Definition
A cockpit instrument display, typically a vertical needle, that shows an aircraft's lateral position relative to a selected course on a VOR, localizer, or GPS navigation system. The needle deflects left or right to indicate which direction the pilot must steer to return to the selected course centerline.
Plain English
A needle on a navigation instrument that tells the pilot whether the airplane is left of, right of, or on the desired track. If the needle moves left, the course is to the left and the pilot needs to fly left to get back on it.
Context Anchor
Seen during instrument navigation when following a VOR course, a localizer approach, or a GPS route.
Derivation
From Latin 'deviare' (to turn aside). The instrument literally indicates how much the aircraft has turned aside from its intended course.
Why Pilots Care
Keeping the CDI centered maintains the planned track, which is essential for staying on course, complying with ATC clearances, and completing instrument approaches safely.
Intuition Check
A CDI does not show which way the airplane’s nose is pointed. It shows whether the airplane is left or right of the selected course or route.
Example Sentence 1
As the pilot intercepted the final approach course, the CDI needle slowly centered, confirming the aircraft was tracking the localizer.
Example Sentence 2
On the GPS approach the pilot kept the CDI centered by making small heading corrections as the wind drifted the airplane off track.