N3854E

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.