What was found
(CAN) IN CRUISE THE CREW NOTICED THAT THE STAB TRIM WAS INOPERATIVE IN NOSE DOWN MODE. FURTHER TROUBLE SHOOTING AFTER LANDING CONFIRMED THE ACTUATOR FAILED INTERNALLY. LOUD GRATING NOISE WHEN TRIMED IN THE NOSE DOWN DIRECTION. ACTUATOR REPLACED AND AIRCRAFT RETURNED TO SERVICE. FURTHER INVESTIGATION OF OTHER ACTUATORS IN THE FLEET SHOWED AN AVERAGE RELIABILITY OF 47 PERCENT TO TBO. THIS ACTUATOR WAS THE HIGHEST TIME ACTUATOR IN OUR HISTORY.