Definition
Visible features outside the airplane that run roughly parallel to the natural horizon and are used by the pilot to judge the airplane's pitch attitude and bank angle during visual flight. The most important horizontal reference line is the natural horizon itself; in its absence, distant ridgelines, cloud layers, coastlines, or the far edge of a wide field can serve the same purpose.
Plain English
Long, level-looking lines you can see out the windscreen — like the horizon, a distant shoreline, or a flat cloud deck — that you use to tell whether the nose is level, pitched up, or pitched down, and whether the wings are level.
Context Anchor
Used in straight-and-level flight when looking outside the windshield and comparing the airplane’s attitude to the horizon and other level-looking cues.
Why Pilots Care
They provide a reliable way to hold attitude without constant instrument scanning, especially important for new pilots developing visual flying skills.
Intuition Check
Do not assume these must be painted lines. In this context, a line can be any clear visual edge that helps you judge level, including the horizon or part of the airplane’s structure.
Example Sentence 1
On a clear day over flat terrain, the student used the natural horizon as her primary horizontal reference line to hold straight-and-level flight.
Example Sentence 2
Adjusting seat position changes how the horizontal reference lines appear against the horizon.