What was found
AIRCRAFT DEPARTED AT 0210 CST & WAS CLIMBING THROUGH FL230 WHEN THE CREW RECEIVED A PITCH TRIM WARNING, FOLLOWED BY EICAS MSGS, WARNING, PTRIM MAIN INOP, WARNING PTRIM BACKUP INOP, CAUTION, PTRIM FO SWITCH FAIL & THE AUTO PILOT DISENGAGED. THE CAPTAIN CALLED FOR THE QRH & CONFIRMED LOSS OF PITCH TRIM. THE CAPTAIN DECLARED AN EMER & RETURNED BACK. THE ACFT LANDED UNEVENTFULLY. IT WAS DETERMINED THE HORIZONTAL STAB ACTUATOR HAD FAILED. THE ACTUATOR WAS R & R ON 10 JAN 2016 & THE ACFT WAS RELEASED FROM A FUNCTIONAL EVALUATION FLIGHT. THE FLIGHT WAS FLOWN & THE CAPTAIN HAD NO ISSUES WITH THE PITCH TRIM AS DEBRIEFED UPON LANDING. THE ACFT WAS RELEASED BACK TO FULL SERVICE.