ANC01LA016

During on-demand air taxi passenger flight with four passengers, the commercial certificated pilot described the weather conditions along the route of flight as 4,000 to 5,000 feet overcast, visibility 10 miles, temperature 35 to 40 degrees F, with areas of snow showers above 2,000 feet, and rain showers below 2,000 feet. During a left turn from the base leg to the final approach heading of the landing pattern at the destination airport, the engine sputtered. The pilot confirmed there were no caution lights on the instrument panel. He confirmed the fuel selector was on the right wing tip fuel tank. He then selected carburetor heat, and the engine lost almost all power. From about 200 feet above the ground, he pilot selected an emergency landing area on the bank of a river, short of the landing threshold. During the ground collision, all of the landing gear were sheared off, and the left wing was buckled. A postaccident examination of the airplane did not reveal any preimpact mechanical malfunctions. According to published carburetor icing charts, the meteorological conditions were conducive to severe icing under descent power.

On November 9, 2000, about 1320 Alaska standard time, a Piper PA-32-260 airplane, N4803S, sustained substantial damage during a forced landing short of runway 09 at Lime Village, Alaska. The airplane was being operated as a visual flight rules (VFR) on-demand passenger flight under Title 14, CFR Part 135, when the accident occurred. The airplane was operated by Bidzy Ta Hot Aana Inc., doing business as Tanana Air Service Inc., Fairbanks, Alaska. The commercial certificated pilot, and the four passengers, were not injured. Visual meteorological conditions prevailed. A VFR flight plan was filed. The flight originated at the Nikolai airport, Nikolai, Alaska, about 1230.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on November 9th, the pilot reported he departed McGrath, Alaska, about 1145 to position the airplane to Nikolai. The four passengers boarded at Nikolai, and he departed for Lime Village. After arriving over Lime Village, he began the landing approach to runway 09. During a left turn from the base leg of the landing pattern to the final approach heading, the engine sputtered. The pilot confirmed there were no caution lights on the instrument panel, and the fuel selector was on the right wingtip fuel tank. He then selected carburetor heat, and the engine lost almost all power. From about 200 feet above the ground, he selected an emergency landing area on the bank of a river, short of the landing threshold. During the ground collision, all of the landing gear sheared off, and the left wing was buckled.

After the airplane was recovered, it was examined by the NTSB IIC at Big Lake, Alaska. The airplane contained fuel that was free of visual contaminants. The engine had gear and valve train continuity. The spark plugs had a gray appearance and were dry. The carburetor contained fuel. The carburetor inlet screen was free of contaminants. The air cleaner was unobstructed. Upon movement of the carburetor heat control, the carburetor heat mechanism functioned properly.

In the "Approach and Landing" portion of the airplane's owner's handbook, the "Before landing check list" states, in part: 1) Mixture RICH. 2) Propeller set. 3) Carburetor heat OFF (unless icing conditions exist).

The pilot described the weather conditions as 4,000 to 5,000 feet overcast; temperature, 35 to 40 degrees F; visibility, 10 miles with areas of snow showers above 2,000 feet, and rain showers below 2,000 feet.

According to the FAA's, Aviation Weather for Pilots and Flight Operations Personnel, advisory circular AC 00-6A, the amount of water vapor in the air is measured by either relative humidity, or the dew point temperature. Relative humidity expresses the actual water vapor present at a given temperature. One hundred percent relative humidity means the air is saturated. For air that is less than one hundred percent saturated, the dew point is the temperature to which the air must cool to become one hundred percent saturated.

The FAA's, Flight Training Handbook, AC 61-21A, Chapter 2, "Carburetor Ice," states, in part: "The formation of ice in the carburetor has always been a problem in those airplanes equipped with a float-type carburetor. It may restrict the power output of the engine or even cause the engine to quit operating. ...In moist air or high relative humidities, the moisture in the air entering the carburetor condenses and because of the lowered temperature (at the narrowest portion of the carburetor venturi), may result in the formation of ice."

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."

The closest weather station that recorded weather data on the date of the accident was Sleetmute, Alaska, located 52 nautical miles west-northwest of Lime Village. At 1255, an automated weather observation system (AWOS) was reporting, in part: Wind, calm; visibility, 10 statute miles; clouds and sky condition, few clouds at 2,300 feet, few at 3,200 feet, 4,800 feet broken; temperature, 34 degrees F, dew point temperature, 28 degrees F; altimeter, 29.79 inHg.

At 1355, the AWOS at Sleetmute was reporting, in part: Wind, calm; visibility, 3 statute miles; clouds and sky condition, 2,000 feet broken; temperature, 32 degrees F, dew point temperature, 28 degrees F; altimeter, 29.76 inHg; remarks, 0.01 inches of precipitation in the previous hour.

According to published carburetor icing charts, the atmospheric conditions at the time of the accident (35 to 40 degrees F, and visible moisture), reflected the probability of serious icing using descent power.