A C525 AT FL370 DECLARED AN EMER AND DIVERTED DUE TO AUTOPLT OUT OF TRIM WARNING. WITH DISENGAGED AUTOPLT THE ACFT PITCHED UP AND NEEDED EXCESSIVE FORCE TO MAINTAIN LEVEL FLT.

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WHILE ENRTE FROM ZZZ1 TO ZZZ2 AT 37000 FT MSL; I OBSERVED A BOXED 'E' ON MY PFD (PRIMARY FLT DISPLAY). THIS INDICATED THAT THE AUTOPLT WAS OUT OF TRIM. IT EXTINGUISHED AFTER APPROX 5 SECONDS. I THEN ASKED MY COPLT TO PULL OUT THE ABNORMAL CHKLIST IN ORDER TO REVIEW THIS ABNORMALITY. WHILE REVIEWING THE ABNORMALITY; THE BOXED 'E' AGAIN APPEARED ON MY PFD. WE BEGAN TO EXECUTE THE AUTOPLT OUT OF TRIM CHKLIST; WHICH CALLED FOR THE AUTOPLT TO BE DISENGAGED. UPON DISENGAGING THE AUTOPLT; THE ACFT PITCHED UP AND REQUIRED EXCESSIVE FORCE TO MAINTAIN LEVEL FLT. BOTH THE ELECTRIC AND MANUAL TRIM WERE INOPERATIVE. AFTER CONSULTING WITH MY COPLT; WE AGREED WE SHOULD DECLARE AN EMER AND LAND AT THE NEAREST SUITABLE ARPT; IN THIS CASE; ZZZ3. MY COPLT THEN DECLARED AN EMER WITH ZZZ CTR AND WE BEGAN AN IMMEDIATE DSCNT. IN ORDER TO EXPEDITE OUR DSCNT; I EXTENDED THE SPEED BRAKE AND THE THROTTLES WERE AT IDLE. THIS STILL REQUIRED EXCESSIVE FORWARD PRESSURE ON THE YOKE TO DESCEND. I DIRECTED MY FO TO CONTACT OUR MAINT DEPT VIA COCKPIT PHONE FOR ANY ADDITIONAL INPUT THAT MAY HELP US WITH OUR SIT. THEY HAD NO SUGGESTIONS AT THE TIME. I THEN INSTRUCTED MY COPLT TO REBRIEF OUR PAX ON THE EMER EVACUATION PROCS AND ANSWER ANY QUESTIONS THEY MAY HAVE. WHILE ON APCH FOR LNDG; I REQUESTED THAT THE EMER EQUIP BE STANDING BY. PRIOR TO LNDG WE RAN THE JAMMED ELEVATOR TRIM TAB CHKLIST. AT APPROX 2000 FT AGL; I WAS ABLE TO MOVE THE MANUAL TRIM WHEEL AND THE LNDG TURNED OUT TO BE NORMAL. OVERALL; THIS INCIDENT WENT WELL. I ALSO PULLED THE PITCH TRIM CIRCUIT BREAKER TO AVOID ANY FURTHER COMPLICATIONS WITH THE SYSTEM. THIS WAS NOT ON THE CHKLIST; BUT USING MY EMER AUTHORITY I FELT IT WAS A PRUDENT THING TO DO. THE ACFT WAS WASHED THE NIGHT PRIOR TO THE FLT. I FEEL IT MIGHT BE POSSIBLE THAT THERE MIGHT HAVE BEEN SOME RESIDUAL WATER IN THE TAIL SECTION THAT MIGHT HAVE FROZEN THE CONTROL CABLES TO THE PITCH TRIM SYSTEM. THIS WOULD EXPLAIN THE RETURN OF MANUAL TRIM AT THE LOW ALTS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED DURING THE DSCNT TO THE DIVERSION FIELD AT 2000 FT; THE MANUAL TRIM WHEEL COULD BE MOVED AND THE LNDG TURNED OUT TO BE NORMAL. THE RPTR SAID THE STABILIZER TRIM WAS CHECKED OUT AND WAS NORMAL. THE RPTR STATED THE AIRPLANE HAD BEEN WASHED THE NIGHT PRIOR TO THE FLT AND BELIEVES WATER GOT INTO THE TAIL SECTION AND FROZE ON THE PITCH TRIM CABLES. THE RPTR SAID THAT EXPLAINS THE MANUAL PITCH TRIM OP AT THE LOWER ALT.