The Horizontal Situation Indicator (HSI) is an avionics instrument that displays an aircraft’s position relative to waypoints using a directional gyroscope, heading error, and VOR/ILS. It transmits location to ground navigation facilities and includes course deviation indicators, glide deviation scales, and a 360-degree compass. HSIs provide pilots with precise control, guidance, and navigation parameters, especially in severe weather conditions. They can also include additional features such as forward speed and aeronautical charts with Doppler radar information.
A Horizontal Situation Indicator, or HSI for short, combines multiple flight instruments that provide a display of the aircraft’s position relative to various waypoints. Among the instruments generally contained within the HSI are a directional gyroscope, a heading error, and a very high frequency instrument/range omnidirectional landing system (VOR/ILS). These aircraft instruments, widely used in avionics, are generally used during takeoff, climb, flight, and landing. Variations of horizontal situation indicator instrumentation are used by fixed-wing aircraft, helicopters, and the space shuttle for manual or automatic piloted flight.
In addition to providing pilots with information about the aircraft, via radio navigation, the horizontal situation indicator transmits the aircraft’s location to selected ground navigation facilities. After programming the course and heading indicators, the team transmits the bearings to two ground stations. The display may also include two four-digit counters. One monitors the aircraft in nautical miles from the takeoff point, while the other counts the nautical miles during the flight until the aircraft lands at the specified location.
The center of the horizontal situation indicator contains a stationary image that represents the aircraft, and around this image, the instrumentation rotates. On the outside of the screen are glide deviation scales, indicated by hash marks, and slider pointers, which indicate distance above or below specific altitudes. Each hash mark indicates a specific number of degrees. Inside this dial is the 360-degree compass, which provides directional data with overlaid pointers indicating your selected heading and selected heading. In the center of the screen, below the aircraft image, are the course deviation indicators, represented by hash marks and a linear bar.
Working together, the instruments indicate the aircraft’s position in relation to takeoff and landing locations, along with the aircraft’s position according to the selected flight path. Horizontal situation indicators also often contain different mode switches to represent the aircraft during takeoff, flight, and landing. These aviation instruments allow pilots and ground stations to visualize flight paths and deviations without having to use mathematical calculations. Especially when an aircraft encounters severe weather conditions, the HSI provides pilots with precise control, guidance, and navigation parameters.
Along with bearings that indicate the distance between two destinations and the predetermined flight path and course deviations, horizontal situation indicators can also be manufactured with additional features. Some might indicate forward speed along with nautical miles and record flight time. Other HSI instruments may include aeronautical charts with Doppler radar information.
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