On May 19, 2006, about 0730 Alaska daylight time, a tundra tire-equipped Piper PA-18 airplane, N4167Z, sustained substantial damage when it collided with brush and the ground following a loss of engine power and subsequent forced landing, about 13 miles north of Naknek, Alaska. The airplane was being operated as a visual flight rules (VFR) cross-country personal flight under Title 14, CFR Part 91, when the accident occurred. The airplane was operated by the pilot. The private certificated pilot, and the two passengers, were not injured. Visual meteorological conditions prevailed. The flight originated at the Naknek Airport about 0710, en route to a lodge on the Alagnak River, and no flight plan was filed. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on May 19, the pilot reported that he was in cruise flight, about 1,000 feet above ground level (agl), when the engine began to lose rpm. The pilot said the engine gauges appeared to be normal, but the rpm decreased from 2,300 to 1,000 rpm. The pilot said he performed all emergency procedures, including application of carburetor heat, but the airplane could not sustain level flight. He turned toward the Diamond J airstrip, located along the Kvichak River, and prepared for an emergency landing. The pilot said the airplane collided with alders and then bounced off the ground, coming to rest near the end of runway 13 at the airstrip, facing the opposite direction of flight. The airplane received structural damage to the fuselage, empennage, and wings. The pilot reported that the weather conditions consisted of overcast skies about 1,100 feet, with a temperature of about 45 degrees F, with no visible moisture. He was unable to determine what precipitated the loss of engine power. The airplane was not examined by NTSB or Federal Aviation Administration (FAA) personnel.