircraft began to veer left of the centerline. The crew attempted to maintain the centerline using right rudder and right differential braking, but at an airspeed of approximately 90 knots, the aircraft performed a hard left turn. The aircraft exited the left side of runway 10, briefly regained a heading parallel to the runway, and then turned left again. During the excursion, the aircraft crossed taxiway Echo and struck a taxiway sign, eventually coming to rest in the grass approximately 144 feet left of the runway edge.
The aircraft sustained minor damage, including skin tears and dents on the left side of the fuselage, damage to the right main gear outboard wheel fairing, and a bent nose wheel drag brace pin.
The investigation
Investigators examined the aircraft's flight data recorder, which captured the landing and rollout. Testing of the aircraft's spoilers, thrust reversers, rudder, and braking system revealed no anomalies. The nose wheel steering system was also found to be functioning normally during post-incident functional tests.
Detailed component-level examinations were conducted on the Steering Manifold Assembly and the Electrohydraulic Servo Valve (EHSV). During testing of the Steering Manifold Assembly, a low flow rate through the bypass valve was discovered, and a piece of clear plastic was found on the bypass valve "C2" filter screen.
Upon disassembly of the EHSV, investigators found that the C1 orifice filter seal showed material deformation and partial separation, while the C2 orifice filter seal was missing material. The void left by the missing material was larger than the nozzle orifice. Additionally, the investigation found that the C1 and C2 filter plug outer diameters were smaller than required for the unit's modification status. This undersized condition increases the gap between the filter plug and the bore, which can cause the orifice filter seals to extrude.