MD-80 Series (DC-9-80) Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Confusion
Aircraft Equipment Problem Critical; Inflight Event / Encounter Other / Unknown
Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft
Climbing thru FL275 we had an un-commanded rudder input. I asked the First Officer (FO) to descend to FL240 and slow to 280 knots. We had no other abnormal indications. The FO was hand flying the aircraft. I used the QRH and turned the Yaw Damper to 'Override'.  Everything was normal; so I returned the Yaw Damper to the 'Normal' position. We [advised ATC of aircraft condition] and proceeded to ZZZ1 with no other issues. Landing was normal. The flight attendants were given the test briefing and instructed to call me if they felt or heard anything abnormal. Maintenance did find a fault in the Restricted Rudder portion of the rudder package.
Reporter stated the First Officer (FO) was hand flying the aircraft in 'Climb Thrust' with the Autopilot 'Off' and climbing through FL275; when suddenly they experienced a hard over Right Rudder; similar to a slam; an impulse move; with the rudder pedals moving. Captain and the FO verified neither had moved the rudder pedals. The aircraft felt like a 12-15 degree hard over had occurred as the MD-80 rolled to the right with the Right rudder pedal not quite full forward. All hydraulic pressures and cockpit indications were good. Nothing to indicate anything had just happened.Reporter stated as he began to input left rudder pedal; he also noticed the left pedal was starting to move on it's own; as if trying to correct for the right hard over. The flight attendants in the aft cabin thought the aircraft had lost an engine. Following their QRH procedures; Captain moved the Yaw Damper switch to the Override (OVRD) position which depowers the Yaw Damper servo. He later moved the Yaw Damper switch to the 'On' position after everything seemed normal. They continued to destination airport; they did not divert. In discussions with Maintenance after they landed; the thought was that perhaps some obstruction; moisture or icing on the Pitot tubes at the Vertical stab could have affected the Rudder Throw Limiter Actuator or linkage.  Maintenance did replace one of the Rudder Limiter switches and also noted a Fault had been recorded involving the Restricted Rudder portion of the rudder PCU. Otherwise; no other discrepancy was found.   Captain reportedly had experienced his first MD-82 Rudder hard over several years ago. In that incidence; the Right pedal did go full right rudder; pedal had bottomed out; with the aircraft in a heavy right roll. Captain credits previous military experience with inflight adverse upset training helped control the MD-80 in a slip configuration until they could get the left rudder pedal to move; which did not happen very fast. In that incident; no cockpit indications or Circuit Breakers (C/Bs) out; nothing. Captain hopes this safety report will help other pilots in the future and help Maintenance determine the cause for these uncommanded rudder movements in the MD-80 aircraft.
Captain reports an uncommanded Right Rudder hard over of approximately 15 degrees and roll had occurred on their MD-80 while climbing thru FL275. Descended to FL240; slowed to 280 knots. Followed QRH and turned Yaw Damper to 'Override'. Descended to FL240. Everything normal.  Returned Yaw Damper to 'Normal' position. Landing normal. Maintenance found fault in the restricted rudder portion of the Rudder PCU and Rudder Limiter switch.