Which fluid type is generally used for anti-icing in a two-step process?

In a two-step anti-icing process, SAE Type II fluid is generally used due to its designed properties that provide extended holdover times and reduced ice adhesion on aircraft surfaces. SAE Type II fluid is particularly effective in preventing ice formation as it has been formulated to remain on the aircraft longer than Type I fluids, which are primarily used in the de-icing phase.

The process typically involves applying a heated Type I fluid for de-icing, which removes existing ice, snow, or frost, followed by the application of Type II fluid to prevent further accumulation of ice. Type II fluids are thicker and provide a protective layer that allows for greater flexibility in operational windows where icing conditions may continue.

Other options may not support the two-step process as effectively. SAE Type I is more focused on the immediate removal of ice and snow but does not offer long-term protection against ice accumulation once applied. Water, while it can serve in some de-icing applications, is not effective as a long-term anti-icing solution since it evaporates quickly and does not prevent ice formation. A mixture of Type I and II fluids is not typically used because it can compromise the effectiveness of each fluid's intended function in both the de-icing and anti-icing steps.