Definition
A cockpit instrument that combines a heading indicator with a course deviation indicator (CDI) on a single display, presenting the aircraft's heading, the selected navigation course, the aircraft's position relative to that course, and (when paired with a glideslope receiver) vertical guidance for an ILS approach. The HSI shows a compass card that rotates with the aircraft's heading, with a course pointer and deviation bar overlaid to indicate the selected VOR or localizer course and how far the aircraft is left or right of it.
Plain English
A single instrument that shows the pilot, at one glance, which way the airplane is pointing and how it is positioned relative to the navigation course they want to fly. It merges two instruments into one picture so the pilot doesn't have to mentally combine them.
Context Anchor
Seen on the instrument panel or electronic flight display during instrument navigation, especially when tracking a selected course or flying an instrument approach.
Derivation
"Horizontal" refers to the horizontal plane — the view as if looking down on the aircraft from above. "Situation" means the aircraft's position and orientation relative to its course. So the name describes exactly what it shows: the aircraft's horizontal situation — where it is and which way it's pointed — relative to the navigation course.
Why Pilots Care
It gives immediate visual awareness of heading and course deviation so the pilot can correct without scanning multiple instruments.
Analogy
Think of it like a top-down map view on a car's GPS that also shows which way the car is pointing — heading and course displayed together so you instantly see whether you're tracking the route or drifting off it.
Grounding Statement
Picture looking down at a simple moving display that shows the airplane’s nose direction and whether the airplane is lined up with the chosen path.
Intuition Check
“Situation” does not mean an emergency here. In HSI, it means the airplane’s position and direction compared with the course the pilot selected.
Example Sentence 1
On the ILS approach, the pilot watched the HSI to keep the course needle centered while also monitoring the glideslope pointer.
Example Sentence 2
During the approach the HSI combined heading and localizer information into one easy-to-read display.