Narrative:

Captain and myself arrived in ZZZ in this aircraft and sat for six hours waiting for our passengers. Weather in the area included low ceilings; rain-showers and scattered thundershowers. It continued to rain all day. We departed and climbing on course through 35000; the captain noticed a jump in the trim wheel. We were aware of previous auto pilot freezing scenarios on this model of aircraft and wanted to make sure that things would be ok. With this in mind; captain disconnected the autopilot to make sure it was functioning normally. It was; so we reconnected the auto pilot and continued our climb to FL410. Upon level off; we completed the cruise check; and accelerated to our cruise speed while still in climb power. Approximately 2-3 minutes after level off; the autopilot pitched the aircraft significantly nose down and started down un-commanded below FL410. The autopilot at this point disconnected from the force being applied to the trim wheel. (Our best estimate of why). Both the captain and I reached for the yoke to bring it back to nose level; at this point the captain regained control and leveled off at an altitude of 40;300 feet. I immediately advised center of our auto pilot issue and reason for our descent. The captain smoothly returned to FL410; re-trimmed the aircraft and reengaged the autopilot system with my assistance. Because of our concern; we disconnected and reconnected the autopilot several times; in cruise to be sure all was operating properly. All autopilot operations seemed normal at this point; so we continued to the destination; advising ATC of its; now normal; operation.it is both the belief of the captain and myself; that the cause of this incident was the freezing of either the autopilot servo; or the cables leading to the elevator trim of the aircraft. We believe this prevented the autopilot from properly trimming the aircraft because of the formation of ice somewhere in the trim system. Once the ice formation broke free; operation remained normal. For myself; this is the second such event in the same model aircraft; following the same scenario; (aircraft sitting in rain all day and then a high altitude trip). This is a reoccurring event in this fleet of aircraft. It happens any time the aircraft sits in all day rain and then climbs to altitude. I believe engineers need to figure out where this water is pooling and freezing in an effort to permanently find a fix using drain holes; or heat to resolve the problem.

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Original NASA ASRS Text

Title: CE560XL flight crew reported an elevator trim wheel malfunction while climbing through FL350. Crew believes the issue may have been frozen cables as aircraft sat in heavy rain before departure.

Narrative: Captain and myself arrived in ZZZ in this aircraft and sat for six hours waiting for our passengers. Weather in the area included low ceilings; rain-showers and scattered thundershowers. It continued to rain all day. We departed and climbing on course through 35000; the Captain noticed a jump in the trim wheel. We were aware of previous auto pilot freezing scenarios on this model of aircraft and wanted to make sure that things would be ok. With this in mind; Captain disconnected the autopilot to make sure it was functioning normally. It was; so we reconnected the auto pilot and continued our climb to FL410. Upon level off; we completed the cruise check; and accelerated to our cruise speed while still in climb power. Approximately 2-3 minutes after level off; the autopilot pitched the aircraft significantly nose down and started down un-commanded below FL410. The autopilot at this point disconnected from the force being applied to the trim wheel. (Our best estimate of why). Both the Captain and I reached for the yoke to bring it back to nose level; at this point the Captain regained control and leveled off at an altitude of 40;300 feet. I immediately advised Center of our auto pilot issue and reason for our descent. The Captain smoothly returned to FL410; re-trimmed the aircraft and reengaged the autopilot system with my assistance. Because of our concern; we disconnected and reconnected the autopilot several times; in cruise to be sure all was operating properly. All autopilot operations seemed normal at this point; so we continued to the destination; advising ATC of its; now normal; operation.It is both the belief of the Captain and myself; that the cause of this incident was the freezing of either the autopilot servo; or the cables leading to the elevator trim of the aircraft. We believe this prevented the autopilot from properly trimming the aircraft because of the formation of ice somewhere in the trim system. Once the ice formation broke free; operation remained normal. For myself; this is the second such event in the same model aircraft; following the same scenario; (aircraft sitting in rain all day and then a high altitude trip). This is a reoccurring event in this fleet of aircraft. It happens any time the aircraft sits in all day rain and then climbs to altitude. I believe engineers need to figure out where this water is pooling and freezing in an effort to permanently find a fix using drain holes; or heat to resolve the problem.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.