I can't be bothered to look it up (sorry), but I recall reading words somewhere to the effect that if the OAT is very cold, there may be ice crystals in the air that will pass harmlessly through the engine (presumably melting in the process), UNLESS you should happen to use enough carb heat to warm the air into the icing range. Probably somewhat unusual though.
Absolutely correct. You can reference the US FAA website (
Tips On Winter Flying - En Route) for clarification, but in a nut-shell... (Remember, the yanks use degrees F)
Three categories of carburetor ice are:
- Impact ice formed by impact of moist air at temperatures between 15-32°F on airscoops, throttle plates, heat valves, etc. Usually forms when visible moisture such as rain, snow, sleet, or clouds are present. Most rapid accumulation can be anticipated at 25°F.
- Fuel ice forms at and downstream from the point that fuel is introduced when the moisture content of the air freezes as a result of the cooling caused by vaporization. It generally occurs between 40-80°F, but may occur at even higher temperatures. It can occur whenever the relative humidity is more than 50%.
- Throttle ice is formed at or near a partly closed throttle valve. The water vapor in the induction air condenses and freezes due to the venturi effect cooling as the air passes the throttle valve. Since the temperature drop is usually around 5°F, the best temperatures for forming throttle ice would be 32-37°F although a combination of fuel and throttle ice could occur at higher ambient temperatures.
Cheers....