Thehorizontal situation indicator (commonly called theHSI) is anaircraftflight instrument normally mounted below theartificial horizon in place of a conventionalheading indicator. It combines a heading indicator with aVHF omnidirectional range-instrument landing system (VOR-ILS) display.[1]
The HSI can reduce 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 aninstrument landing system localizerback course approach. As long as the needle is set to the localizer front course, the instrument will indicate whether to fly left or right, in either direction of travel.
On the HSI, the aircraft is represented by a schematic figure in the centre 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 anautopilot capable of following the heading select bug and of executing anILS approach by following the localizer andglide slope.[1]
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 andTO/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 meansTO, and if it points behind to the side opposite the course selector, it meansFROM. The HSI illustrated here is a type designed for smaller airplanes and is the size of a standard 3 ¼-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 withelectronic flight instrument systems into so-called "glass cockpit" systems.
HSI is part of aremote indicating compass system, which was developed to compensate for the errors and limitations of the older type of heading indicators. The two panel-mounted components of a typical system include the HSI and a slaving control and compensator unit, which pilots can set to auto-correct the gyro error using readings from a remotely mounted magnetic slaving transmitter when the system is set to "slave gyro" mode. In a "free gyro" mode, pilots have to manually adjust their HSI.[2]