Aeronca 7AC N3854E accident near Topsham, Maine

Probable cause

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

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for ERA24LA334 for the full record.