Date: 201307
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: Citation X (C750)
Operator: Fractional
Operating under: Part 91
Flight plan: IFR
Mission: Ferry / Re-Positioning
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Altitude Hold/Capture
Component problem: Malfunctioning
Reporter organization: Fractional
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Time Pressure; Situational Awareness; Confusion
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: ATC cleared us to FL370 on initial descent into our destination. Descending through FL373 for FL370 aircraft appeared that it would not level off at FL370 as set in altitude selector. The Captain disengaged autopilot to level off aircraft manually and aircraft would not level off due to the fact elevator inputs on control wheel were not responding. I was concerned as we were descending through FL370 and reiterated the 'CLIMB' command issued by the RA as there was traffic 1;000 FT below and within 3 miles of our position. At approximately FL367 the Captain arrested descent by physically overpowering the autopilot and was then able to have control authority of elevator and aircraft. ATC was notified of RA event and after flying the aircraft at level flight at FL370; the Captain returned aircraft control to me. After a couple of minutes of manually flying the aircraft at FL370; we agreed to re-engage the autopilot. From that point in flight there were no further issues with the autopilot and the aircraft performed normally until aircraft shutdown.I do not think it would be possible to predict the flight control malfunction and believe the appropriate action was taken by the Captain to return to the assigned altitude and to overpower; what appeared to be; a stuck elevator autopilot servo. However; I am concerned about the aging fleet of our CE-750's as it is noticeable that there seems to be more serious malfunctions of the Citation X in general. In the past 3 tours I have encountered a magnetic chip detect warning light coming on while on descent; a hung start; a FGC failure/ADC mis-compare while on takeoff; and now this issue with the flight controls. It appears that the aging of our Citation X fleet is becoming more noticeable and this concerns me as it will affect the overall reliability of our aircraft and the flight crews' ability to perform their job in a reliable manner. This will cause more undue delays and ultimately affect our ability to give our owners the service and reliability they have grown to love here at the carrier.

Callback: The reporter stated that the force required to overpower the elevator was estimated to be about 25 pounds. The event got the crew's attention but he has not yet heard what the cause was. The reporter speculated that since aircraft had been parked in moisture earlier in the day; some moisture may have entered the control areas and frozen.



Synopsis: A CE-750 autopilot failed to capture FL370 during descent so the Captain disconnected it and manually returned to the selected altitude as the TCAS RA commanded 'CLIMB.'