Citation Excel (C560XL)
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 85; Flight Crew Total 2700; Flight Crew Type 350
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Overcame Equipment Problem; General Maintenance Action
Aircraft
Climbing through approximately FL200; [we] received a Pitch Trim warning indication. I called for the checklist and Pilot Monitoring (PM) referenced the correct checklist. Among other items in the checklist; autopilot was disengaged; pitch trim circuit breaker was pulled; and aircraft was hand flown for the remainder of the flight. Leveling at assigned altitude of FL280 took higher than normal forward yoke pressure and trim wheel would turn but was abnormally resistant. Within 30 seconds to 1 minute of leveling; something in the pitch control of the aircraft seemed to release causing the aircraft to become unstable in pitch and momentarily out of control. Altitude variation was within plus or minus 800 feet and the aircraft was stabilized at FL280. Deviation was reported to ATC. Aircraft was slowed and stabilized at 180 KIAS. Destination weather was forecast 700 foot ceilings and departure airport had weather moving in so I requested ATC provide information on VFR or MVFR options. I did not want to take the chance of having further control issues IMC close to the ground. [An appropriate nearby airport] was selected. I briefed that we should be vigilant while configuring for landing that any further upset could be the result of the last configuration change. When appropriate; I requested and PM selected flaps 7. Immediately the pitch trim wheel increased resistance to movement again (abnormal) and the aircraft tried to pitch up. Holding nose down yoke pressure to maintain assigned altitude the same 'release' of pitch force happened but with significantly less upset. Decision was made to land no flaps which was completed without incident. Aircraft was shut down and secured. Maintenance was dispatched to the aircraft and inspected the aircraft finding no cause.
The reporter stated that upon more thorough inspection Maintenance found evidence of 'moisture contamination' on a trim mechanism that could have frozen in flight and melted during descent.
A C560XL Captain reported pitch trim problems that caused an upset at FL280. The flight diverted to a suitable alternate.