What was found
(CAN) PILOT APPLIED REVERSE POWER TO BACK AWAY FROM THE DOCK. WHEN THE POWER LEVER WAS MOVED BACK TO THE IDLE POSITION, THE ENGINE ACCELERATED UNCONTROLLED. PILOT TRIED TO SLOW THE AIRCRAFT DOWN BY PLACING POWER LEVER BACK INTO BETA RANGE, THE ENGINE DECELERATED. THIS WAS REPEATED A FEW TIMES, AND EACH TIME THE POWER LEVER RETURNED TO IDLE, THE ENGINE WOULD ACCELERATE UNCONTROLLED. BETA RANGE WAS CONTROLLABLE AND WAS USED TO BRING THE AIRCRAFT BACK TO THE DOCK. RIGGING VERIFIED CORRECT WITH NO BINDING CONTROLS, FCU WAS REPLACED. AIRCRAFT RETURNED TO SERVICE WITH NO RECURRENCE OF THE SNAG. (TC# 20190709014)