Definition
A flight instrument that combines the heading indicator and the VOR/ILS course deviation indicator into a single display. It shows the aircraft's heading, the selected course, the deviation of the aircraft from that course, and the to/from indication relative to a navigation station, all referenced to a compass card that rotates with the aircraft's heading.
Plain English
One instrument that shows you which way the aircraft is pointing and, at the same time, whether you are left or right of the route you have selected to fly. It puts heading and course information together so you can see your situation at a glance.
Context Anchor
Seen on instrument panels and electronic flight displays during en route navigation and instrument approaches.
Derivation
"Horizontal" because it shows your situation in the horizontal plane (left/right, heading, course) rather than vertical (climb/descent). "Situation" because it presents the overall navigation picture, not just one piece of it. The name reflects its purpose: a single picture of where you are horizontally in relation to your intended path.
Why Pilots Care
Reduces instrument scan workload and improves situational awareness during instrument approaches and enroute navigation.
Intuition Check
Do not assume “situation” means the instrument shows everything happening around the aircraft. A Horizontal Situation Indicator normally shows heading and course relationship; it is not a traffic, weather, terrain, or attitude display.
Example Sentence 1
After tuning the VOR, the pilot set the desired course on the HSI and turned to intercept it.
Example Sentence 2
During the ILS approach the horizontal situation indicator showed a left deviation that the pilot corrected with a small heading change.