Definition
The banking index is the curved scale at the top of the attitude indicator marked with reference lines at 10°, 20°, 30°, 45°, 60°, and (on some instruments) 90° of bank. A pointer fixed to the instrument's gyro-stabilized inner element moves against this scale as the aircraft rolls, showing the angle of bank in degrees.
Plain English
It is the curved row of marks across the top of the attitude indicator that shows how steeply the aircraft is banked left or right.
Context Anchor
Seen on the attitude indicator during instrument flying, especially while setting, holding, or correcting a turn.
Derivation
‘Bank’ refers to the aircraft’s left-or-right roll angle (from the older sense of leaning into a turn, like a road or racetrack banked on a curve). ‘Index’ comes from Latin index, meaning ‘pointer’ or ‘indicator.’ Together: the pointer-and-scale that shows bank angle.
Why Pilots Care
Allows precise control of bank angle to maintain coordinated flight and desired turn rate without over- or under-banking.
Intuition Check
Do not read “banking” as anything related to money, and do not read “index” as a list in a book. Here, “banking” means aircraft tilt, and “index” means the mark used to read that tilt.
Example Sentence 1
Rolling into the turn, the pilot stopped the bank when the pointer aligned with the 30° mark on the banking index.
Example Sentence 2
During the rollout from the turn, the pilot watched the banking index return to zero to confirm wings level.