After a normal touchdown and initial deceleration, the airplane tracked the runway heading before beginning a right turn to exit on a high-speed taxiway. The flight data recorder (FDR) revealed that, as the right turn developed, the captain released the brakes and the cockpit voice recorder (CVR) revealed that the captain commented about the lack of nosewheel steering. The captain applied full left rudder and used the nosewheel steering tiller but was unable to arrest the right turn, which continued but at a slightly reduced rate, until the airplane departed the paved surface and came to a full stop in the grass to the right of taxiway NK. During the excursion the airplane struck an airport sign and sustained minor damage to the right wing flap skin and fairing. Neither the tiller control input nor the nosewheel steering (NWS) angle were recorded by the FDR.The thrust reversers remained deployed throughout the turn with a slight asymmetry. Although this would have provided some right-turning tendency, lateral control can be maintained with one engine at idle and the other at maximum reverse thrust using rudder and differential braking. The wind gusts recorded at the airport at the time of the incident could have produced a right crosswind component up to 15 knots. Given the airplane’s crosswind component limitation of 27 knots, the slight thrust asymmetry or crosswind component would not have rendered the airplane uncontrollable. Post-incident testing of NWS components revealed no anomalies that would have precluded normal operation of the system. However, the handwheel control unit (tiller) failed the initial return-to center acceptance test. It was then cleaned, retested and passed. Although the FDR recorded a Nose Wheel Steer Status “FAIL/Not Armed” condition, and the crew observed a “STEERING INOP” message on the engine indicating and crew alerting system display, both of those messages occurred after the airplane departed the runway. The cause of these indications could not be determined. One possible condition that would have caused these Page 2 of 13 DCA22LA182indications was rapid tiller inputs, but the investigation could not assess those inputs because they were not recorded on the FDR. Several fault codes were recovered from the NWS electronic control unit (ECU). Although these codes have no date or timestamp, they are stored chronologically. The most recent code recorded was “PILOT_HANDWHEEL_RATE_FAULT,” which, according to the type certificate holder’s Fault Isolation Manual, can occur if (1) the handwheel (tiller) is moved too rapidly or (2) the handwheel is moved (or was in a position) too far from center immediately after the extension of the landing gear. While it is likely that at least one code would have been logged on the ECU due to the recorded “FAIL/Not Armed” condition captured by the FDR, had this been the only code associated with the event, it alone, nor its potential causes, can explain the loss of lateral control. The FDR recorded “FAIL/Not Armed” condition occurred after the airplane departed the runway and may have been caused by the captain’s tiller inputs, or erratic nosewheel movement due to the excursion. Neither of which could be confirmed or refuted. Given that the airplane initially tracked along the runway heading immediately after landing, it is unlikely that the nose landing gear steering angle would have been off center before or upon touchdown. The pilot used some differential braking to counter the right turn, though not to the fullest extent available. The pilot’s left rudder applications, which would have been less effective as the speed decreased, should have also commanded the nosewheel to turn up to 7° to the left of the airplane’s centerline. The pilot reported that he also used the tiller in an attempt to arrest the right turn, though a lack of recorded data precluded a determination of when and how much tiller command was applied. Post accident maintenance evaluation of the nosewheel steering system by the operator and subsequent component testing at manufacturers’ facilities found no evidence of malfunctioning or inoperative components. The cause of the right turn and why the pilot’s inputs were insufficient to arrest it, could not be determined.