Definition
The skill of reading the Course Deviation Indicator (CDI) needle on a VOR display to determine the aircraft's position relative to the selected course. The pilot interprets three pieces of information together: the OBS (Omni Bearing Selector) setting, which course the needle's deflection direction (left or right), and the TO/FROM flag, to establish where the aircraft is in relation to the VOR station and the chosen radial.
Plain English
Reading the VOR needle correctly so you know where you are compared to the course you've chosen. The needle tells you which way the course is, the flag tells you whether the station is ahead of you or behind you, and together they place you in the picture.
Context Anchor
Seen during VOR navigation, especially when using the OBS knob and CDI needle to stay on or intercept a selected course.
Derivation
CDI stands for Course Deviation Indicator. 'Deviation' comes from Latin deviare, meaning 'to turn off the way.' The needle literally shows how far you have turned off the course you selected.
Why Pilots Care
Correct interpretation prevents off-course flight and enables precise navigation to waypoints or approaches.
Grounding Statement
The CDI does not show where the airplane is pointed; it shows where the selected course is in relation to the airplane.
Intuition Check
Do not assume the CDI needle is simply pointing to the station. It shows left or right of the selected course, and the TO/FROM indication must be included to understand it correctly.
Example Sentence 1
After tuning the VOR and setting the OBS to 360, the student practiced CDI interpretation by identifying which side of the course the aircraft was on and whether the station lay ahead or behind.
Example Sentence 2
During the approach, accurate CDI interpretation kept the aircraft aligned with the final course.