The pilot reported that he had completed two previous approaches to the private grass airstrip without incident. During the third approach, the airplane struck unmarked power lines located just prior to the approach end of the runway. The airplane subsequently impacted the ground and sustained substantial damage to the underside of its fuselage, rudder and both wings. The pilot stated that the he did not experience any mechanical malfunctions with the airplane. A Federal Aviation Administration inspector reported that the runway previously contained a marked 670-foot displaced threshold to compensate for the power lines. The airport manager reported that the markings were made of wood and had deteriorated away over years.