Illustration of the face of a horizontal situation indicator showing the key elements of the display.
The horizontal situation indicator (commonly called the HSI) is an aircraft instrument normally mounted below the artificial horizon in place of a conventional heading indicator. It combines a heading indicator with a VOR/ILS display, reducing pilot workload by lessening the number of elements in the pilot's instrument scan to the six basic flight instruments. Among other advantages, the HSI offers freedom from the confusion of reverse sensing on a localizer backcourse approach. On a front course approach, the HSI needle is set to the inbound track; on a back course approach, the HSI needle is set to the outbound rather than the inbound track, causing needle deflection that mimics a front course approach instead of displaying the normal reverse sensing.
On the HSI, the airplane is represented by a schematic figure in the center of the instrument – the VOR/ILS display is shown in relation to this figure. The heading indicator is usually slaved to a remote compass, and the HSI is frequently interconnected with an autopilot capable of executing an approach by following the localizer and glide slope.
On a conventional VOR indicator, left/right and to/from must be interpreted in the context of the selected course. When an HSI is tuned to a VOR station, left and right always mean left and right and TO/FROM is indicated by a simple triangular arrowhead pointing to the VOR. If the arrowhead points to the same side as the course selector arrow, it means TO, and if it points behind to the side opposite the course selector, it means FROM. The HSI illustrated here is a type designed for smaller airplanes, and is the size of a standard 3-1/4 inch instrument. Airline and jet aircraft HSIs are larger and may include more display elements. The most modern HSI displays are electronic and often integrated with electronic flight director systems into so-called "glass cockpit" systems.