Ignition, Induction and Carburetor Ice

Aircraft piston engines use dual ignition: two independent, engine-driven magnetos, each firing one of the two spark plugs in every cylinder. Because a magneto generates its own spark, the engine keeps running even with a dead battery, and the second magneto is a built-in backup that also improves combustion. During run-up you check each magneto alone and expect a small RPM drop; too large a drop or none at all points to a problem. Air and fuel reach the cylinders through the induction system. A float-type carburetor mixes fuel and air using a venturi, which is simple and cheap but prone to icing. Fuel injection instead sprays fuel at each intake port, giving more even distribution, better performance, and far less icing risk. Carburetor ice is a special hazard: the pressure drop and fuel vaporization in the venturi cool the air, and moisture can freeze even on a warm, humid day, choking the mixture. The test asks for the specifics: float-type carburetors are markedly more susceptible than fuel-injected engines, the risk is highest at outside air temperatures of roughly 20 to 70 degrees Fahrenheit with high humidity or visible moisture, and in an airplane with a fixed-pitch propeller the FIRST indication of carburetor ice is a decrease in engine RPM (with a constant-speed propeller it shows as a drop in manifold pressure). Carburetor heat melts and prevents the ice, though it reduces power and enriches the mixture — expect a further small RPM drop when heat is first applied, then a rise as the ice melts — so a gradual RPM loss and rough running should prompt its use.