In non-icing conditions, the anti-ice system should be used only at ram air temperatures of:

• A. 20 °C or less
• B. 10 °C or less
• C. 0 °C or less
• D. -10 °C or less

Answer: (B)

The key idea is when anti-ice protection is actually needed based on outside air temperature. On the Beechjet 400A, the anti-ice system is designed to be used when there’s any possibility of ice forming in the air—typically in conditions where visible moisture could exist at temperatures where ice can still accumulate. The practical threshold chosen is 10°C or below, because at or below this ram air temperature there’s a real icing risk if moisture is present, so turning on anti-ice protects the surfaces.

Using anti-ice above 10°C is generally unnecessary and wastes bleed-air, reducing efficiency and increasing wear without providing real benefit. So, even in non-icing conditions, the guideline is to activate anti-ice only if the ram air temperature is 10°C or lower.