Definition
A cockpit instrument that combines a heading indicator with a course deviation indicator (CDI), showing the aircraft's heading along with its position relative to a selected navigation course (VOR radial, ILS localizer, or GPS track) on a single display.
Plain English
An instrument that puts your compass heading and your navigation course on the same dial, so you can see at a glance which way you're pointed and whether you're on track.
Context Anchor
Seen on instrument panels and glass cockpit displays, especially during navigation and instrument approach work.
Derivation
From 'horizontal situation' — meaning the aircraft's situation in the horizontal plane (heading and course position over the ground), as opposed to its vertical situation (pitch and altitude). The instrument shows where you are horizontally relative to your intended path.
Why Pilots Care
It reduces instrument scan workload and improves situational awareness by presenting heading and course information together.
Intuition Check
Do not read “situation” as a general status report. In HSI, it means the aircraft’s direction and position relative to the selected navigation path.
Example Sentence 1
She set the course pointer on the HSI to the inbound radial and watched the needle center as the aircraft intercepted the airway.
Example Sentence 2
As the aircraft intercepted the localizer, the HSI course deviation needle began to center.