Aircraft engines with carburetors have traditionally all equipped with up draft carburetors. One reason being a downdraft carb would have to be mounted on top of the engine. In case of a stuck float or an overflow the engine would be covered with raw fuel with the possibility of an engine fire in flight. Carburetor heat is controlled by the pilot from the cockpit with a cable attached to a flapper valve in a carb heat box attached to the bottom of the carb. This box is supplied with hot air from a muff surrounding an exhaust pipe. It doe's NOT use exhaust gas. Carb heat is not normally used on takeoff because it typically lowers RPM. During runup before takeoff the carb heat control is operated to make sure it is working. The way you can tell it is working is that the tach should show an RPM drop. Depending on the particular engine the drop can be from 50 to 150 RPM. Carb heat is almost always used on landing because this is most likely when the carburetor would ice up because the engine is at idle RPM. In case of a botched approach or landing the throttle is then opened and carb heat shut off and the engine will go back to its takeoff RPM. The carb can ice at partial or full throttle also depending on the humidity.
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