On November 6, 2015, about 1205 central standard time, a Cessna 120 airplane, N2942N, impacted terrain shortly after departing the Louisiana Regional Airport (L38), Gonzales, Louisiana. The commercial rated pilot was seriously injured and the airplane was substantially damaged during the accident. The airplane was registered to and operated by Graham Aviation LLC, Baton Rouge, Louisiana, under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed at the time. The pilot reported that he had been airborne about 25 minutes and was on final approach to the runway. The engine began to lose power, so he applied carburetor heat and switched fuel tanks. The engine did not recover, so he made a forced landing in a field. The pilot added that the engine's carburetor had been replaced the day prior. The responding Federal Aviation Administration (FAA) inspector reported that the airplane had crashed in a field near the airport. A visual examination of the engine noted that fuel was present on site. The airplane came to rest upright, with the main landing gear collapsed. Heavy impact damage was noted to the airplane's fuselage. After the airplane was recovered, an examination of the wreckage was conducted by the FAA inspector. The examination found that the castellated nut holding the carburetor's throttle arm had backed off, allowing the engine to go to idle. Additionally, the nut was missing the associated cotterpin.