On October 17, 2003, approximately 1200 central daylight time, a Prill RV-6 single-engine experimental airplane, N3XS, registered to and operated by a private individual, was substantially damaged during a forced landing following a severe vibration while descending near Snyder, Texas. The private pilot and his passenger sustained minor injuries. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 personal flight. The cross-country flight departed from the Fort Worth Alliance Airport (AFW), near Fort Worth, Texas, at 0800, with the Las Cruces International Airport (LRU), near Las Cruces, New Mexico, as their destination. The 727-hour pilot reported to the NTSB investigator-in-charge that he planned use the Winston Field Airport (SNK), near Snyder, Texas, as his intermediate refueling point for his flight to Las Cruces. The pilot stated that the engine developed a "severe" vibration when the airplane was descending thought about 5,000 feet agl, approximately 15 miles east from the airport. The pilot added that he pulled the throttle back in order to stop the vibration to no avail. He subsequently pulled the mixture to the closed position and then pulled the nose up to stop the rotation of the propeller, which stopped the vibration. The pilot located an open pasture and elected to line-up the airplane for a forced landing. The terraces that were cut in the pasture were not visible from the airplane. During the landing roll, the main landing gear collided with the first set of terraces and the airplane became airborne. During the collision with the second terrace, the left main landing gear was "ripped-back." According to the FAA inspector, who responded to the accident site, a tip of one of the wooden propeller blades was missing and that could not be found. Additionally, the left main landing gear was pushed aft, the engine firewall was bent and the engine mounts were damaged. The remaining pieces of the Aymar-Demuth (model 3600) wooden propeller were sent to the National Transportation Safety Board's Materials Laboratory, in Washington D.C., for examination; however, the Lab was unable to determine the failure mode of the propeller due to the limited amount of pieces provided. The pilot/builder of the aircraft reported that the propeller had accumulated a total of 284 hours since new.