Icing conditions exist when visible moisture is present at a TAT of ____ or below, except when the SAT is ____ or below.

• A. 0°C and -20°C
• B. 10°C and -40°C
• C. 5°C and -50°C
• D. 15°C and -30°C

Answer: (B)

Icing conditions arise when there is visible moisture and the temperature allows liquid water to exist on the aircraft’s surfaces. The practical rule is that icing can occur when the total air temperature (TAT) is at or below 10°C, because at or below this temperature supercooled liquid water droplets can persist and freeze on contact with the aircraft.

However, there’s an important exception: if the standard outside air temperature (SAT) is -40°C or colder, the moisture is almost entirely in the form of ice crystals rather than liquid water, so icing conditions do not exist even if the TAT is low. This is why the best match is a TAT threshold of 10°C with an SAT threshold of -40°C.

Why the other options don’t fit: a lower TAT limit (like 0°C) would miss many potential icing scenarios; a higher limit (like 15°C) would include conditions where liquid water isn’t present or doesn’t stick as efficiently; and a different SAT limit (less cold than -40°C) wouldn’t correctly capture the point where liquid water can no longer exist.