Regional Jet 700 ER/LR (CRJ700)
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Critical
Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport
Aircraft
On climb out passing 18000 we started getting erratic uncommanded rudder movement.  First to the right then to the left. We checked our flight control page on the EICAS and noticed everything was normal on the page with the rudder indication dead center but the rudder was still moving as the brick was moving back and forth and the aircraft was yawing back and forth.  It was not violent; but you could feel it moving. We slowed down to to 250 knots initially to avoid aggravating the situation as every rudder input you have given to the aircraft slightly aggravated the rudder. We were worried that we had a PCU that was going bad and it could possibly hardover on us.  We opted to return [to the departure airport] since it was closest with most CFR in the area and the longest runways.  We told the controllers what was happening and told them we needed to slow down and descend.  We proceeded to complete the checklists and headed back. On the descent the aircraft was still moving back and forth with the rudder still moving erratically.  We asked the controllers for wide shallow turns on to final and a slower speed which made the ride back much better for us. Throughout the remainder of the flight; the rudder indication stayed centered and normal even though the actual aircraft was still swaying from rudder movement. We had no other indication or any alerted messages.  After landing we taxied to the gate and parked without incident; shut the aircraft down and went outside to look at the rudder for any damage.  When we got outside to look at the rudder; it was still moving back and forth with the engines shut down and the hydraulic pumps still on.
The reporter has not been informed of the cause of these rudder oscillations but he believes that a PCU anomaly is the only unannunciated fault that could cause the rudder to move uncommanded.
CRJ700 Captain reported passing FL180 experience erratic uncommanded rudder movement. With the aircraft yawing back and forth; no movement is visible on the EICAS flight control page. The crew elects to slow down; and return to the departure airport for an uneventful landing. After parking at the gate with the engines shut down but with the hydraulic pumps on the rudder was still moving back and forth.