Accident Details
On July 15, 2010, at 1137 Alaska daylight time, a Piper PA-14, registration N5126H, experienced a loss of engine power and collided with terrain during a subsequent forced landing along a stream bank approximately 6 miles north of King Salmon, Alaska. The airline transport pilot was operating the aircraft under 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.
Pilot Report and Sequence of Events
The pilot reported to the Safety Board investigator that the weather had deteriorated to a ceiling between 500 and 700 feet with 10 miles visibility as he approached King Salmon. Air traffic control instructed him to hold north of the King Salmon class D airspace and to expect a special VFR clearance in 30 minutes. During the hold, the pilot intermittently cycled the carburetor heat on and off. After approximately 25 minutes of holding, he decided to reposition to an area between King Salmon and his alternate, Naknek Airport. As he began proceeding toward the new position, the engine started losing rpm and running rough. The pilot applied carburetor heat and full mixture, but the engine continued running rough. He conducted a forced landing along the edge of a stream, and the airplane came to rest on an embankment.
Weather Conditions
The King Salmon METAR for 1154 recorded wind from 210 degrees at 8 knots, visibility 7 statute miles, ceiling 500 feet overcast, temperature 7 degrees Celsius, dew point 7 degrees Celsius, and altimeter 30.20 inHg.
Examination and Findings
An FAA inspector performed an on-site examination. The engine was placed on a hoist, the bottom spark plugs were removed, and the engine was rotated by hand. Continuity, compression, and valve action were observed. The carburetor was removed, and during disassembly, the inner walls of the carburetor throat were noted to be clean. The carburetor bowl contained a small drop of fuel, and both the fuel screen and fuel line were clean. The carburetor floats and internal accelerator pump appeared to function normally. The fuel had drained from the airplane's tanks at the accident site, so the quantity on board could not be verified. The pilot reported having approximately one-quarter tank of fuel remaining in both tanks before 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 can occur at temperatures above freezing when visible moisture or high humidity is present; vaporization of fuel and expansion of air through the carburetor (the venturi effect) cause sudden cooling, allowing ice to build rapidly. 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.