FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A pilot reviews a carburetor icing probability chart during preflight planning. Under which condition can carburetor icing occur even though the outside air temperature (OAT) is as high as 70°F (21°C)?
- AWhen relative humidity is below 30 percent
- BWhen relative humidity is high, with visible moisture or high humidity present
- COnly when the OAT is below freezing
- DOnly during high-power climb settings
Show answer & explanationAnswer & explanation
Correct answer: B. When relative humidity is high, with visible moisture or high humidity present
Carburetor icing charts show that ice can form across a wide range of temperatures and even at fairly high OATs (up to around 100°F) when relative humidity is high, because the venturi effect causes rapid cooling and condensation/freezing of moisture in the induction air. Low humidity greatly reduces this risk regardless of temperature.
Why the other options are wrong
- A. Low humidity air holds little moisture, reducing icing risk.
- C. Icing is actually most common between 20°F and 70°F, not just below freezing.
- D. Icing risk is often highest at low power (descent/idle), not high power settings, due to lower induction air temperature.
Carburetor Icing Conditions
Carburetor ice can form in a wide temperature range (roughly 20°F–70°F, extending to near 100°F) whenever relative humidity is high, due to rapid cooling from fuel vaporization and venturi pressure drop.
- Most likely with visible moisture or high humidity
- Can occur even on a warm, humid day
- Descent/low power settings increase risk (reduced airflow, cooler venturi)
Memory trick: 'Humid air hides ice, even when it's warm'