During de-ice/anti-ice application, why are hold times observed?

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Hold times are set to make sure the applied de-ice/anti-ice fluid actually stays where it belongs and works as intended. The fluid needs time to wet and spread over every surface, including joints, edges, and complex shapes, so it forms a continuous protective film rather than just a thin smear. If the fluid isn’t allowed to sit long enough, it can run back or pool and be carried away by gravity or air flow, leaving bare patches or causing contamination on surfaces where protection is most needed. By allowing proper adhesion, the coating remains in place through handling and on-wings warm-up, giving the aircraft the intended protection until takeoff. The hold time can vary with the type of fluid and the environmental conditions, but the core purpose is to ensure adhesion and prevent runback, not to dry the aircraft or change its appearance.