Definition
The movement of the course-deviation needle on a navigation indicator (such as a VOR or localizer display) away from its centered position, showing that the aircraft is laterally displaced from the selected course. The direction the needle moves indicates which way the course lies relative to the aircraft, and the distance it moves indicates how far off course the aircraft is.
Plain English
The needle on the navigation instrument is showing how far off course you are, and which direction to fly to get back on course. A centered needle means you are on course; a needle to one side means the course is over there.
Context Anchor
Seen while using cockpit navigation instruments to follow a radio navigation path, especially during tracking practice in instrument flying.
Derivation
“Deflection” comes from words meaning “to bend or turn aside.” That fits the aviation use: the needle turns aside from its normal or desired position to show that the airplane is off the intended path.
Why Pilots Care
The direction and amount of deflection tells the pilot exactly which way and how much to turn to regain or maintain the desired track.
Intuition Check
Do not read needle deflection as a command to turn the airplane by the same amount the needle moved. It is an indication that the airplane and the desired navigation path are not lined up, so the pilot must interpret it and make a suitable correction.
Example Sentence 1
As the aircraft drifted south of the airway, the pilot noticed a half-scale needle deflection to the left and corrected back toward the course.
Example Sentence 2
As the heading correction took effect, the needle deflection gradually reduced until the needle centered.