High Quality Content by WIKIPEDIA articles! 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.