Definition
A fixed reference mark on an instrument or aircraft component used as the zero or starting point from which measurements, alignments, or readings are taken. On an attitude indicator, for example, the index point is the small symbolic aircraft against which pitch and bank are read.
Plain English
A small mark or pointer that acts as the 'you are here' spot on a gauge or part. Everything else is measured from it.
Context Anchor
Seen in maintenance instructions, instrument markings, adjustment procedures, and chart or scale references.
Derivation
From the Latin index, meaning 'pointer' or 'one that points out' (the same root as 'index finger'). An index point is literally the spot the instrument is pointing to as its reference.
Why Pilots Care
Proper alignment at the index point ensures instruments give accurate altitude, RPM, and performance data needed for safe flight.
Analogy
It is like the zero mark on a ruler. The ruler only gives the right measurement if you start from the correct mark.
Intuition Check
Index does not mean a book list here. It means a fixed reference point used for reading, setting, or alignment.
Example Sentence 1
The pitch ladder on the attitude indicator moves up and down behind the index point, which represents the aircraft's nose.
Example Sentence 2
During the magneto check the mechanic verified the tachometer needle was steady at the index point for the correct RPM.