MULTIPLE CREW ALERTING SYSTEM MESSAGES REGARDING PITCH ATTITUDE CONTROL CAUSE FLT CREW OF UNIDENTIFIED ACR ACFT TYPE TO DECLARE EMERGENCY AND RETURN TO THEIR DEP ARPT.

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CLBING FROM ZZZ TO ZZZ1 THROUGH ABOUT FL140 WE RECEIVED THE FIRST CAS MESSAGES: MACH TRIM; STABILIZER TRIM. RAN ASSOCIATED CHKLIST WHICH DIRECTED US TO DISCONNECT AUTOPLT; THEN RE-ENGAGE AUTOPLT. A MODERATE NOSE-UP CONDITION WAS NOTED AT THAT TIME. WE THEN CONTACTED MAINT CTL VIA RADIO TO INFORM THEM OF OUR SITUATION. IN THE MEANTIME WE THEN RECEIVED THE AUTOPLT PITCH TRIM; AUTOPLT TRIM IS NEW CAS MESSAGES ALONG WITH THE ORIGINAL 2 MESSAGES. WE ADVISED MAINT CTL OF THE ADDITIONAL MESSAGES AND THEY ADVISED US TO CONTINUE ON TO ZZZ1. WE THOUGHT THIS WAS ILL-ADVISED AS TO THE UNKNOWN FUNCTION ABILITY OF OUR FLT CTL SYS (THE PITCH WAS FROZE AT 3.4 NOSE DOWN). WE HAD LEVELED OFF AT FL230 TO PRECLUDE POSSIBLY RETURNING TO ZZZ. AT THAT TIME MAINT CTL STATED THAT IT WOULD BE 'ON US' IF WE ELECTED TO RETURN TO ZZZ. WE INFORMED MAINT THAT WE WOULD RETURN TO ZZZ. ZZZ2 ATC WAS ADVISED OF OUR INTENTIONS AND THEY VECTORED US TOWARDS ZZZ. UPON CONTACTING ZZZ APCH; WE DID DECLARE AN EMER DUE TO THE UNKNOWNS ASSOCIATED WITH THE FLT CTL PROB. THE LNDG WAS UNEVENTFUL AND WE WERE SWAPPED INTO ANOTHER ACFT. MAINT CTL'S ADVICE TO CONTINUE TO DEST NEEDS TO BE ADDRESSED. WE FELT THAT THE ACFT WOULD BE BROKEN HARD IN ZZZ1 AND THAT PITCHING MOMENTS UPON AUTOPLT DISCONNECT MANDATED THAT WE RETURN TO ZZZ.