At what altitude does the gross flight path start on a wet runway?

The gross flight path on a wet runway begins at an altitude of 15 feet. This specific altitude is significant because it's the point at which aircraft usually transition from ground effect to free flight. Ground effect can have a substantial impact on performance during takeoff, particularly on wet runways, where the characteristics of lift and drag may differ due to increased moisture on the surface. At 15 feet, any residual water on the surface affects tire performance and overall aircraft handling. This altitude is critical because it allows for adequate clearance over any potential obstacles while still being influenced by runway surface conditions.

The other altitude options do not align with standard operational practices and guidelines regarding the gross flight path on a wet runway. For instance, starting at 0 feet would imply no takeoff clearance, while other higher altitudes such as 35 feet or 50 feet would not adequately reflect the specific dynamics at the very initiation of the flight path during takeoff when challenges from wet conditions are most pronounced.