Final answer:
The first sign of carburetor ice in an airplane with a fixed-pitch propeller is typically a slight decrease in engine RPM or an increase in manifold pressure. Carburetor heat should be applied in conditions that favor the formation of ice.
Step-by-step explanation:
The first indication of possible carburetor ice for airplane equipped with a fixed-pitch propeller would typically be a gradual decrease in engine RPM, which is sensed due to the blockage of the air-fuel mixture as ice begins to form within the carburetor. If the airplane has a manifold pressure gauge, you may instead notice a slight rise in manifold pressure before a change in RPM occurs. It is essential to apply carburetor heat as recommended by the manufacturer when operating in conditions conducive to carburetor icing, which include high humidity and temperatures between 20°F and 70°F (-6°C and 21°C). Early detection and action are crucial to avoid engine failure.