On July 26, 2004, at 1326 Pacific daylight time, a WSK PZL Mielec M-18A [Dromader], N495WP, was substantially damaged when it struck a fence post during a forced landing attempt near Burns, Oregon. The commercial pilot was not injured. Western Pilot Service Inc., Maricopa, Arizona, was operating the aircraft for the United States Department of Agriculture, United States Forest Service. Visual meteorological conditions prevailed for the fire retardant application flight that originated approximately 20 minutes before the accident. A company VFR flight plan was on file. The pilot reported that he was attempting to climb following a retardant drop. When he moved the throttle forward, the engine did not respond. He said the airplane started loosing altitude, and pumping the throttle had no effect. The pilot saw a plateau just southwest of the fire, and he performed a forced landing to it. During the landing, the throttle would not retard all the way. In order to land the airplane where he wanted to, he turned the fuel selector to the off position. During the landing roll, the left wing impacted a fence pole bending ribs and wrinkling the aileron. Post accident examination revealed that the bolt normally connecting the throttle linkage clevis to the throttle arm was missing. The investigating team examined a sister aircraft from the same provider. They found that a nylon lock nut had been installed on the clevis bolt, and it was loose enough to be removed without the aid of any tools. The industry standard, found in FAA Advisory Circular AC43.13-1B, chapter 7, section 4, paragraph 7-64 states that nylon locking nuts should not be used with rotating components or assemblies. It further states that nylon and/or fiber nuts are highly susceptible to wear from repetitive use and from high heat applications.