Which of the following could indicate that a re-application of de-icing fluid is needed?

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The presence of visible frozen contamination on surfaces, such as wings or other critical areas, is a strong indicator that a re-application of de-icing fluid is necessary. This contamination can arise from snow, ice, or frost that could impair the aircraft's performance by disrupting airflow. Proper aerodynamics is crucial for safe operations, and any frozen material could pose a risk during takeoff and flight. If visible frozen contamination exists, it signifies that the existing de-icing protection has failed, necessitating additional treatment to ensure safe and efficient operation.

Clear wings, although they might suggest that no contamination is currently present, do not necessarily indicate if the de-icing fluid has lost its effectiveness over time or under certain operational conditions. Wind blowing snow can create a situation where snow accumulates again, but it is not a direct indicator of the necessity to reapply de-icing fluid. Dry pavement conditions suggest that there is no freezing rain or snow which would affect the aircraft; thus, they do not require de-icing interventions either. Hence, only visible frozen contamination is a definitive signal that reapplication is needed.