Overview
On September 14, 2016, at approximately 0910 central daylight time, a Stinson V-77 airplane, registration 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 Title 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed, and no flight plan was filed. The local flight departed South St. Paul Municipal Airport–Richard E Fleming Field (SGS), Minnesota, at about 0820.
Engine Failure
The pilot was conducting the flight to put additional break-in hours on a newly installed engine, which had about 10 total hours since overhaul and 2 hours since the last inspection at the time of the accident. About 30 minutes after takeoff, while at 2,000 feet above ground level, 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 by switching fuel tanks, adjusting the engine throttle, fuel/air mixture, and propeller controls, and selecting individual magneto positions. There was no subsequent increase of engine power or reduction of backfiring.
Emergency Landing
The pilot performed an emergency landing to a soft field, where the airplane nosed over during landing. The rudder sustained substantial damage.
Post-Accident Examination
An on-scene examination by a Federal Aviation Administration (FAA) inspector revealed the fuel selector was in the left tank position, the magneto switch in the "BOTH" position, the primer in and locked, the mixture in the "RICH" position, the throttle in the "CLOSED" position, and the propeller control approximately one-quarter back from the full-forward position.
A subsequent examination on October 5, 2016, found the Bendix dual magneto timing was 34° before top dead center (BTC) on the left and 30° BTC on the right. All spark plug leads were tight and in like-new condition. The magneto switch operated normally, and magneto lead continuity was confirmed. The magneto points opened normally while rotating the propeller, and both magnetos sparked. Both coils appeared to have a good spark. The magneto was removed and bench tested with no anomalies noted.
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.
Carburetor Icing Conditions
The pilot reported a temperature of 50°F at the time of the accident and did not know the dewpoint at the accident location. At 0853, the temperature and dewpoint at Minneapolis-St. Paul International/Wold-Chamberlain Airport (MSP), located 25 miles northeast of the accident site, were reported as 54°F and 45°F, respectively. 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.