Horizontal situation indicator

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. This reduces 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 Instrument landing system localizer back 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.