Definition
On a VOR Course Deviation Indicator (CDI), full needle deflection is the condition in which the course needle has moved to its maximum displacement from center, indicating the aircraft is at or beyond 10° (or more) off the selected course. At full deflection, the indicator can no longer show how far off course the aircraft actually is — only that it is off course by at least the maximum the instrument can display.
Plain English
The needle on the VOR has swung all the way to one side as far as it can go. It tells you that you are well off the course you selected, but it can't tell you exactly how far off — once it's pinned to the side, it stops giving you fine information.
Context Anchor
Seen when using a VOR receiver and watching the CDI during navigation or course interception.
Derivation
Deflection comes from an older word meaning to bend or turn aside. That helps here because the CDI needle has turned aside from the centered position, showing that the aircraft is not on the selected course.
Why Pilots Care
It tells you that you have lost usable course guidance and must correct heading promptly to re-intercept the radial.
Analogy
It is like a meter whose pointer has gone all the way to the edge. You know the value is high enough to hit the limit, but the meter is not showing how much farther past the limit it may be.
Intuition Check
Do not read full needle deflection as an exact amount of distance from the course. In a VOR indication, it means the needle has reached its display limit, about 10 degrees or more off the selected course.
Example Sentence 1
After turning to the wrong heading, the pilot saw full needle deflection on the CDI and realized the aircraft had drifted well left of course.
Example Sentence 2
During the VOR approach the needle reached full deflection, prompting an immediate missed approach.