In flight with visible moisture, anti-ice is required when total air temperature is within which range?

• A. 10°C to -39°C
• B. 0°C to 10°C
• C. -20°C to 0°C
• D. -40°C to -10°C

Answer: (A)

Ice can form on aircraft surfaces only when there is visible moisture and liquid water droplets can exist in the air. The anti-ice system should be used in flight with visible moisture when the total air temperature is in a window that starts at +10°C and goes down to about -40°C. Within this range, supercooled liquid droplets are present and can freeze on contact, so anti-icing is needed to prevent ice buildup. Above +10°C, icing from visible moisture is unlikely, and below about -40°C, there aren’t liquid droplets in the clouds, so icing risk is negligible. That’s why this temperature band is the best match.