Which surfaces should be free of frost, ice, snow, or slush to comply with the clean aircraft concept?

The clean aircraft concept mandates that all critical surfaces must be free of frost, ice, snow, or slush before takeoff to ensure the aircraft's performance and safety. This includes not just the wings but also the horizontal and vertical stabilizers, control surfaces, and any other parts that affect the airflow over the aircraft. Contaminants can significantly alter the aerodynamics, leading to a loss of lift and control issues. Therefore, ensuring that all critical surfaces are clear of any frost, ice, snow, or slush is essential for maintaining safe and optimal flight conditions.