A CANADAIR CL65 IN CRUISE AT FL280 EXPERIENCED AUTOPLT PITCH TRIM AND MAIN ELECTRIC STABILIZER TRIM IN NOSE DOWN DIRECTION.

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'AUTOPLT PITCH TRIM' CAUTION MESSAGE WAS DISPLAYED ON EICAS. FO HAD CTL OF ACFT; AUTOPLT ENGAGED. CAPT DIRECTED FO TO MAINTAIN ACFT CTL WHILE THE QRH WAS REFED FOR CORRECTIVE PROC. FO NOTICED NO ABNORMALITIES IN ACFT ATTITUDE; AUTOPLT REMAINED ENGAGED. QRH PROC WAS PERFORMED; AND PROB PERSISTED; SO IN ACCORDANCE WITH THE QRH; THE AUTOPLT WAS DISCONNECTED AND THE ACFT WAS HAND-FLOWN BY THE FO. NOTICEABLE PITCH-UP FORCES WERE FELT ON THE CTLS; WHICH REQUIRED A SMALL AMOUNT OF FORCE TO COUNTERACT AT APPROX 310 KIAS. ATTEMPTS TO TRIM AWAY THIS FORCE WERE UNSUCCESSFUL; AS THE PITCH TRIM WOULD NOT OPERATE ANY FARTHER IN THE NOSE-DOWN DIRECTION (STICK AT AN INDICATION OF '3'). DISPATCH AND MAINT WERE CONTACTED BEFORE ATTEMPTING ANY AIRSPD CHANGES. WHEN AIRSPD WAS DECREASED TO ABOUT 290 KIAS; THE NOSE DOWN FORCE REQUIRED FOR LEVEL FLT WAS REDUCED TO ZERO AT THE CURRENT STABILIZER TRIM SETTING. AS AIRSPD WAS FURTHER REDUCED WHILE DSNDING INTO DEST (250-270 KIAS) THE ACFT REQUIRED NOSE-UP TRIM; WHICH WAS EASILY ATTAINABLE. THE STABILIZER TRIM THEN WORKED WELL IN THE NOSE-UP AND NOSE-DOWN DIRECTION; AT SETTINGS HIGHER THAN AT THE TIME OF THE ORIGINAL CAUTION MESSAGE. ALL SUBSEQUENT AIRSPD CHANGES AND CONFIGN CHANGES WERE MADE IN 10-20 KIAS INCREMENTS; AND ALL CONTROLLABILITY CONTINGENCIES (TO INCLUDE A GAR) WERE FULLY BRIEFED BTWN THE CAPT AND FO. A LONG; UNEVENTFUL STRAIGHT-IN FINAL APCH WAS FLOWN TO DEST; WHERE A SAFE FULL STOP LNDG WAS MADE. ON THE GND; IT WAS NOTED THAT THE STABILIZER TRIM COULD NOT BE ADJUSTED TO ANY SETTING LOWER THAN '3' IN THE NOSE DOWN DIRECTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR REFUSED TO ANSWER ANY AND ALL QUESTIONS PERTAINING TO THE PITCH TRIM AND MAIN ELECTRIC TRIM FAILURE UNTIL THE ASRS IDENT STRIP IS RETURNED.