Definition
The vertical needle on a course deviation indicator (CDI) that shows the aircraft's lateral position relative to the selected VOR radial or other navigation course. When the needle is centered, the aircraft is on the selected course; when it deflects left or right, the selected course lies in that direction relative to the aircraft.
Plain English
A moving line on a cockpit instrument that tells you whether you are left of your chosen course, right of it, or right on it. To get back on course, you fly toward the side the needle is pointing.
Context Anchor
Seen on the instrument panel when using a course deviation indicator for navigation or an instrument approach.
Derivation
Course comes from an older word meaning a path or running direction. Deviation means moving away from a path. Needle refers to the thin pointer on the instrument, so the term literally points to how far you have moved away from the selected path.
Why Pilots Care
It provides immediate visual guidance to correct heading and stay on the desired track, essential for accurate enroute navigation and instrument approaches.
Grounding Statement
When the needle is centered, the aircraft is on the selected course; when it moves left or right, the selected course is on that side.
Intuition Check
Do not think of “course” as a training class or a general direction. Here, it means the specific navigation path the pilot selected to follow.
Example Sentence 1
As the pilot tracked the VOR radial, the course deviation needle drifted slightly left, so she made a small heading correction to recenter it.
Example Sentence 2
During the ILS approach, keeping the course deviation needle centered ensured the airplane remained aligned with the runway centerline.