ANC10LA061

The pilot reported that the weather at his destination airport had deteriorated to a 500-700 foot ceiling with 10 miles of visibility. He was instructed by air traffic control personnel to hold outside the airport's controlled airspace, and to expect a special visual flight rules clearance in 30 minutes. Throughout the hold, the pilot cycled the carburetor heat intermittently on and off. After about 25 minutes holding, he decided to reposition to a location between his destination and alternate airport. As he started to fly towards this new position, the engine began losing rpm and running rough. He applied carburetor heat and full mixture, but the engine continued to run rough. He made a forced landing along the edge of a stream, during which the right wing and fuselage received substantial damage. About the time of the accident, the destination airport reported both the temperature and dew point as 07 degrees Celsius. According to the Federal Aviation Administration Carburetor Icing Probability Chart, an aircraft encountering those ambient conditions could expect serious carburetor icing while at cruise power. Examination of the airplane and engine revealed no mechanical failures or malfunctions that would have precluded normal operation.

On July 15, 2010, at 1137 Alaska daylight time, a Piper PA-14, N5126H, had a loss of engine power and collided with terrain during the subsequent forced landing along a stream bank, about 6 miles north of King Salmon, Alaska. The airline transport pilot operated the airplane under the provisions of 14 Code of Federal Regulations, Part 91. The pilot and three passengers were not injured. The airplane sustained substantial damage to the right wing and fuselage. Instrument meteorological conditions prevailed, and no flight plan had been filed. The flight originated at Igiugig, Alaska, at 1030. The pilot reported to the Safety Board investigator that the weather had deteriorated between a 500 to 700 foot ceiling and 10 mile visibility as he approached King Salmon. He was instructed to hold north, outside the King Salmon class D airspace, and to expect special VFR clearance in 30 minutes. Throughout the hold the pilot cycled the carburetor heat intermittently on and off. After about 25 minutes holding, he decided to reposition to a location between King Salmon and his alternate, Naknek Airport. As he started to proceed towards this new position, the engine began losing rpm and running rough. He applied carburetor heat, full mixture, but the engine continued to run rough. The pilot made a forced landing along the edge of a stream and the airplane came to rest on the embankment. The King Salmon METAR for 1154 recorded winds from 210 degrees at 8 knots, 7 statute miles visibility, 500 foot overcast, temperature 7 degrees Celsius, dew point 7 degrees Celsius, and altimeter 30.20 in hg. An FAA inspector performed an on-site exam, both of which had clean fuel. The carburetor floats and internal accelerator pump appeared to function normally. The fuel had drained out from the airplane's fuel tanks at the accident site, therefore, the fuel quantity on board the airplane could not be verified. The pilot reported having about 1/4 tank fuel remaining in both tanks prior to the accident. First responders reported seeing fuel leaking from the damaged wing. The FAA published Special Airworthiness Information Bulletin (SAIB) CE-09-35 on June 30, 2009, regarding carburetor ice prevention. The SAIB noted that carburetor icing does not just occur in freezing conditions; it can occur at temperatures well above freezing temperatures when there is visible moisture or high humidity. It states that icing can occur in the carburetor at temperatures above freezing. Vaporization of fuel combined with the expansion of air as it flows through the carburetor (the venturi effect) causes sudden cooling. A significant amount of ice can build up within a fraction of a second. The SAIB contains a graph that illustrates the probability of carburetor icing for various temperatures, dew points, and relative humidity conditions. According to the SAIB, a pilot encountering the ambient conditions reported by the King Salmon METAR could expect serious carburetor icing while at cruise power.