On October 21, 2013, about 0800 mountain daylight time, a Cessna 172M, N80257, was substantially damaged during an emergency landing following reported engine roughness while in cruise flight, about 5 miles northwest of Hardin, Montana. The airplane was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91. The commercial pilot sustained minor injuries and was the sole person on board. Visual meteorological conditions prevailed and no flight plan was filed. The personal flight departed Broadus Airport, Broadus, Montana, about 0730, with a planned destination of Billings Logan International Airport (BIL), Billings, Montana.

In a written statement to the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), the pilot reported a rough running engine and partial power loss while in cruise flight about 1,500 feet, above ground level, and he decided to conduct an emergency landing to a nearby field. The pilot reported that during the landing flare, a gust of wind caused an abnormal landing which resulted in the airplane nosing over. During the accident sequence, the airplane sustained substantial damage to the engine firewall. Local law enforcement reported that fuel was observed leaking out of the wings at the accident site.

The airplane was recovered to a secure storage facility for further examination.

In a conversation to the NTSB IIC, the pilot reported that after the engine began running rough, he applied carburetor heat, but only for a short duration of a few seconds, while trouble shooting the engine. Despite his efforts the engine was still running rough and the pilot elected to perform an emergency landing.

Examination of the recovered airframe by the NTSB IIC revealed that both wings were detached from the fuselage during the recovery efforts. Control continuity was established from the carburetor heat, throttle, and mixture controls to their respective linkages on the engine.

Examination of the recovered engine revealed that it remained attached to the fuselage via its mounts. The engine mounting structure was displaced downward about 45 degrees. All engine accessories remained attached to the engine. The magnetos were removed from the engine. The magneto drive shafts were rotated by hand and a blue colored spark was produced on all ignition leads. The crankshaft was rotated by hand by use of the propeller and mechanical continuity was established throughout the engine and valve train. Cylinder compression and suction was obtained on all four cylinders.

Fuel system continuity was obtained from both wing tanks inlet lines to the carburetor and no evidence of fuel leakage or loose lines was observed. The carburetor was removed, disassembled and was unremarkable. The gascolator was removed, examined and was unremarkable. About 2 ounces of a blue colored fuel was drained from the gascolator and was clear of debris.

The spark plugs were removed from the engine and revealed normal operating and wear signatures.

Examination of the recovered airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.

Weather conditions recorded at the BIL, located about 34 miles northwest of the accident site, at 0753, were wind calm, visibility 7 statute miles, clear sky, temperature 45 degrees Fahrenheit, dew point 34 degrees Fahrenheit, and an altimeter setting of 30.08 inches of mercury. According to the Federal Aviation Administration Special Airworthiness Information Bulletin CE-09-35, entitled Carburetor Icing Prevention, the temperature and dew point were conducive to the formation of icing at cruise power.