Primary finding
Probable cause
The total loss of engine power due to carburetor icing, which resulted from the pilot's failure to use carburetor heat while operating at low power settings in conditions conducive to the formation of carburetor ice.
Investigator assessment
Analysis narrative
According to the pilot, while maneuvering the airplane in mountainous terrain, he reduced engine power and initiated a descent; the engine then began to cough and run roughly. He applied full throttle, and the engine subsequently lost total power. In an attempt to restore power, he applied carburetor heat and switched fuel tanks while initiating an emergency descent. The engine restarted; however, it continued to run roughly and would not develop full power. He continued the emergency descent and landed the airplane on a mountain ridge. Upon touchdown, the left main landing gear separated from the fuselage and the left wing struck the ground, which resulted in substantial damage to the left wing. A postaccident examination of the engine, which included an engine test run, revealed no evidence of any preimpact mechanical malfunctions or failures that would have precluded normal operation. According to a 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. It is likely that, when the pilot reduced engine power for the descent, carburetor ice formed and resulted in the loss of engine power.
Source record
Factual narrative
On September 30, 2018, about 1330 Alaska daylight time, a tundra tire and tailwheel-equipped Piper PA-18 Super Cub airplane, 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), Delta Junction, Alaska about 1130 for the upper Salcha River area with a planned return to D66. 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 ground resulting in substantial damage to the left wing. 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. 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."