Definition
The vertical pointer on a Course Deviation Indicator that shows the aircraft's lateral position relative to the selected course. When the needle is centered, the aircraft is on course. When it deflects left or right, the aircraft is off course in that direction, and the pilot must steer toward the needle to return to the selected track.
Plain English
The line on the navigation display that tells you whether you are on the path you chose. If it sits in the middle, you are on the path. If it leans left or right, you have drifted that way and need to turn toward it to get back.
Context Anchor
Seen on navigation instruments during enroute navigation and instrument approaches when tracking a selected course.
Derivation
Needle comes from the older idea of a thin pointer on an instrument face. Even on a screen, pilots still call the moving course pointer the needle because it acts like the old physical pointer.
Why Pilots Care
It provides immediate visual feedback for course corrections, preventing drift during instrument flight and approaches.
Analogy
It is like a lane-centering guide in a car: if the marker shows you are drifting left or right of the lane, you correct back toward the center.
Intuition Check
The CDI needle does not point directly to a place on the ground. It shows whether you are left or right of the course you selected.
Example Sentence 1
As the aircraft drifted south of the airway, the CDI needle moved left, prompting the pilot to correct with a slight left turn.
Example Sentence 2
With the CDI needle steady in the center, the aircraft remained on the final approach course.