Definition
On an attitude indicator, the curved scale at the top of the instrument that shows the airplane's bank angle. Fixed reference marks are typically placed at 0°, 10°, 20°, 30°, 60°, and 90°, with the 45° position usually shown as an inverted triangle or by the corner of the scale itself. A pointer (or the top of the miniature airplane symbol) moves against this scale to indicate how many degrees the aircraft is banked left or right.
Plain English
The bank-angle scale at the top of the attitude indicator. You read your bank angle by seeing which mark on the scale lines up with the pointer.
Context Anchor
Seen on the attitude indicator during instrument flying, especially when setting, holding, or correcting a turn by reference to instruments.
Derivation
Roll' is the aircraft's rotation about its longitudinal axis -- what produces bank. 'Index' comes from the Latin 'index' meaning 'pointer' or 'indicator,' the same root as 'index finger.' So a roll index is literally a pointer scale for roll (bank).
Why Pilots Care
Allows precise setting of bank angle for turns and heading control when outside visual references are unavailable.
Intuition Check
Do not think of index as a book index or list. Here, index means a pointer or reference mark on the instrument.
Example Sentence 1
Entering the turn, she rolled until the pointer lined up with the 30° mark on the roll index.
Example Sentence 2
During the turn, the instructor checked that the roll index stayed steady at the desired angle.