On August 24, 2009, about 1720 Alaska daylight time, a wheel-equipped Cessna 180 airplane, N4601A, sustained substantial damage during a forced landing, about 5 miles northeast of the Birchwood Airport, Chugiak, Alaska. The airplane was being operated as a visual flight rules (VFR) personal flight under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91, when the accident occurred. The airline transport pilot and three passengers all sustained minor injuries. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated at the Birchwood Airport, about 1600. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on August 24, the pilot noted that the purpose of the flight was to show his passengers some Alaskan scenery. He said that as he approached his destination airport in level cruise flight at 1,200 feet agl, the engine began to run rough and lose power. He noticed that the carburetor air temperature was just above the carburetor ice zone marked in red on the gauge, so he applied full carburetor heat, which was followed by a complete loss of engine power. After completing the emergency procedures for a loss of engine power, the pilot said he was unable to restart the engine, and he selected a marshy tree-covered site as a forced landing area. During the forced landing, the airplane collided with trees, and sustained substantial damage to the fuselage and wings. The airplane was equipped with a naturally aspirated Teledyne Continental Motors O-470-K engine. The closest official weather observation station is at the Birchwood Airport. At 1753, an Aviation Routine Weather Report (METAR) was reporting, in part: Wind, 300 degrees (true) at 4 knots; visibility, 10 statute miles; clouds and sky condition, clear; temperature, 57 degrees F; dew point, 45 degrees F; altimeter, 29.58 inHg. The Birchwood Airport is situated adjacent to the ocean waters of the Cook Inlet. The temperature and dew point were entered into a carburetor icing probability chart, which coincided with the "serious icing-cruise power" category. On September 15, 2009, a Federal Aviation Administration (FAA) airworthiness inspector from the Anchorage Flight Standards District Office traveled to the Big Lake Airport, Big Lake, Alaska, and examined the airplane after it was recovered. The inspector reported that he was unable to find any preaccident mechanical problems with the airplane. ADDITIONAL INFORMATION The FAA's Advisory Circular (AC) 61-23C, Pilot's Handbook of Aeronautical Knowledge, Chapter 2, Carburetor Icing, states, in part: "... if the temperature is between -7 degrees C (20 degrees F) and 21 degrees C (70 degrees F), with visible moisture or high humidity, the pilot should be constantly on the alert for carburetor ice. During low or closed throttle setting, an engine is particularly susceptible to carburetor icing.” The FAA's AC 20-113, Pilot Precautions and Procedures to be taken in Preventing Aircraft Reciprocating Engine Induction System and Fuel System Icing Problems, 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."