What was found
CREW MADE AN AIR RETURN, DUE TO THE ALTITUDE SELECTOR KNOB ON THE FLIGHT CONTROL UNIT BEING STUCK ON AN ALTITUDE, RENDERING THE AUTO-PILOT, AUTO-THRUST AND FLIGHT DIRECTORS TO AN UNAVAILABLE STATE. MX REMOVED AND REPLACED THE FLIGHT CONTROL UNIT AND PERFORMED OPS TEST, BITE TEST AND CAT III LAND TEST IAW AMM 22-81-12 PB401, 22-96-00-710-001 AND 22-97-00-710-001.