ERA24LA334

A total loss of engine power due to carburetor icing as a result of the pilot’s delayed application of carburetor heat.

The pilot was on a local flight following an annual inspection. For the first 45 minutes of flight, the pilot remained in the traffic pattern where he operated from a grass runway. He then departed the airport traffic pattern and then flew at about 1,500 ft above ground level (agl) over rivers and open water for another 45 minutes. He began a climbing 180° turn back toward the airport at full power when the engine lost partial power. The pilot attempted to troubleshoot the loss of engine power by applying carburetor heat and manipulating the throttle without success. He was unable to maintain altitude and, with no desirable locations available to land, the pilot executed a forced landing to a river, which resulted in substantial damage to the left wing. The airplane was recovered from the river and a postaccident examination of the airplane’s fuel system and engine was conducted. Water was present in the fuel system and disassembly of the engine’s carburetor revealed a “white gummy substance” throughout the unit that was blocking the main fuel jet. There were no other mechanical malfunctions or anomalies with the airframe or engine that would have precluded normal operation. A sample of the unknown substance from the fuel system was analyzed and found to most likely be a plant or other naturally occurring cellulosic material. A review of the maintenance logbooks for the airplane revealed that the fuel tank was emptied and flushed about one month before the accident, during the most recent annual inspection. The fuel strainer, carburetor inlet screen, carburetor bowl was “inspected and cleaned” at the same time. According to the owner, this inspection and cleaning was the result of a fuel contamination issue following the restoration of the airplane. Considering that the carburetor was inspected and cleaned during the annual inspection, just a month before the accident, and that the engine had operated normally for about 90 minutes of flight, it is likely that the contamination found in the carburetor was the result of the water immersion in the river. The temperature and dew point at the time of the accident were conducive to the formation of serious icing at glide engine power settings. Considering that the airplane operated from a grass runway and flew at low altitudes over rivers and open water for about 90 minutes, it is likely that the airplane encountered higher relative humidity than anticipated, which resulted in a partial loss of engine power due to carburetor icing. Additionally, because the pilot did not activate the carburetor heat until after the engine had begun losing power, the likelihood that the icing could be remediated and engine power restored was substantially reduced.

On June 16, 2024, at 1048 eastern daylight time, an Aeronca 7AC, N3854E, was substantially damaged when it was involved in an accident near Topsham, Maine. The pilot received minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. According to the pilot, the airplane was fueled before the flight and the fuel gauge confirmed that the tank was full. The flight departed from Brunswick Executive Airport (BXM), Brunswick, Maine about 0915. The pilot reported that he remained within the traffic pattern and operated from the grass runway for about 45 minutes to ensure the engine and other systems were operating normally. He then proceeded to fly south over the Androscoggin and Kennebec rivers at about 500 ft to 600 ft agl toward Popham Beach before flying over open water to Two Bush Islands, where he increased his altitude to about 1,500 ft agl. He then turned north again and followed the Androscoggin River again until he passed the dam near Downeaster. After about 90 minutes of uneventful flying, the pilot decided to return to the departure airport. He began a climbing 180° right turn with the engine at full power; about 90° through the turn, the engine began to “hesitate” and the engine rpm was “decreasing.” The pilot reported that the engine lost thrust, but it continued to idle. The pilot attempted to troubleshoot the loss of engine power by applying carburetor heat and manipulating the throttle; however, engine power did not return. The pilot elected to perform a forced landing in the Androscoggin River based on the airplane’s altitude and the unfavorable surrounding terrain. As he descended towards the river, the pilot recalled cracking open the cabin door, tightening the shoulder harness, and reducing airspeed. The airplane impacted the water and immediately nosed over. The cabin quickly filled with water, but the pilot was able to orient himself and escape. The airplane was recovered from the river, which revealed substantial damage to the left wing. A postaccident examination of the airplane’s fuel system and engine was conducted. The engine crankshaft was initially seized from corrosion; however, after moving the crankshaft back and forth to break up the corrosion, the crankshaft was able to rotate fully. Compression on all four cylinders was established, and a borescope inspection did not identify any preaccident discrepancies. Both magnetos were removed and the left magneto produced spark during impulse coupling operation. The spark plugs were removed and were corroded consistent with water immersion. There were no obstructions within the induction system. The fuel tank contained ¼ of a tank of fuel. Water was present in the fuel system. The carburetor was disassembled, and an unknown “white gummy” substance was found within the fuel bowl, which was also obstructing the main fuel jet. A sample of the unknown substance was sent to the NTSB Materials Laboratory for analysis. The sample was examined using a Fourier Transform Infrared (FTIR) spectrometer. The residue displayed spectral peaks that corresponded to particular functional groups found within molecular structure of the unknown material. The spectrum was consistent with presence of an amine and a carbohydrate. A spectral library search was performed on the unknown spectrum. The spectral search found a few spectral similarities to cellulose. A mixture search was also performed. There were matches to several other natural nitrogen-containing material sources. Based on the combination of spectral patterns and similarities to several natural amino-carbohydrates and other materials, the analysis concluded that the unknown material was most likely plant material or other naturally occurring cellulosic material. Plant or leaf material consists of a combination of carbon, nitrogen, carbohydrates, organic acids, mineral substances, and water. Molecular bonds for these materials were all present within the unknown spectrum. The presence of the elemental carbon in the sample indicates that the material had sustained some type of thermal exposures carbonizing the material. The owner stated that the accident flight was the first flight after the airplane’s second annual inspection, after an extensive airframe restoration and major engine overhaul. A review of the maintenance logbooks for the airplane revealed that the fuel tank was emptied and flushed during the most recent annual inspection on May 6, 2024, just over a month before the accident. The logbook entry stated, “Inspected and cleaned fuel strainer, carb inlet screen and flushed carb fuel bowl as needed. Leak check performed; no leaks noted.” At the time of the accident, the air temperature was 66.2°F and the dewpoint was 42.8°F, which were conducive to the formation of serious icing at glide power according to a carburetor icing probability chart.