Definition
On a Horizontal Situation Indicator (HSI), the movable bar that represents the selected course (such as a VOR radial or localizer) and shows the aircraft's lateral displacement from that course. The bar swings left or right of center to indicate the direction and amount of deviation from the desired track.
Plain English
It's the line on the HSI that shows whether you are left or right of the course you've chosen to fly. If the bar is centered, you're on course. If it sits to one side, you need to steer that way to get back on track.
Context Anchor
Seen on the horizontal situation indicator during instrument navigation, especially when following a selected route or instrument approach path.
Derivation
"Deviation" comes from Latin deviare, meaning "to turn off the way." The bar literally shows how far the aircraft has turned off the chosen way (course).
Why Pilots Care
It provides immediate visual feedback so the pilot can make small heading corrections to stay on the desired course, preventing large track errors especially in instrument conditions.
Intuition Check
Do not read the course deviation bar as a heading pointer. It does not show which way the airplane’s nose is pointed; it shows whether the selected course line is left or right of the airplane.
Example Sentence 1
As she intercepted the inbound radial, the course deviation bar slid toward center and she rolled out on heading.
Example Sentence 2
With the course deviation bar centered, the aircraft remained on the inbound localizer course during the ILS approach.