A CE560XLS Instructor pilot discovered that his IOE student adjusted the rudder trim inflight to the limit with no rudder response. A frozen rudder was suspected because the aircraft had been in heavy rain prior to departure; followed by a cold high altitude flight.

Narrative

[The] rudder possibly [became] frozen in place during flight. During flight at FL390; rudder trim was selected full nose left without any change in yaw. Autopilot and yaw damper were off. The trim knob moved in a normal manner (no binding or difficulty in movement). Rudder defection via rudder pedals was not attempted during flight and no problems noted on ground or while on approach. Aircraft was in moderate rain for several hours on the ground prior to departure. The temperature at takeoff was approximately 60 degrees F. The landing temperature was approximately 80 degrees F. The rain was so heavy during preflight that a large accumulation of water splashed the crew while taking off the engine covers. Water was also seen leaking in the maintenance inspection compartment during preflight. On ground (post flight); trim tab was seen moving left and right when selected with normal movement of trim knob (no binding or difficulty in movement). However; trim tab was not in neutral position when cockpit trim indicator was in neutral position. The trim tab also did not have the same range of deflection in both directions. The following discrepancy was entered in the aircraft logbook: 'Rudder trim acted as if frozen in place. During flight at FL390 rudder trim was selected full nose left without any change in yaw. Autopilot and yaw damper were off. On ground (post flight) trim tab was seen moving left and right when selected. However; trim tab was not in neutral position when cockpit trim indicator was in neutral position. The trim tab also did not have the same range of deflection in both directions. Aircraft was in moderate rain for several hours on the ground prior to departure. Rudder defection via rudder peddles (sic) was not attempted during flight and no problems noted on ground.'