Which procedure must be followed if the crew is unable to visually determine that aircraft surfaces are free from contamination?

The procedure that must be followed when the crew is unable to visually determine that aircraft surfaces are free from contamination is to perform a tactile pre-takeoff contamination check. This procedure is essential because visual checks alone may not be sufficient under certain conditions where ice, snow, or other contaminants could be present, but are not visibly apparent. A tactile check involves physically feeling the surfaces to ensure there is no frost, ice, or snow that could affect the performance and safety of the aircraft during takeoff.

Conducting this tactile check helps ensure that all critical surfaces such as wings, tail, and control surfaces are properly cleaned and safe for flight, which is vital for maintaining aerodynamic efficiency and preventing potential accidents. Adhering to this procedure reinforces the best practices in aviation safety, emphasizing that no departure should take place unless the aircraft is ensured to be in a safe condition for flight.