Horizontal situation indicator
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.
The HSI is typically mounted below the artificial horizon, where it takes the place of a conventional heading indicator. On the HSI, the aircraft is represented by a schematic figure in the center of the instrument. The VOR-ILS (Very High Frequency Omi-Directional Range Instrument Landing System) system is shown in relation to this figure. In most configurations, the heading indicator is linked to a remote compass and the HSI is frequently interconnected with an autopilot. While an HSI is a highly-intricate tool, it is relatively easy to understand when you break it down to its base parts. This blog will explain what an HSI is, what it does, and the main components of one.
Simply put, an HSI is a combination of two very common cockpit instruments: the directional gyro with a heading bug and the VOR-ILS indicator. The combination of the directional gyro with the NAV indicator greatly reduces the pilot’s workload by providing them with heading, course reference, course deviation, and glide slope information in a convenient, all-in-one visual aid. Additionally, the HSI also makes it easier to understand the aircraft’s position relative to the desired course or holding pattern. The split needle configuration resulting from the course and reciprocal pointers and the VOR/LOC deviation indicators clearly indicated both selected course and course deviation.
The HSI also provides standard sensing and course deviation indication during back-course ILS approaches. It can do this as long as the front course heading is set under the head of the course pointer and the aircraft is flying in the direction of the course deviation indicator. HSIs feature 45 degree tic marks to provide visual representation of procedure turns and reciprocals, allowing a pilot to fly without having to memorize outbound & inbound headings or add & subtract 45 degrees for intercepts or offsets. The HSI also provides a heading bug for autopilot coupling or, in aircraft not equipped with autopilot, as a heading reminder.
While all parts of an HSI have important roles, the most critical are the compass card, warning flags, course deviation bar, and the to/from indicator. The compass card, driven by the internal gyro, shows the magnetic heading of the aircraft and can automatically correct them in certain setups. The warning flags serve a number of purposes, ranging from indicating that the HSI course deviation indicator is not functioning properly, to alerting the pilot that the speed of the directional gyro is too low. The course deviation bar, as its name suggests, indicates when the aircraft is off a selected course, and how far off it is. The to/from indicator is used to show whether the selected course will lead the aircraft to or from the VOR station.
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