Definition
A form of induction icing that forms on or near the throttle valve (butterfly) of a carburetor when the partially closed throttle restricts airflow, causing a localized pressure drop and temperature decrease that freezes water vapor onto the metal surfaces of the throttle assembly.
Plain English
Ice that builds up around the throttle valve inside a carburetor. As air squeezes past the partly-closed throttle, it cools sharply, and any moisture in that air can freeze onto the metal — slowly choking off the airflow to the engine.
Context Anchor
Seen in carburetor icing discussions, especially during low-power operation such as descent, approach, or prolonged idling.
Derivation
Throttle' comes from an old English word meaning 'to choke' — the same root as 'throat.' That is exactly what the throttle does: it chokes or opens the airflow into the engine. When ice forms there, it physically chokes the engine even further.
Why Pilots Care
Throttle ice can restrict airflow enough to cause engine roughness, power loss, or complete stoppage if carburetor heat is not applied promptly.
Grounding Statement
Picture a partly closed plate inside the carburetor getting cold enough that moisture in the passing air freezes onto it.
Intuition Check
Throttle ice does not mean ice on the throttle lever in the cockpit. It means ice forming inside the carburetor around the throttle valve, where it can affect engine airflow.
Example Sentence 1
During a long descent at low power, the pilot applied carburetor heat to prevent throttle ice from forming.
Example Sentence 2
In humid conditions the risk of throttle ice rises whenever the throttle is held partially closed for extended periods.