On August 16, 1997, approximately 2030 central daylight time, an Air Tractor AT-301, N2316X, was substantially damaged during a forced landing following partial loss of engine power near Vernon, Texas. The commercial pilot, the sole occupant of the airplane, was not injured. The airplane was operated by Vernon Ag Service under Title 14 CFR part 137. Visual meteorological conditions prevailed for the local aerial application flight that originated from Vernon, Texas, approximately 10 minutes before the accident. No flight plan had been filed. The pilot reported to the Investigator-In-Charge (IIC) that he was flying an application swath, at dusk, in a cotton field when his airplane's engine "lost most of its power." He stated that he attempted to land straight ahead but "had to pop-up over a road into the next field." The field was terraced and the airplane impacted one, which subsequently "flipped the airplane over on its back." The farmer, whose field was being sprayed, reported to the pilot that "there was a vertical piece of irrigation pipe in the field that was bent." The repair facility, where the engine was sent, reported that the number 5 cylinder failed due to being "apparently struck or hit by something."