A DHC8-400 HAD COMPLETE LOSS OF PITCH TRIM CTL ON GND. CLRED WITH SVC LETTER ACTION. SECOND LEG IN AIR AT 3000 FT; LOST PITCH CTL AGAIN. CAUSE UNKNOWN.

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DURING PAX BOARDING IN ZZZ; WE GOT A PITCH TRIM FAIL CAUTION LIGHT; AUTOTRIM FAIL ON BOTH PFD'S (PRIMARY FLT DISPLAY); FULL NOSE UP PITCH INDICATION; AND NO TRIM AVAILABLE. ACR MAINT CTL SAID THERE WAS A SVC LETTER THAT CALLED FOR PULLING THE PITCH TRIM CIRCUIT BREAKER; RESETTING; AND THEN RUNNING THE TRIM FULL DEFLECTION ON BOTH DIRECTIONS. THIS FAILED TO RESTORE ANY OF THE SYMPTOMS; SO WE WERE UNABLE TO MOVE THE TRIM. HE THEN SAID TO PWR THE ACFT DOWN; AND THIS DID CLR THE FAULTS. HE ASKED IF I WAS COMFORTABLE WITH THAT; AND I SAID NO; THAT WE HAD CLRED THE FAULT; BUT DONE NOTHING TO FIX THE PROB. IT WOULD PROBABLY HAPPEN AGAIN; AND QUITE POSSIBLY IN THE AIR NEXT TIME. HE ASKED IF I WOULD BE HAPPIER IF THEY GOT A MECH OUT TO CHK THE FAULT CODES; AND I SAID YES. HE CALLED BACK 10 MINS LATER TO SAY NO MECH WAS AVAILABLE; BUT THAT THE MANUFACTURER REPRESENTATIVE HAD SAID JUST TO RUN IT. WE DID THIS FOR JUST A FEW MINS AND THE PROB DID NOT REOCCUR. HE THEN HAD ME SIGN OFF THE DISCREPANCY PER A SVC LETTER. WE THEN FLEW TO ZZZ1 UNEVENTFULLY. ON THE ZZZ1-ZZZ2 FLT; WE ONCE AGAIN GOT THE PITCH TRIM FAIL CAUTION; AUTOTRIM FAIL MESSAGES ON BOTH PFD'S; FULL NOSE UP TRIM INDICATION; AND COMPLETE LOSS OF PITCH TRIM CTL. WE RAN THE CHKLIST AND LANDED UNEVENTFULLY. THE FAILURE OCCURRED DSNDING THROUGH ABOUT 3000 FT; CLEAN; ABOUT 230 KTS AND WHILE THE TRIM BUTTON WAS NOT BEING USED. THE AUTOPLT WAS ON AT THE TIME OF THE FAILURE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED MAINT DID NOT RELEASE ANY DETAILS ON THE PITCH TRIM FIX. THE RPTR SAID ON ARR; THE AIRPLANE WAS REMOVED FROM SVC AND THE CREW WAS PUT ON ANOTHER AIRPLANE AND DISPATCHED.