The Horizontal Situation Indicator or HSI, one of the most important tools for any pilot, provides a basic horizontal view of an aircraft’s navigation picture. For example, in the F-15E Eagle fighter jet, the HSI can provide navigation data to selected ground navigation facilities like Instrument Landing Systems or onboard navigation systems such as the Inertial Navigation System. The HSI is a critical tool during instrument flying, wherein the pilot’s visibility is so poor that he or she must rely solely on their instruments. Because of this, the HSI has become more and more popular in general aviation over time.