Definition
A small horizontal bar on the face of the attitude indicator that represents the aircraft's wings. It stays fixed relative to the airplane while the artificial horizon line behind it moves, showing the aircraft's pitch and bank attitude relative to the natural horizon.
Plain English
It is the little bar on the attitude indicator that stands for your wings. You read your pitch and bank by seeing where this bar sits compared to the horizon line behind it.
Context Anchor
Seen on a mechanical attitude indicator on the instrument panel, especially when learning how to read the instrument display.
Derivation
Called a 'reference arm' because it is the fixed reference against which the moving horizon is read. 'Horizon' identifies what it is being compared to. The name describes its job: an arm you reference the horizon against.
Why Pilots Care
It allows accurate pitch control when outside visual references are lost, directly supporting safe instrument flight and attitude awareness.
Intuition Check
Do not think of this as the real outside horizon or as a physical arm on the airplane. It is an instrument reference line used to compare the airplane symbol against an artificial horizon.
Example Sentence 1
In a level climb, the horizon reference arm sits just above the artificial horizon line.
Example Sentence 2
During a climb entry the horizon reference arm appeared below the miniature airplane symbol.