What is a sign that the frost on an aircraft may be permissible?

The presence of frost on an aircraft can significantly impact its performance, particularly during takeoff and landing. The correct choice indicates that the frost is thin enough to allow the underlying paint features to be distinguished. This is crucial because a thin layer of frost may not significantly alter the aircraft's aerodynamic characteristics compared to thicker frost, which can disrupt airflow and lead to decreased lift and increased drag.

When the frost is thin, it suggests that it may not have enough mass or irregularity to create a major hazard, as opposed to thick and opaque frost, which would likely cause more serious aerodynamic issues. Additionally, frost only covering the wings or being evenly distributed across all surfaces does not address the critical aspect of its thickness and potential aerodynamic effect. Therefore, the thickness and transparency of the frost in relation to visual inspection is an important factor in determining whether it is permissible for flight.