The pilot's failure to maintain adequate airspeed while turning to join the final approach leg of the airport traffic pattern with full airplane-nose-up trim, which resulted in the airplane exceeding its critical angle of attack and experiencing an aerodynamic stall. Contributing to the accident were the pilot's mis-use of the forward elevator flight control input with the autopilot engaged, which resulted in the full airplane-nose-up trim; his failure to recognize and correct the mis-trimmed airplane per the emergency procedures; and the excessive control forces required to maintain control of the airplane in the mis-trimmed condition, which resulted in pilot fatigue.