Date: 201602
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Night
Aircraft: Citation Excel (C560XL)
Operator: Air Taxi
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Aeroplane Flight Control
Component problem: Malfunctioning
Reporter organization: Air Taxi
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Detector: Person Flight Crew
When detected: In-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew Returned To Clearance
Contributing factors: Aircraft; Weather
Primary problem: Aircraft

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.


Second narrative: The aircraft sat on the ground for 5+ hours in pouring rain at above freezing temps before the event. I have been assigned to the aircraft for three days without event. The aircraft is also fairly recently out of paint. We took off with 2 passengers. Climbing thru the mid-thirties; we experienced a 'jump' in the elevator trim wheel and a very slight 'feel' of the same. I momentarily disconnected the autopilot and actuated the elevator trim both electrically and manually. I re-engaged the autopilot and stated to my partner that there did not appear to be any binding or abnormality at all. We continued the climb to FL410 unrestricted and leveled off. As we accelerated in the climb detent to Maximum Operating Mach Number (MMO); the temp was -37 degrees Ram Air Turbine and we were VMC on top. As we approached MMO; the autopilot abruptly disconnected and the chime sounded as the yoke rapidly moved towards the panel. The First Officer and I both reached for the yoke as I retarded the throttles to avoid the over speed. We recovered from the pitch upset 700 feet later. We then ran the trim again electrically and mechanically and determined it was functioning normally. After satisfying our curiosities; we turned the autopilot back on as it was trimmed for hands off flight. It seemed that something had frozen and the autopilot to no longer 'muscle' the force necessary to hold our pitch attitude. After much discussion and troubleshooting; we determined if was safe for us to return to FL410 and continue to our destination. No other anomalies were noted for the rest of the trip. We did not get any warning (i.e. PITCH MSTRIM) or anything. The autopilot just disconnected and the yoke went forward.


Synopsis: 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.