Definition
A cockpit instrument that combines the heading indicator and the navigation course display into a single presentation. It shows the aircraft's current heading on a rotating compass card while also displaying the selected navigation course, course deviation, and to/from information from a VOR, GPS, or similar navigation source.
Plain English
An instrument that puts your heading and your navigation course on the same dial, so you can see where the airplane is pointing and where the chosen course lies in one quick glance.
Context Anchor
Seen on the instrument panel when using heading information and navigation guidance, especially during instrument flying and course tracking.
Derivation
The 'horizontal situation' refers to the airplane's situation in the horizontal plane — its heading and its position relative to a course line — as opposed to the vertical plane shown by the attitude indicator. The instrument gives a top-down picture of that horizontal situation.
Why Pilots Care
It reduces the need to cross-check separate instruments, lowering workload and helping maintain accurate course tracking in instrument conditions.
Grounding Statement
Picture a compass-like display that also shows whether your selected route is centered, left of you, or right of you.
Intuition Check
Do not confuse “horizontal” here with the artificial horizon or pitch-and-bank display. In HSI, “horizontal” means a top-down navigation view of heading and course.
Example Sentence 1
After tuning the VOR, the pilot set the desired course on the HSI and watched the course needle center as the aircraft rolled out on heading.
Example Sentence 2
During the missed approach the HSI helped the pilot immediately turn to the assigned heading.