An HSI, or Horizontal Situation Indicator, is a combination of two familiar cockpit instruments: the directional gyro with a heading bug and a VOR/ILS indicator.
Combining the directional gyro and the NAV indicator into one instrument reduces pilot workload by providing heading, course reference, course deviation, and glideslope information all in one visual aid. In addition, an HSI:
Heading Select Knob: Rotating this knob sets the heading bug and also aligns a heading transformer for coupled autopilot use to the selected heading. Pushing this knob in cages the gyro.
Heading Flag: This red warning flag indicates loss of vacuum and/or electrical power to the gyro. Heading information is then unusable, but all course information (comparable to a standard VOR/ILS) remains valid.
Course Select Knob: Rotating this knob sets the course pointer to a selected course, and if so equipped, a course transformer for coupled autopilot use.
Course Pointer: This pointer indicates the selected course. Turning the course select knob will rotate the course pointer (VOR/LOC deviation indicator) and course reciprocal around the compass card. As the aircraft's heading changes, the course pointer rotates with the compass card to indicate the difference between the course (under the course pointer) and the actual aircraft heading (under the lubber line). The course selector may also be coupled to an autopilot or flight director. When coupled, "off course" signals are generated which direct the autopilot to maintain or acquire the selected course.
VOR/LOC Deviation Indicator: The center portion of the course pointer needle moves to indicate deviation from selected course. A series of dots provides a linear indication of how far the aircraft is off course. Each dot represents a percentage of the total course width. In VOR use, the course width is a total of 20 degrees, 10 degrees either side of center. When being used to fly the localizer, it shows runway center line and percentages of the total course width which is 2-1/4 either side of center. In either case, the actual linear distance off course depends upon the distance from the station. When used to display RNAV or LORAN, each dot will represent a percentage of the total course width in miles. An "on course" condition is indicated when the course pointer, the course deviation bar, and the course reciprocal are all in line.
To-From Indicator: This indicator is a white triangle and appears underneath the VOR/ILS deviation indicator. It shows whether the selected course will take the aircraft either to or from the VOR station.
Reference Aircraft: Representing the actual aircraft, this symbol is fixed and is located in line with the lubber line. Lubber Line: This orange line, located at the top of the display, indicates the aircraft's magnetic heading on the compass card. The lubber line is in line with the reference symbol to reinforce this association.
Compass Card: This card, located beneath the lubber line, indicates the aircraft's current heading. The card is mechanically coupled to the compass card set knob and, at the beginning of each flight, must be set by the pilot to agree with the magnetic compass heading or to the slaving meter, if installed. As the flight progresses and headings change, the directional gyro rotates the card to indicate the current heading. As with any standard unslaved DG, some gyro precession will occur. Therefore, it is necessary to check and reset the compass card at periodic intervals.
Slaving Meter: A slaving meter is located in the upper right-hand corner of the instrument face on slaved models. If equipped with a slaving feature, an HSI will automatically update its compass card with a magnetic sensor to correct for precession. In flight, the meter needle will oscillate slowly when the compass card is properly aligned with the magnetic compass.
Heading Bug: The selected heading is marked by an orange heading bug which can be moved to any point about the perimeter of the compass card. As the aircraft's heading changes, the bug rotates with the compass card, thus alerting the pilot to the difference between the selected heading (located under the bug) and the actual aircraft heading. The heading bug may also be coupled to an autopilot or flight director system. When coupled, "off heading" signals will be generated causing the autopilot to fly the aircraft so as to maintain the selected heading.
NAV Flag: This red warning flag indicates inadequate VOR or LOC signal or loss of power to meter circuits. Under these conditions, course information is unusable; however, all heading information remains valid.
Glideslope Deviation Indicator: This yellow wedge relates the vertical glide path centerline to the aircraft's position. The aircraft is "on glide slope" when the wedge covers the horizontal index. Each dot on this vertical scale represents approximately 0.4 degrees vertical deviation from the centerline. An up deviation is a fly up command while a down deviation means to fly down in order to place the aircraft "on glide slope."
Glideslope Mask: This mask will cover the glide slope deviation indicator in the absence of a usable signal or when a VOR frequency is selected.