What was found
DURING REPAIR OF HORIZ STAB, FOUND STRUCTURAL NUTPLATES CRACKED THROUGH THREADS. NUTPLATES ARE USED TO ATTACH ELEVATOR HINGE BRACKETS AND FORWARD STAB SPAR TO FUSELAGE. THESE PARTS CANNOT BE SEEN DURING ROUTINE INSPECTIONS. ATTEMPTED TO REUSE ONE PREVIOUSLY UNCRACKED NUTPLATE WHICH CRACKED WHEN STANDARD TORQUE WAS APPLIED TO BOLT. INSTALLED ALL NEW NUTPLATES AND TORQUES TO STANDARD TORQUE. SUBMITTER STATED FAILURE OF THESE NUTPLATES COULD LEAD TO LOSS OF ELEVATOR AND/OR HORIZ STABILIZER.