What direction do water lines extend when viewed from the top of the aircraft?

Water lines on an aircraft are a crucial reference used in aircraft design and maintenance. When viewed from the top, water lines extend parallel to the longitudinal axis of the aircraft. The longitudinal axis runs from the nose to the tail, and water lines provide a way to denote specific levels vertically across the aircraft's structure.

Understanding this orientation is essential for various maintenance tasks, as it aids technicians in assessing aspects like weight distribution, structural integrity, and design features meant to accommodate fluids, including fuel and hydraulic systems. By maintaining the relationship of components to these water lines, technicians can ensure appropriate balance and functionality throughout the aircraft's systems.

In summary, the correct answer highlights how water lines are aligned with the longitudinal axis, reinforcing the understanding of aircraft design and maintenance environments.