What was found
ON RUN UP & PREPARING FOR FLIGHT, ELEVATOR DOWN AUTHORITY COULD NOT BE ACHIEVED UNDER NORMAL CONTROL PRESSURE. PILOT RETURNED AND GROUNDED THE AIRCRAFT FOR MAINTENANCE. INVESTIGATION IDENTIFIED BINDING TO BE LOCATED WITHIN THE PITCH TRIM CARTRIDGE ASSEMBLY PN 10681004. DUE TO FOREIGN OBJECT INTRUSION INTO THE PITCH TRIM CARTRIDGE CUTOUT OPENING IN THE FORM OF A SINGLE DISCARDED COTTER PIN TAILING. WHILE THE PRE-TAKEOFF PITCH TRIM TEST WAS BEING PERFORMED, AND THE ELEVATOR PLACED IN THE NOSE UP ATTITUDE, THE COTTER PIN LODGED. THE AFT SPRING RETAINER IN THE PLACE NOT ALLOWING IT TO RETURN WHEN TRIM WAS COMMANDED DOWN.