CEN16LA133

During a local flight, the pilot noticed the engine was running rough when he added power. He attempted to troubleshoot the problem; however, the engine continued to run rough, and the pilot performed a forced landing to a field. The landing gear collapsed, and the airplane came to rest upright. A postaccident examination of the airplane and engine found no mechanical malfunctions or failures that would have precluded normal operation. The pilot reported that he routinely flew with a partially-open carburetor heat setting. Since the weather conditions around the time of the accident were conducive to the formation of carburetor ice at glide and cruise power, it is likely that the partial carburetor heat setting and the lower power setting increased the engine's susceptibility to the formation of carburetor ice, resulting in the rough-running engine. When the pilot changed the throttle setting, full power was not available due to the ice formation.

On March 27, 2016, about 1910 central daylight time, an amateur-built Mathieu Cassutt airplane, N78RM, was substantially damaged during a forced landing to a field near Louisville, Nebraska. The private pilot sustained serious injuries. The personal flight was conducted under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no Federal Aviation Administration (FAA) flight plan had been filed for the flight. The local flight departed Millard Airport (MLE), Omaha, Nebraska, about 1850. The pilot reported that he had been flying for about 20 minutes and decided to return to the airport. He "gave it some throttle" at which time the engine started running rough. The pilot troubleshot the engine roughness by priming the engine and adjusting the throttle position without resolve. During the forced landing to the field the main landing gear collapsed and the airplane came to rest upright. Both wings and the fuselage were substantially damaged. A farmer in the area heard the engine "cutting in and out and sputtering". The airplane was observed to circle around a field and the famer lost sight of the airplane after it descended below his view. The injured pilot was not able to exit the airplane without assistance. First responders cut into the airplane and fuselage to extricate the pilot. Fuel was observed leaking from the airplane. A post accident examination of the airplane and engine, conducted by an FAA inspector, revealed no preimpact malfunctions or failures that would have precluded normal operation. During the examination the inspector noted that the mixture control was wired in the full rich position. The carburetor heat was supplied by unfiltered air from inside the lower engine cowling. The pilot reported to the inspector that he routinely flew with the carburetor heat 1/2 way out. The closest weather reporting station was located at Plattsmouth Municipal Airport (PMV), Plattsmouth, Nebraska, 15 miles east of the accident location. The weather report taken at 1915 reported a temperature of 45 degrees Fahrenheit (F) and a dew point temperature of 23 F. The carburetor icing probability chart from the FAA special Airworthiness Information Bulletin (SAIB): CE-09-35 Carburetor Icing Prevention, illustrated a probability of icing at glide and cruise power at the temperature and dew point temperatures reported at the time of the accident. The FAA Pilot Handbook of Aeronautical Knowledge states that "partial heat or leaving heat on for an insufficient time might aggravate the situation."