CAPT OF AN MDT LOST ELEVATOR CTL OF ACFT DURING CRUISE AND DECLARED AN EMER AND DIVERTED TO A NEAR BY ARPT.

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WHILE AT 25000 FT THE ADVISORY DISPLAY PANEL INDICATED AN ELEVATOR TRIM PROB. I ATTEMPTED THE USE OF THE STANDBY ELEVATOR TRIM; TO NO AVAIL. AUTOPLT USAGE PROVIDED SOME DSCNT; BUT ERRATIC. DECLARED AN EMER AND DIVERTED TO MDT. UPON REACHING APPROX 3000 FT (WITH AN AMBIENT TEMP OF PLUS 1 DEG C) THE ELEVATOR TRIM 'UNFROZE' AND A NORMAL LNDG WAS ACCOMPLISHED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE DHC-8 HAS ELEVATOR TRIM TABS THAT NOT ONLY ARE USED FOR TRIMMING THE ELEVATOR POS; BUT ARE USED TO MOVE THE ELEVATOR (COMMONLY REFERRED TO AS 'FLYING TABS'). THE 2 OUTBOARD ARE FOR TRIM SETTING AND THE 2 INBOARD ARE FOR MOVING THE ELEVATOR. THERE IS ALSO AN 'OFF ARM' TRIM SWITCH FOR ALTERNATE TRIM CTL ON THE PLT'S PEDESTAL FOR THE PURPOSE OF DISCONNECTING THE TRIM SERVO FOR MANUAL CTL OP. THIS WAS ATTEMPTED BUT DID NOT HELP RELIEVE THE STUCK TRIM OR ELEVATOR MOVEMENT. THE AUTOPLT ON WAS THE ONLY ACTION GIVING MINOR RESULTS. HE STATED THAT HE FINALLY DECLARED AN EMER AFTER ATC COMPLAINED ABOUT HIS SLOW DSCNT. HE RESTATED THE FACT THAT WHEN HE DSNDED TO A LOWER ALT WHEREIN THE TEMP WAS UP TO 1 DEG C; THE TRIM STARTED WORKING NORMALLY. THE RPTR FURTHER STATED THAT THE NTSB AND FAA INVESTIGATED THIS INCIDENT AND FOUND THAT WATER DRAINED OUT OF THE ELEVATOR TRIM TAB AREA AFTER THE ACFT WAS PUT INTO A HANGAR FOLLOWING THIS INCIDENT. THE FAA ALSO IS TRYING TO GET THE MANUFACTURER TO PROVIDE A MODIFICATION TO THE OLDER ACFT; SUCH AS THIS ONE; THAT IS CURRENTLY BEING INSTALLED ON THE NEW ACFT TO PREVENT THIS PROB. THEY HAVE ASKED THE RPTR FOR HIS STATEMENT TO HELP BRING THIS ABOUT THROUGH EITHER THE MANUFACTURER'S VOLUNTARY EFFORT OR THROUGH A MANDATORY AIRWORTHINESS DIRECTIVE BY THE FAA.