Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Critical
General Maintenance Action
Aircraft; Weather
Enroute; three hours into flight at FL360; we noticed the rudder trim  was slightly out of balance. When we attempted to trim the rudder; we found the rudder trim knob was frozen and would not move.  We very carefully moved the rudder pedals to confirm the rudders were still functional and found they were.  We notified maintenance of the issue and continued on to destination without declaring an emergency; since all primary flight controls were working normally.  The crosswinds were less than 3 knots.  The approach and landing were uneventful.  After landing and clearing the runways; we again attempted to move the rudder trim knob and found it still frozen.  At block-in we tried the knob once more just before shutting down and found the knob was functioning normally.  We wrote a maintenance discrepancy and debriefed with maintenance personnel in the cockpit after shutdown.  We also included in the maintenance write-up that the First Officer observed a significant amount of water draining from the airframe during his exterior pre-flight.  Suspected cause is ice buildup but is not confirmed.
The reporter did not remain with the aircraft after the incident and does not know if there were any maintenance findings.  A thunderstorm had passed over the field just prior to departure and a great deal of water was running of the air frame; in addition heavy rain was encountered during departure.
MD11 flight crew experiences frozen rudder trim at FL360.  Flight continues to destination for a normal landing with rudder trim still frozen.  Just prior to engine shut down the trim is found to be free.