What does the term "horizontally" refer to in relation to flight?

The term "horizontally" in relation to flight specifically refers to a flight level being parallel to the horizon. This means that when an aircraft is flying horizontally, it maintains a flight attitude that is level with the Earth's surface, providing a constant altitude above the ground. This is crucial for ensuring consistent airspeed and avoiding unwanted altitude deviations during flight, as well as contributing to overall aircraft stability.

When an aircraft is flying in a horizontal manner, it is neither climbing nor descending, allowing for smoother flight operations. This level flight is essential for navigation, as it helps pilots maintain their intended course and altitude while considering factors such as wind drift and air traffic.