What must be the minimum freeze point of blended type I fluid when applied to the aircraft?

The minimum freeze point of blended type I fluid must be 18 degrees below the outside air temperature (OAT) when applied to the aircraft. This regulation is in place to ensure that the de-icing fluid remains effective during operation, providing adequate protection against ice formation. If the fluid's freeze point is not sufficiently lower than the OAT, there's a risk that the fluid will not effectively prevent ice accumulation, which can significantly impair the aircraft’s performance and safety.

The recommended freeze point ensures that the fluid maintains its liquid state under colder temperatures, allowing it to function as intended during critical flight operations. By specifying a freeze point that is 18 degrees below the OAT, it provides a buffer against temperature fluctuations that might occur at high altitudes or during flight.

The significance of maintaining this temperature differential lays in ensuring that the de-icing material remains effective throughout its application and does not freeze or become ineffective, potentially leading to hazardous conditions.