On October 22, 2006, at 0741 Pacific daylight time, a Cessna 152 airplane, N45929, nosed over during a forced landing near Prunedale, California. A loss of engine power preceded the forced landing. The airplane was operated by United Flight Services of Watsonville, California, under the provisions of 14 CFR Part 91 as an instructional flight. The certified flight instructor sustained minor injuries and the student pilot was not injured. The airplane was substantially damaged. Visual meteorological conditions prevailed and a flight plan was not filed. The flight originated from Watsonville, California, about 25 minutes prior to the accident, and was destined for Salinas, California. According to the flight instructor, he and his student departed on the flight to Salinas when, about halfway to their destination, the engine cowling and instrument panel began to shake violently. He adjusted the mixture setting and scanned the instruments and noted that the engine oil temperature and pressure were still indicating in the normal range. Shortly after that, the engine began shaking and then stopped entirely and gray smoke filled the cockpit. The flight instructor noted flames coming from the nose area of the aircraft and attempted to find a suitable landing site through the smoke and haze. The flight instructor elected to land in an agricultural field. The airplane touched down on the main landing gear, and as the nose wheel touched down it dug into the dirt and the airplane nosed over. When the instructor and student exited the airplane, they again observed a small fire near the front engine area, but it extinguished itself. According to a Federal Aviation Administration (FAA) inspector, the engine was examined and a large hole was found on the bottom side of the engine crankcase between the number 1 and number 2 cylinders. A concentration of oil was found on the inside of the cowling at the same approximate location as the crankcase hole. FAA inspectors discovered that the number 1 connecting rod cap had separated from the connecting rod during engine operation. The reason for this occurrence is undetermined.