General Aircraft Technical Knowledge Practice Test

Throttle icing happens when the air is being restricted by a nearly closed throttle, causing a sharp pressure drop and cooling inside the carburetor as air accelerates and fuel vaporizes. In this low-power region, the intake surfaces are cold and there isn’t enough engine heat reaching the throttle area to melt ice as it forms. Moisture in the humid air condenses and freezes on the throttle plate and in the venturi, progressively constricting the passage and making icing worse. When the throttle is wide open and the engine is at higher power, air flows more vigorously, the pressure drop is reduced, and any forming ice is more readily blown away or melted by the warmer, more energetic flow. That’s why throttle icing is most likely at low throttle settings, including idle.