Date: 201411
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: Regional Jet 200 ER/LR (CRJ200)
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Rudder Trim System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Human factors: Situational Awareness
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Ground Incursion Taxiway
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; General Declared Emergency
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: We were descending into ZZZ and I was making my arrival PA to the passengers. As I was looking down to switch back to COMM 1; I felt a violent yaw motion in the aircraft. As I was looking up; I saw the rudder trim was full nose left and the Captain (CA) had his hand on the rudder trim. I immediately asked him what happened and he said that the rudder trim had not stopped while he was trying to trim up the aircraft. He then continued to try to re-trim the aircraft; and there were a few more very noticeable yaw moments; but not as bad as the first one. I stated that this is an uncommanded yaw moment; and asked if we should disconnect the yaw dampers as per the memory items; and the CA said not to worry about it. He then said we should write it up; and called the FA to make sure she was OK with everyone in the back. When he got off the call; he then said he was seriously thinking of declaring an emergency. I told him; as far as I know this is an uncommanded yaw moment and is a flight control failure. He said he needed a second to think about it; and then; after about 30 seconds; said he was going to tell ATC what was going on and ask for priority. I told him this is a flight control failure again; and an emergency needed to be declared and he agreed. As we continued; the brick continued to bounce back and forth between a half scale deflection on either side; and was uncontrollable. We declared an emergency and were already set up for XXL; which was the closest to us; so decided it was best to continue for that runway. I; again; asked him if we should disconnect the yaw dampers as per the memory items; and he; again; said no and said he was not aware of the memory item for the uncommanded yaw motion. We were on approach at this time; and had to configure the aircraft; so had no more time to discuss this. We never did run the QRH due to time constraints. We landed and taxied to the gate. During the taxi; as a result of all the chaos; we missed a turn and ended up in the deice pad. Ground asked us to ask iceman for permission to continue to 'K'; which we got; and continued to the gate. The CA had me write the write up; as he was calling Flight Control. He told me to put his number down; as required; and told me what to write. I wrote what he said; and placed the AFDL on the pedestal against the throttle quadrant. He read the write up; and asked me if there was a QRH procedure. I said I believe so; as I know for sure there were memory items which I was questioning him about; looked in the QRH to verify there is a procedure; and told him there was. He then told me to add in to the write up that they had ran and complied with the QRH; which we did not. I added it at his request; as he was on the phone with flight control.


Second narrative: While I was descending into ZZZ somewhere between 14;000 to 18;000 feet I adjusted the rudder trim slightly to the left. After taking my hand off the trim the rudder continued full left. I overrode the malfunction and then ran the QRH. The rudder 'brick' continued vacillating right and left after I had centered the 'brick'. I advised ATC and landed in ZZZ. We landed without an event. After clearing the runway we taxied to the gate. While taxing I followed the wrong yellow line onto the de-ice pad. I stopped the aircraft on the pad and notified ground of my mistake. Ground had us contact the Ice-man for taxi instructions. After approaching the end of the de-ice pad we contacted ground for taxing instructions to the gate. We were about 100 yards away from the actual de-icing that was going on.


Synopsis: CRJ-200 flight crew experienced a rudder trim issue causing the aircraft to yaw to the left during the descent phase.  The crew landed at their destination airport. During taxi the crew followed the wrong yellow line and ended up on the de-ice pad.