Accident Details
On September 30, 2018, at about 1330 Alaska daylight time, a tundra tire and tailwheel-equipped Piper PA-18 Super Cub, registration N5907Y, sustained substantial damage during a forced landing following a partial loss of engine power while maneuvering near Delta Junction, Alaska. The private pilot was not injured. The airplane was registered to and operated by the pilot as a 14 Code of Federal Regulations Part 91 visual flight rules (VFR) flight. Visual meteorological conditions prevailed, and no flight plan had been filed. The local area flight departed Delta Junction Airport (D66) at about 1130 for the upper Salcha River area with a planned return to D66.
Pilot Account
According to the pilot, while maneuvering in mountainous terrain, he reduced engine power and initiated a descent when the engine began to cough and run rough. He applied full throttle, and the engine subsequently lost all power. In an attempt to restore power, he applied carburetor heat and switched fuel tanks while initiating an emergency descent to a mountain ridge. The pilot stated that the engine appeared to restart; however, it continued to run rough and would not develop full power. He elected to continue with the emergency descent and landing on the mountain ridge. Upon touchdown, the left main landing gear separated from the fuselage, and the left wing struck the ground, resulting in substantial damage to the left wing.
Engine Examination
On October 12, the engine, while still mounted on the accident airplane's airframe, was operated under the direction of the NTSB IIC. The engine accelerated normally, and a series of power adjustments from idle to full power were conducted without any hesitation, stumbling, or interruption in power. During the engine test, the magnetos were checked, and a drop of about 25 rpm was noted for the left and right magnetos.
Carburetor Icing Considerations
According to the carburetor icing probability chart, an airplane operating in the ambient conditions at the accident site could expect carburetor icing while at cruise power and at glide power. A Lycoming Service Instruction states in part: "Landing Approach – In making a landing approach, the carburetor heat is generally in the 'Full Cold' position. However, if icing conditions are suspected apply 'Full Heat.' In the case that full power needs to be applied under these conditions, as for an aborted landing, return the carburetor heat to 'Full Cold' after full power application."