CHI97LA037

While in cruise flight, the pilot said his RPM suddenly dropped and the engine began to make a '...stuttering...' noise. He said he applied carburetor heat and positioned the mixture to full rich. The engine continued to make the stuttering noise and have '...drops in the RPM.' As he approached an alternate airport he said he was informed the airport had no winter maintenance. He said he selectd a field that appeared flat with a little snow on it for a landing. As he approached the field he said it was a hill and landed next to it. An on-scene investigation revealed no mechanical or fuel anomalies that would cause the engine to stop running. The nose gear crushed the bottom of the firewall when it was bent aft during the landing. Ice crystals were found in the carburetor venturi and float bowl.

On November 27, 1996, at 1250 central standard time, a Canadian registered Cessna 150, C-GOCS, piloted by a private pilot, was substantially damaged during an off-airport landing in 18-inch deep snow following a partial loss of engine power. The 14 CFR Part 91 instructional flight was operating on a VFR flight plan. The pilot and passenger reported no injuries. Visual meteorological conditions prevailed at the time of the accident. The flight departed Minot, North Dakota, exact time unknown.

The pilot reported the airplane was at 6,500 feet above mean sea level when he "...noticed a sudden drop in the [engine] RPM and a stuttering noise." He said he applied carburetor heat and placed the mixture control to full rich. He said, "We continued to have stuttering along with drops in the RPM."

During an interview, the pilot said he was informed the airport had no winter maintenance as he made his initial approach to the Bowbells Municipal Airport, Bowbells, North Dakota. The pilot said he headed toward what he believed was a flat landing surface with a little sanow on it. As he approached the chosen field, he said he noticed it was a hill and landed in a field next to it.

The Cessna 150 Pilot Operating Handbook section on the use of carburetor heat states, "Carburetor ice.... can be removed by application of full carburetor heat. Upon regaining the original RPM (with heat off), use the minimum amount of heat (by trial and error) to prevent ice from forming. Since the heated air causes a richer mixture, readjust the mixture setting when carburetor heat is to be used continuously in cruise flight."

The on-scene investigation was conducted by a Federal Aviation Administration Principal Maintenance Inspector (PMI). According to the PMI, there were no mechanical or fuel anomalies that would have caused the engine to stop running. He said the nose gear had been pushed aft into the bottom of the firewall. According to the PMI, fuel samples taken from the wing tanks did not have ice crystals in them. The PMI said he "...found ice crystals in the carburetor's venturi/injector section and float section... ."