On July 6, 1999, approximately 1930 hours mountain daylight time, a Grumman-Schweizer G-164A "AgCat," N7869, registered to Donald Taylor, being operated by Taylor Aviation, and being flown by a commercial pilot, was substantially damaged when the aircraft collided with terrain during the landing roll following a power loss while maneuvering on an aerial application flight one mile north of New Plymouth, Idaho (refer to CHART I). The pilot was uninjured. Visual meteorological conditions existed and no flight plan had been filed. The flight, which was being conducted for the purpose of insecticide/fungicide dispensation over an onion field, was to have been operated under 14CFR137, and originated from a landing strip approximately one mile north of spray site. An inspector from the Federal Aviation Administration's Boise Flight Standards District Office reported that the pilot departed with 140 gallons of insecticide. Immediately after takeoff the pilot sensed a loss of engine performance and attempted to return to the departure airstrip. Unable to maintain altitude, he dumped the chemical load and then initiated a forced landing to an agricultural (sugar beet) field. The pilot was unable to stop the aircraft before reaching an irrigation ditch and impacted the upsloping terrain at the ditch's opposite side (refer to photograph 1). The temperature at the crash site at the time of the accident was estimated to be 90 degrees Fahrenheit. The operator was contacted twice requesting the completion of NTSB Form 6120.1/2 (Pilot/Operator Aircraft Accident Report) but failed to provide the required documentation (refer to ATTACHMENT L-I). A partial engine disassembly was conducted under the oversight of an inspector from the Federal Aviation Administration's Boise Flight Standards District Office. The inspector reported the following: "The supercharger and accessory section was removed as a unit from the back of the engine. Upon removal, a gear tooth was found in the blower drive section. Further investigation found the tooth to be from [the] impeller shaft P/N 6514." No other pre-impact failure or malfunction was noted during the disassembly (refer to ATTACHMENT FAA-I).