On May 29, 1996, at approximately 0930 Pacific daylight time, N48559, a Schweizer G-164B operated by Fountain Inc. of Moscow, Idaho, a 14 CFR 137 agricultural aircraft certificate holder, was substantially damaged in a forced landing following an engine malfunction while conducting a 14 CFR 137 agricultural aerial application flight near Moscow. The commercial pilot of the single-seat aircraft was not injured. Visual meteorological conditions prevailed and no flight plan had been filed. The pilot reported: "While applying herbicide on wheat field...the windscreen suddenly became ob[s]cured by smoke and oil. Attempted landing on ridge top in hilly country while forward visibility was poor." FAA inspectors from Spokane, Washington who examined the aircraft wreckage after the accident reported that an access plug was missing from the front of the crankcase of the aircraft's Pratt & Whitney R-1340 radial engine. Photographs taken by the inspectors also showed oil staining on an aircraft component. The operator did not furnish any information regarding the maintenance, service or inspection history of the aircraft or engine with his accident report.