On April 29, 2017, about 1015 Pacific daylight time, a Yakovlev Yak 52 airplane, N132MD, experienced a total loss of engine power and was substantially damaged during the off airport forced landing in a field near Porterville, California. The airline transport pilot sustained minor injuries and the one passenger was uninjured. The airplane was registered to and operated by the pilot as a 14 Code of Federal Regulations Part 91 personal flight. Visual meteorological conditions prevailed at the time of the accident, and no flight plan was filed. The local flight originated from Porterville Municipal airport (PTV) at 0935. The pilot reported that the purpose of the flight was to practice a mass formation flight. As planned, the pilot separated from the mass group as a formation of four to land. Shortly after separating from the larger formation, the engine started to run rough and intermittent. Unable to maintain level flight, the pilot detached from the four-airplane formation. He noted that 29 gallons of usable fuel was remaining, he checked the magnetos, ignition switch, generator, battery, and all indications were normal. The engine continued to lose power, and about 500 ft above the ground, he elected to land in a vineyard. During landing, the airplane touched down, came to an abrupt stop, and nosed over coming to rest inverted. During a postaccident examination of the airplane and engine by a mechanic and Federal Aviation Administration inspector, the spark plugs were removed and exhibited signatures consistent with normal wear and operation. The engine was rotated by hand and thumb compression was established in all cylinders. The oil screen was removed and was clear of debris. The fuel tanks were empty, and the fuel system was continuous throughout. The fuel screen exhibited some small pieces of unknown fabric. In addition, a small about of metal shavings from an unknown source and a small chunk of metal were found in the fuel intake and inlet screen. The carburetor was removed and examined. The carburetor throttle plate moved from stop to stop. The fuel metering section was heavily damaged and fracture separated; the needle valve moved freely. There were no signs of corrosion throughout. The fuel inlet screen was clear of debris with the exception of small metal shavings and a small chunk of metal. The origin of the metal pieces was not determined.