On September 14, 2016 about 0910 central daylight time (CDT), a Stinson V-77 airplane, N715FB, was substantially damaged during an accident near East Jordan, Minnesota. The airline transport pilot was not injured. The airplane was privately registered and operated under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, and no flight plan was filed. The local flight departed South St. Paul Municipal Airport-Richard E Fleming Field (SGS), Minnesota, about 0820.
The pilot was conducting the flight to put additional break-in hours on a newly-installed engine. The engine had about 10 total hrs since overhaul and 2 hrs since the last inspection at the time of the accident. About 30 minutes after take off, and at 2,000 ft above the ground (AGL), the engine started to constantly backfire and lost power. The airplane was unable to maintain level flight due to the power loss. The pilot attempted to regain engine power; he switched fuel tanks, adjusted the engine throttle, fuel/ air mixture, and propeller controls, and selected the individual magneto positions. There was no subsequent increase of engine power or reduction of backfiring. The pilot performed an emergency landing to a soft field where the airplane nosed over during landing. The rudder sustained substantial damage.
An on-scene examination of the airplane by the Federal Aviation Administration (FAA) inspector revealed that the fuel selector was in the left tank position, the magneto switch was in the "BOTH" position, the primer was in and locked, the mixture was in the "RICH" position, the throttle was in the "CLOSED" position, and the propeller control was approximately ¼ back from the full-forward position.
An additional examination was conducted by the FAA inspector on October 5, 2016. The Bendix dual magneto timing was 34º before top-dead center (BTC) on the left and 30º BTC on the right. All spark plug leads were tight and in like-new condition. The magneto switch was checked and appeared to operate normally. Magneto lead continuity was confirmed. The magneto points were checked while rotating the propeller and they opened normally. Both magnetos sparked. Both coils appeared to have a good spark. The magneto was removed and bench tested. No anomalies were noted with the magneto.
The fuel system was examined and small amounts of liquid consistent with 100LL fuel were recovered from the carburetor bowl and the airframe gascolator. The carburetor was examined and no anomalies were noted. No pre-impact anomalies were noted with the airframe or engine that would have contributed to a partial loss of power.
The pilot reported a temperature of 50° F at the time of the accident. He did not know the dewpoint at the accident location. At 0853, the temperature and dewpoint at Minneapolis-St. Paul International/Wold-Chamberlain Airport (MSP), Minneapolis, Minnesota, located 25 miles northeast of the accident site, were reported as 54° F and 45° F respectfully at 0853. According to the carburetor icing probability chart in FAA Special Airworthiness Information Bulletin CE-09-35, these conditions were conducive to serious icing at cruise power.