Carburetor ice can easily occur in light aircraft up to 70 degrees F in high humidity conditions and according to the FAA it can occur up to 100 F. The air in the carburetor can drop in temperature 30 – 40 degrees when it is accelerated through the carburetor's throat. The venturi effect in the carburetor accelerates the air and cools it. When moisture laden air reaches the freezing point at 32 degrees F it begins to form ice particles which stick to the throttle plate. If enough ice forms here, the airflow becomes restricted.
Carb Ice can be detected by a loss of power, as indicated in a gradual drop in RPM with a fixed pitch propeller, or a drop in manifold pressure with a constant speed propeller.
Carburetor heat or “carb heat” in light aircraft is the system that is used to prevent carburetor icing in light aircraft. A knob inside the cockpit controls a flange which blocks off the air coming in directly from the outside through the air filter. The air is heated by the engine before it enters the carburetor and will then melt the ice.