Definition
The small fixed reference mark at the top center of the heading indicator (directional gyro) that represents the nose of the aircraft. The heading currently aligned with the nose index on the rotating compass card is the aircraft's present heading.
Plain English
A little marker at the top of the heading instrument that stands for the front of your airplane. Whatever number on the dial is sitting under that marker is the direction you are pointed.
Context Anchor
Seen during instrument takeoff when the pilot uses the heading indicator to keep the airplane aligned with the runway after outside visual cues become limited.
Derivation
‘Nose’ refers to the front of the aircraft; ‘index’ comes from the Latin ‘index’ meaning a pointer or indicator. Together: a pointer that stands in for the airplane's nose on the instrument face.
Why Pilots Care
During an instrument takeoff or while flying without outside reference, the nose index is how a pilot reads heading and detects drift. Keeping the desired heading aligned under the nose index is the basic technique for tracking runway centerline and holding a course.
Intuition Check
Do not think of “index” here as a list in a book. In this cockpit use, an index is a fixed pointer or reference mark.
Example Sentence 1
After lift-off, the pilot kept the runway heading aligned under the nose index to detect any drift.
Example Sentence 2
Once airborne the pilot keeps the nose index aligned with the desired pitch mark to maintain a steady climb.