ANC01LA029

The private certificated pilot was in cruise flight about 1,000 feet above ground level (agl). He observed an area of snow showers ahead of the airplane, and decided to descend to about 500 feet agl. The pilot said he reduced the engine throttle without first applying carburetor heat. When he added engine throttle to level the airplane at 500 feet, the engine began sputtering. He then applied carburetor heat, but the engine did not respond. The engine continued to run rough, and the airplane could not sustain level flight. The pilot selected a snow-covered gravel bar for a forced landing. The airplane touched down in snow that was about 18 inches deep. During the landing roll, the airplane nosed over, and received damage to the right wing lift strut and the rudder. The pilot reported the weather conditions in the area of the accident as 1,200 to 1,500 feet overcast; temperature 35 to 38 degrees F; light to moderate snow showers. According to published carburetor icing charts, the atmospheric conditions at the time of the accident (38 degrees and visible moisture) reflected the probability of serious icing utilizing descent power. A postaccident inspection disclosed no mechanical anomalies with the engine.

On January 13, 2001, about 1430 Alaska standard time, a tundra tire-equipped Piper PA-18 airplane, N2191J, sustained substantial damage during a forced landing, about 32 miles west of Anchorage, Alaska. The airplane was being operated as a visual flight rules (VFR) local area personal flight when the accident occurred. The airplane was operated by the pilot. The private certificated pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed. The flight originated at the Birchwood Airport, Chugiak, Alaska, about 1330.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), January 14, 2001, the pilot reported that he was in cruise flight about 1,000 feet above ground level (agl). He observed an area of snow showers ahead of the airplane, and decided to descend to about 500 feet agl. The pilot said he reduced the engine throttle without first applying carburetor heat. When he added engine throttle to level the airplane at 500 feet, the engine began sputtering. He then applied carburetor heat, but the engine did not respond. The engine continued to run rough, and the airplane could not sustain level flight. The pilot selected a snow-covered gravel bar for a forced landing. The airplane touched down in snow that was about 18 inches deep. During the landing roll, the airplane nosed over, and received damage to the right wing lift strut and the rudder.

At 1453, an Aviation Routine Weather Report (METAR) at Anchorage was reporting in part: Wind, 276 degrees at 7 knots; visibility, 10 statute miles; clouds and sky condition, few at 2,500 feet, 7,000 feet overcast; temperature, 29 degrees F; dew point, 27 degrees F; altimeter, 29.52 inHg.

In the Pilot/Operator report (NTSB Form 6120.1/2) submitted by the pilot, he reported the weather conditions in the area of the accident as 1,200 to 1,500 feet overcast; temperature 35 to 38 degrees F; light to moderate snow showers.

The owner's handbook for the accident airplane states in part: "Cruising: ...Unless icing conditions in the carburetor are severe, do not cruise with the carburetor heat on. Apply full carburetor heat only for a few seconds at intervals determined by icing severity. Approach and Landing: ...The carburetor heat need not be used unless icing conditions prevail, but the engine should be cleared occasionally by opening the throttle."

The FAA's, Pilot's Handbook of Aeronautical Knowledge, AC 61-23C, states, in part: "Conditions conducive to carburetor icing: ...if the temperature is between 20 degrees F and 70 degrees F, with visible moisture or high relative humidity, the pilot should be constantly on the alert for carburetor ice. During low or closed throttle settings, an engine is particularly susceptible to carburetor icing."

The FAA's, Pilot Precautions and Procedures to be Taken in Preventing Aircraft Reciprocating Engine Induction System and Fuel System Icing Problems, AC 20-113, states, in part: Vaporization icing may occur at temperatures from 32 degrees F to as high as 100 degrees F with a relative humidity of 50 percent or above… Since aviation weather reports normally include air temperature and dew point temperature, it is possible to relate the temperature/dew point spread to relative humidity. As the spread becomes less, relative humidity increases and becomes 100 percent when temperature and dew point are the same. In general, when the temperature/dew point spread reaches 20 degrees or less, you have a relative humidity of 50 percent or higher and are in potential icing conditions."

According to published carburetor icing charts, the atmospheric conditions at the time of the accident (38 degrees and visible moisture) reflected the probability of serious icing utilizing descent power.

In a telephone conversation with the pilot on June 11, 2001, he reported that no mechanical malfunctions were found after the airplane was recovered.