Definition
On an attitude indicator, the fixed reference line that separates the simulated sky (upper half, usually blue) from the simulated ground (lower half, usually brown or black). It represents the actual horizon and serves as the baseline against which the miniature airplane symbol is compared to determine the airplane's pitch and bank attitude.
Plain English
It's the line on the attitude indicator that stands in for the real horizon outside. The little airplane symbol on the instrument is positioned above, below, or tilted relative to this line, and that tells you how the airplane is pitched and banked.
Context Anchor
Seen on the attitude indicator during instrument flying, especially when the real outside horizon is hard to see or cannot be seen at all.
Derivation
Horizon comes from a Greek word meaning “boundary” or “dividing line.” That fits the aviation use: the horizon line is the dividing reference between sky and ground on the attitude indicator.
Why Pilots Care
When you can't see the real horizon outside (in cloud, at night, in low visibility), the horizon line on the attitude indicator becomes your primary reference for keeping the airplane right-side up. Misreading it, or losing it from your scan, is a leading cause of spatial disorientation accidents.
Grounding Statement
When you look at the attitude indicator, the horizon line is the stable-looking reference that tells you how the airplane is positioned compared with level flight.
Intuition Check
Do not assume the horizon line is the actual outside horizon. Here, it means the instrument’s displayed reference line that represents the horizon for attitude control.
Example Sentence 1
Entering the clouds, she shifted her scan to the attitude indicator and kept the miniature airplane aligned with the horizon line to maintain level flight.
Example Sentence 2
In the turn the horizon line tilted while the miniature airplane stayed level with the wings.