On September 29, 2002, about 1630 Alaska daylight time, a tundra tire-equipped Piper PA-18 airplane, N723C, had an in-flight separation of a propeller blade while in cruise flight about 12 miles southeast of Ekwok, Alaska. The airplane received minor damage during a forced landing. 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 commercial certificated pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed. The flight originated at the Ekwok Airport about 1615. No flight plan was filed, nor was one required. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on October 2, 2002, the pilot reported he was en route to Naknek, Alaska, and was in cruise flight about 700 feet agl. The airplane suddenly developed a severe vibration. He reduced engine power to idle, and then shutdown the engine. He said he landed on tundra-covered terrain. During the landing roll, the airplane nosed down, but received only minor damage to the air intake housing. After the emergency landing, the pilot discovered that about 10 inches of one propeller blade separated from the rest of the blade. He described the separation as a straight chordwise fracture.