Date: 201502
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: Citation Excel (C560XL)
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZJX
ATC / advisory: Center ZJX
Aircraft component: Rudder Control System
Component problem: Malfunctioning
Reporter organization: Fractional
Reporter function: First Officer; Pilot Not Flying
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While cruising at FL400 the Pilot Flying (PF) attempted to input left rudder trim with the rudder trim knob since the inclinometer indication was slightly left. He noted that the rudder trim knob would not move. We both tried moving the rudder trim knob left and right independently and it would not move either direction. I pressed on each rudder pedal and I could yaw the aircraft very slightly with the autopilot on.  We discussed our options and the PF communicated with our company in order to utilize all of our resources and comply with the FOM. We made a plan in the event that we did not regain rudder control and a plan in the event rudder control was restored.Once the descent was started the PF tested rudder authority several times during the decent and at FL230 (temp -11 C) rudder control and trim were restored. The rest of the flight proceeded uneventfully. The Pilot in Command (PIC) did note that rudder forces were higher than normal during the taxi after landing. The PIC made a maintenance entry noting the issue.We were both aware of frozen rudder events from reading the ASAP newsletter and considered that as a possible explanation of the issue. I had noted water draining from a drain hole in the bottom of the rear access compartment (hellhole) in Boca Raton that morning while the APU was running. I did not find that to be unusual since it was humid and assumed the condensation was draining out like it should. It had made a puddle on the ground about a 1.5 feet in diameter. There was no water draining from the departure airport for the whole time we were there (6 hours) and it was approximately 65 degrees F.After landing in DAL I immediately checked the rear access compartment for evidence of moisture but none could be seen in accessible compartments. There was a drop of water on the drain hole I had previously seen draining. The temperature [at our destination] was 36 degrees F.


Second narrative: On our flight to ZZZ crew noticed after 1 hour in flight that the airplane needed to have the rudder trimmed to the left. Crew attempted to use the rudder trim. The rudder trim would not move left or right. Crew turned autopilot off to see if the trim would move and the airplane was flying correctly and safe to continue. Crew contacted the company; dispatch; and Chief Pilot. We all agreed it was safe to continue the flight to ZZZ where it was warmer and we could burn off more fuel. On descent thru FL230 the crew turned off the autopilot and was able to move the rudders and rudder trim. On landing the pilot flying noticed the rudders were stiffer than normal but; easily controllable for landing. On the taxi in to the FBO the rudder pedals used for taxing the airplane were also stiffer than normal.  Crew again contacted the Chief Pilot and Maintenance upon arrival to the FBO.


Synopsis: A Citation air crew noticed after about an hour's flight at FL400 that the rudder trim knob would not move and that inputs to the rudder pedals resulted in only very slight yaw. They continued to their destination and found that rudder trim and rudder inputs were restored to normal operation after they descended through lower; warmer altitudes.