CEN24LA362

The pilot reported that during the flight, at an altitude of 1,800 ft above the ground, she encountered an area of light rain, so she applied carburetor heat as a precaution. Once carburetor heat was applied, the engine began to run rough and lose power. The pilot turned the carburetor heat on and off a few times because she was not sure if the introduction of the carburetor heat was making the loss of power worse. The pilot also completed the checklist for engine roughness. Unable to clear up the engine roughness and unable to maintain airplane altitude, the pilot performed a forced landing. The fuselage and right wing sustained substantial damage when the airplane impacted trees and terrain. A postaccident examination of the engine and fuel system did not reveal any preexisting anomalies or failures that would have precluded normal operation. Ample fuel drained from the wing fuel tanks before the wreckage was recovered; however, the amount of fuel drained was not documented. The recorded temperature and dew point near the accident site were conducive to “serious icing” at cruise engine power settings. Based on this information, it is likely that carburetor ice formed and there was insufficient time/altitude for the carburetor heat to melt the accumulated ice.

On September 17, 2024, about 1055 mountain daylight time, a Cessna 172N, N739JU, was substantially damaged when it was involved in an accident near Arobles, Colorado. The pilot sustained minor injuries, and the passenger was seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot was taking a friend on a local flight when, about 1,800 ft above ground level, the airplane flew into an area of light rain. The pilot applied carburetor heat as a precaution. Despite having the carburetor heat engaged while the airplane flew in the rain, the engine began to run rough and lose power. The pilot turned the carburetor heat on and off a few times because they were not sure if the introduction of the carburetor heat was making the situation worse. The pilot also completed the checklist for engine roughness. Unable to clear up the engine roughness and unable to maintain airplane altitude, the pilot initiated an off airport forced landing, and the airplane fuselage and right wing sustained substantial damage when it impacted trees and terrain. The pilot estimated that the flight departed with a 26-gallon fuel load. The accident occurred about 35 minutes after departure, and according to airplane performance charts, the airplane’s expected fuel consumption rate was 5 to 8.5 gallons per hour. After the accident, ample fuel was drained from the wing tanks before the wreckage was recovered; however, the amount of fuel drained was not documented. The recorded temperature and dew point near the accident site were conducive to “serious icing” at cruise engine power settings. The pilot stated that the carburetor heat was working properly when checked before departure. According to FAA Advisory Circular 20-113, "To prevent accident due to induction system icing, the pilot should regularly use [carburetor] heat under conditions known to be conducive to atmospheric icing and be alert at all times for indications of icing in the fuel system." The circular recommended that when operating in conditions where the relative humidity is greater than 50 percent, "…apply carburetor heat briefly immediately before takeoff, particularly with float type carburetors, to remove any ice which may have been accumulated during taxi and runup." It also stated, "Remain alert for indications of induction system icing during takeoff and climb-out, especially when the relative humidity is above 50 percent, or when visible moisture is present in the atmosphere."