Definition
A reference mark on the face of the turn-and-slip indicator (or turn coordinator) showing the position of the indicator's pointer or miniature aircraft when the airplane is turning at a standard rate of three degrees per second. Aligning the pointer with this index produces a 360-degree turn in two minutes.
Plain English
It is the small mark on the turn instrument that shows you where the needle should sit when you are turning at the normal rate used in instrument flying — three degrees every second, or a full circle in two minutes.
Context Anchor
Seen on the face of a turn-and-slip indicator or turn coordinator, especially during instrument flying practice, timed turns, and instrument scan discussions.
Derivation
"Index" comes from Latin index, meaning a pointer or indicator — something that points the way. On this instrument, the index is the fixed mark the pilot lines the moving pointer up with to fly a standard rate turn.
Why Pilots Care
It lets the pilot set and hold a consistent turn rate without watching the clock or computing bank angle, producing predictable heading changes during instrument procedures.
Analogy
It is like a tick mark on a speedometer: the mark is not the motion itself, but it tells you when you have reached the target amount.
Intuition Check
Standard does not mean any smooth or comfortable turn rate; here it means the specific rate of 3 degrees per second. Index does not mean a list in a book; here it means a reference mark on the instrument.
Example Sentence 1
Entering the hold, the pilot rolled into a turn and adjusted bank until the pointer aligned with the standard rate turn index.
Example Sentence 2
During the procedure turn, alignment with the standard rate turn index produced the required three-degree-per-second rate without further adjustment.