A horizontal situation indicator, or HSI for short, combines multiple flight instruments that provide a visual display of the aircraft’s position in relation to various navigation points. Among the instruments generally contained within the HSI are a directional gyroscope, a heading bug, and a very high frequency omnidirectional range/instrument landing system (VOR/ILS). These aircraft instruments, widely used in avionics, are generally employed during take-off, ascension, flight, and landing. Fixed wing aircraft, helicopters and the space shuttle all use variations of horizontal situation indicator instrumentation for manual or automatic piloted flight.
In addition to providing pilots with information concerning the aircraft, through radio navigation, the horizontal situation indicator transmits the aircraft’s location to selected ground navigational facilities. After programming the course and the heading indicators, the equipment relays the bearings to two ground stations. The display might also include two four-digit counters. One monitors the aircraft in nautical miles from the point of take-off while the other counts down the nautical miles during flight until the craft lands at the specified location.
The center of the horizontal situation indicator contains a stationary image representing the aircraft, and around this image, the instrumentation rotates. On the outer part of the display are the glidescope deviation scales, denoted by hash marks, and the glidescope pointers, which indicate the distance above or below specific altitudes. Each hash mark indicates a specified number of degrees. Located on the inside of this dial is the 360 degree compass, providing directional data with overlaid pointers indicating the selected heading and the selected course. In the very middle of the display, under the airplane image, lie the course deviation indicators, depicted by hash marks and a linear bar.
Working together, the instruments indicate the position of the aircraft in relation to take-off and landing locations, along with the craft’s position according to the selected flight path. Horizontal situation indicators also usually contain different mode switches that represent the aircraft during take-off, 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 adverse weather conditions, the HSI provides pilots with precise control, guidance, and navigational parameters.
Along with bearings indicating the distance between two destinations and the predetermined flight path and course deviations, horizontal situation indicators might also be manufactured with additional features. Some might indicate ground speed along with nautical miles and clock the flight time. Other HSI instruments might include aeronautical maps with Doppler radar information.