Definition
A cockpit instrument that combines a heading indicator with a course deviation indicator on a single display. It shows the aircraft's current magnetic heading along with its position relative to a selected navigation course, presenting both pieces of information in one integrated picture.
Plain English
An instrument that shows you which way the aircraft is pointing and, at the same time, whether you are left, right, or on the navigation course you have chosen to follow.
Context Anchor
Seen on the instrument panel during navigation, instrument flying, and maintenance checks of navigation displays.
Derivation
Horizontal because the display represents the view looking down on the aircraft from above (a top-down, horizontal-plane picture). Situation because it tells the pilot the aircraft's situation relative to a course and a heading, all in one place.
Why Pilots Care
Allows the pilot to maintain accurate course tracking and situational awareness by consolidating multiple pieces of navigation information in one place.
Intuition Check
Do not read “horizontal” as meaning the airplane must be flying level. Here it means the instrument presents the navigation situation in a top-down, map-like way.
Example Sentence 1
The pilot scanned the HSI to confirm the aircraft was tracking the centerline of the approach course.
Example Sentence 2
As the airplane turned onto final approach, the HSI localizer needle centered smoothly.