Definition
On a Horizontal Situation Indicator (HSI), the course deviation bar is the movable center segment of the course arrow that shifts left or right of center to show the aircraft's lateral position relative to the selected VOR radial or other navigation course. When the aircraft is on course, the bar aligns with the fixed course arrow ends to form a single straight line. When off course, the bar slides sideways to indicate which direction the aircraft must turn to return to the selected course.
Plain English
It is the moving middle part of the course line on an HSI. If the bar is sitting straight in the middle, you are on the line you chose. If it has slid to one side, you have drifted off that line and need to steer toward the bar to get back on.
Context Anchor
Seen on an HSI during instrument navigation, especially when tracking a VOR course, GPS course, or instrument approach course.
Derivation
Deviation comes from Latin words meaning “off the way.” That fits the aviation use: the bar shows how far you are off the course you meant to follow.
Why Pilots Care
It shows exactly how far off the desired course the aircraft is so the pilot can correct promptly and stay on track.
Analogy
Think of it like a road centerline shown on a display. If the line appears left of you, the path you want is left of your present position.
Intuition Check
Do not read “bar” as just a fixed mark on the instrument. In this context, the bar moves, and its movement shows where the selected course is relative to the aircraft.
Example Sentence 1
As the aircraft drifted right of the selected radial, the course deviation bar slid to the left, prompting the pilot to make a small left correction.
Example Sentence 2
With the course deviation bar centered, the aircraft remained aligned with the navigation path.