What action should be taken with critical surfaces before takeoff according to the clean aircraft concept?

The clean aircraft concept emphasizes that critical surfaces of an aircraft must be completely free of frost, ice, snow, or slush before takeoff to ensure optimal aerodynamic performance and control. These critical surfaces include wings, control surfaces, and other areas that significantly influence the aircraft's ability to generate lift and maintain stability. Any contamination can adversely affect the aircraft's handling characteristics and increase the risk of accidents during takeoff and flight.

By adhering to this principle, pilots and ground crews ensure that the aircraft can perform as intended and that safety is maintained. The presence of any frost or ice can disrupt airflow over the wings and control surfaces, leading to reduced lift and increased drag, which can compromise the aircraft's ability to take off safely.

Therefore, ensuring that these surfaces are clear of any contaminants is crucial to upholding safety standards in aviation operations.