Horizontal Situation Indicators
The horizontal situation indicator (commonly called the HSI) is an aircraft instrument normally mounted below the artificial horizon in place of a conventional directional gyro (DG). It combines both the DG and the VOR display, reducing pilot workload by lessening the number of elements in the pilot's instrument scan. The HSI instrument also includes a glide slope needle so that an ILS instrument approach can be flown with reference only to the six basic flight instruments. Among other advantages, the HSI offers freedom from the confusion of reverse sensing. This is not the case when on a localizer back course. The back course outbound bearing must be set to prevent reverse sensing.
On the HSI, the airplane is represented by a schematic figure in the center of the instrument. The VOR/ILS localizer is shown in relation to the airplane figure. The heading indicator is slaved to a remote compass-integrating the heading indicator and VOR/ILS (with glide slope). It is frequently interconnected with an autopilot with altitude hold and the capability of following the glide slope all the way to the decision height and beyond.
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 selecter arrow, it means TO, and if it points behind to the side opposite the course selecter, it means FROM.