The pilot's improper decision to return to the runway instead of landing straight ahead when the engine lost power and his failure to maintain adequate airspeed while maneuvering for an emergency landing, which resulted in an exceedance of the airplane's critical angle of attack and an aerodynamic stall. Contributing to the accident was the pilot's failure to properly secure the left main fuel tank cap after refueling, which resulted in a loss of engine power due to fuel starvation during the takeoff climb.