horizontal situation indicator (HSI)


The HSI provides a basic horizontal view of the aircraft's navigation picture. In the F-15E, it can provide navigation data to selected ground navigation facilities (TACAN or Instrument Landing System (ILS)) or to onboard navigation systems like the Inertial Navigation System (INS).

Instrument flying without an HSI requires a fair amount of mental gymnastics; for this reason, the HSI is becoming more prevalent in general aviation since it provides an excellent picture for precise navigation.

The HSI may display bearing and distance to a navigation aid, magnetic heading, track/ track deviation and course/course deviation.

Keyboard commands:

  • Left and Right arrows to make turns.-
  • Up and Down arrows to move the heading indicator
  • PGUP and PGDWN keys to move the instrument cursor

WHAT'S AN HSI?

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. 

WHAT DOES AN HSI DO FOR THE PILOT?

Combining the directional gyro and the NAV indicator into one instrument reduces pilot workload by providing heading, course reference, course deviation and glide slope information - all in one visual aid. In addition, an HSI makes it easier to visualize the aircraft's position with reference to the selected course or holding patterns. The "split needle" presentation made up of the course and reciprocal pointers and the VOR/LOC deviation indicators, clearly shows both selected course and course deviation.

It also gives standard sensing and course deviation indication on back course ILS approaches provided the front course heading is set under the head of the course pointer and you fly toward the course deviation indicator. Provides convenient 45 tic marks to help visualize procedure turns and reciprocals so that pilots need not memorize outbound/inbound headings or add/subtract 45 for intercepts or offsets. The HSI provides a heading bug for autopilot coupling or as a heading reminder in aircraft not equipped with autopilots.

Heading Select Knob

Rotating this knob sets the heading bug and will also align a heading transformer for coupled autopilot use, to the selected heading. Pulling this knob out will cage the gyro.

Heading Flag

This red warning flag indicates loss of 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 will rotate 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 will be generated which direct the autopilot to maintain or acquire the selected course.

VOR/LOC Deviation Indicator

The centre 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." In VOR use, each dot represents 5 degrees; when being used to fly the localizer, it shows 1 1/4 degrees per dot; for RNAV "APPR" mode, 0.625 nm per dot; and for RNAV "Enroute" mode it indicates 1.25 nm per dot. 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 start of each flight, must be set by the pilot to agree with the magnetic compass heading. 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.

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.

Glide Slope Deviation Indicator

This yellow wedge relates the vertical glide path centreline 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 four-tenths degrees vertical deviation from the centreline.

Glide Slope Mask

This mask will cover the glide slope deviation indicator in the absence of a usable signal or when a VOR frequency is selected.

RNAV

The legend "RNAV" will appear in the lower right corner of the instrument face when the HSI is part of an area navigation system and that mode is engaged.

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 the magnetic compass to correct for precession. The meter needle will oscillate slowly when the compass card is properly aligned with the magnetic compass.

REMEMBER:

* To function properly, an HSI's compass card must be properly set, and selected course and heading information applied.

* Because the HSI is a gyroscopic device, it must be set to the magnetic heading and checked periodically. Even in a slaved system, the action of the slaving meter and compass card correspondence with the magnetic compass should be checked periodically.

* For back course approaches, set in the ILS front course heading and then fly "the picture" using normal response to needle movements (i.e., needle left, correct left; needle right, correct right).

* Don't confuse a radial (which radiates outward from a VOR) with an inbound course (i.e., the reciprocal).

* HSIs are equipped with "TO-FROM" NAV function, heading and glide slope warning flags. Improper performance of each function is indicated by its respective warning flag.