Definition
On a Horizontal Situation Indicator (HSI), the course deviation scale is the row of dots arranged on either side of the course pointer that shows how far the aircraft is laterally displaced from the selected course. Each dot represents a fixed amount of deviation, with the meaning depending on the navigation source: typically 2° per dot for VOR tracking, about 1° per dot when tuned to a localizer, and a fixed distance per dot when using GPS in certain modes.
Plain English
The dots beside the course line on the HSI that tell you how far off course you are. The further the course needle moves toward the outer dots, the further you are from the course you set.
Context Anchor
Seen on the Horizontal Situation Indicator (HSI), next to the moving course deviation indicator.
Derivation
‘Deviation’ comes from the Latin deviare, meaning ‘to turn off the way.’ The scale literally measures how far the aircraft has turned off the chosen way (course).
Why Pilots Care
It lets the pilot see and correct course errors immediately to maintain accurate instrument navigation.
Analogy
Think of it like a small ruler with the desired path at the center. The farther the indication moves from the center mark, the farther you are from where you meant to be.
Intuition Check
Do not read “scale” as a weighing device here. In this context, it means a marked measuring reference. “Course” here means the selected path or direction to follow, not a school class or a meal course.
Example Sentence 1
Halfway down the approach he noticed the needle had drifted to the second dot on the course deviation scale and made a small correction back toward centerline.
Example Sentence 2
With the course deviation scale centered, the aircraft remained on the localizer inbound.