What is the first indication of carburetor icing in an aircraft with a fixed-pitch propeller?

The first indication of carburetor icing in an aircraft with a fixed-pitch propeller is often a decrease in engine RPM. Carburetor icing occurs when moisture in the air freezes in the carburetor, which restricts airflow and disrupts the fuel-air mixture. As a result, the engine does not receive an adequate amount of air, leading to a drop in RPM. In many instances, pilots may notice a slight reduction in power output, which is evidenced by the engine RPM decreasing, even before any other symptoms manifest.

This phenomenon can be particularly pronounced during certain atmospheric conditions, such as high humidity and low temperatures, which are ideal for the formation of ice in the carburetor. Recognizing this initial sign allows pilots the opportunity to respond appropriately by applying carburetor heat, helping to dissolve the ice and restore engine performance.

The other indications that might arise as icing progresses, such as increased fuel consumption, reduced airspeed, or engine roughness, are often secondary effects or relate to more advanced stages of icing when the issue has worsened, making the initial RPM drop a critical warning sign for pilots.