Definition
The course deviation indicator (CDI) on a VOR display — a vertical needle that swings left or right of center to show whether the aircraft is left or right of the selected VOR course. When the needle is centered, the aircraft is on the chosen course; full-scale deflection means the aircraft is at or beyond the edge of the course width (typically 10° either side of the selected radial).
Plain English
The pointer on the VOR instrument that tells you whether you are on the line you've chosen to fly, or off to the left or right of it. Center means on course; needle leaning left or right means the course is over there.
Context Anchor
Seen on VOR navigation displays and in discussions comparing VOR indicators with ADF needles.
Derivation
VOR stands for very high frequency omnidirectional range, a radio navigation system that gives course information from a ground station. Needle comes from older instruments that used a thin moving pointer; in modern displays it may look more like a vertical line than a physical needle.
Why Pilots Care
The VOR needle is the primary visual cue for staying on a selected course. Misreading which way it points — or chasing it the wrong direction — leads to off-course flying, busted approaches, and on instrument checkrides, a failed task. Knowing what full-scale deflection means is also essential for judging how far off course you actually are.
Intuition Check
Do not assume the VOR needle points directly to the station like an ADF needle. A VOR needle shows whether you are left or right of the course you selected.
Example Sentence 1
As the aircraft drifted in the crosswind, the VOR needle moved slowly to the left, so the pilot corrected back onto course.
Example Sentence 2
A right deflection on the VOR needle prompted a turn to rejoin the course.