Citation Excel (C560XL)
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
Aircraft Equipment Problem Dissipated; General Maintenance Action
Aircraft; Weather
While climbing out of FL350 in IMC with anti-ice on at 1;000 FT per minute; the aircraft suddenly pitched down into a descent. Upon recognizing the uncommanded change in the aircraft attitude; the autopilot was disengaged and the aircraft was leveled off at FL350. ATC was informed of the need for us to maintain FL350 until an appropriate assessment could be made to determine the proper action to be taken as a result of the malfunction. After disengaging the autopilot it was determined that the control wheel required excessive force to change the pitch of the aircraft. I requested that the SIC contact ATC to request a lower altitude and ATC complied with our request. While descending I observed that the aircraft elevator control was effected and it was abnormally difficult to move the aircraft controls and upon application of forward and/or back pressure on the control wheel the aircraft had a tendency to move to an exaggerated nose angle (up or down) depending on the control input; so very minor and slow corrections needed to be accomplished in order to have the required effect. In our initial descent I utilized the electric trim and noticed that the trim wheel moved as commanded; however using the trim had the same undesirable effect of causing the aircraft to move to more exaggerated pitch angle than was commanded. As a result; the trim wheel was used to a limited extent and only manually. At no time during the flight did the elevator trim become jammed or inoperative. After descending to below the freezing level the pitch controls seemed to at one point to suddenly become less difficult to operate and eventually returned to having a normal tactile feel and normal pressures could be used to control aircraft pitch and no excessive pitch changes occurred when using pitch commands. Upon our initial approach; with normal indications regarding the use of the flight controls; the autopilot was re-engaged and a normal approach and landing were executed safely and uneventfully.It seemed as though the flight conditions at the time of the event contributed to the incident. The event occurred in icing conditions with temperatures below -20C and the malfunction was resolved upon our descent below the freezing level. This indicates that formation and the existence of ice on the control cables or actuators which are associated with the elevator contributed to the initial incident and the abnormal pressures required to change the aircraft pitch. My recommendation is that the matter be investigated to ascertain the cause of the malfunction in order to take the appropriate action necessary to remedy this problem in order to prevent this event from occurring again in the future on this and other aircraft of this type.
The reporter states that the cause of the incident is being investigated and that no definitive cause has been reported to him. All indications are that ice on the elevator control cables from water collected in the belly of the aircraft may have been the culprit.
CE560 Captain experiences aircraft pitch down climbing out of FL350 and disengages the autopilot to find the elevator controls to be very stiff. Once out of freezing conditions control forces return to normal and an uneventful landing ensues.