© Hal Stoen

30 August, 2001

minor revision, 8/23/2005


To help the reader understand this marvelous instrument, how it works, and how to use it. We'll first look at the instrument as a whole, then take it apart, piece by piece to help make it more understandable.

The horse and the cart, or the cart and the horse.

There's an old saying: "Don't put the horse before the cart." There are so many things in aviation and airplane instrumentation that are inter-related. If you find some of the discussion below confusing when it comes to the "VOR modes", you may wish to refer to Understanding VOR's, VORTAC's and How To Use Them (Just click on the link.)


Horizontal Situation Indicator The all-in-one instrument on your panel. Let's take a look at all of the things that are displayed on this nifty little instrument.

Heading That yellow hash mark at the top, known as a "lubber line" is your heading. In this case it is 000 (or 360, your call) degrees, due North.

The compass rose The compass rose is the 360 degree circle around the HSI. It is broken down into 5 degree divisions. This is a marvelous orientation device. Given your current heading of 000 degrees, which way is East (090 degrees)? Taking a look at the compass rose shows us that a right turn would take us to that heading.

Heading bug Located on the compass rose is a movable heading bug, the orange triangle that is pointing at 340 degrees. This bug is moved around the rose by rotating the knob on the lower right. The heading bug can be used several ways. When hand flying the aircraft turn the bug to your desired heading. This way you will have a constant visual reminder. If Air Traffic Control gives you a new heading move the heading bug to the new heading and you have your visual reminder.

When using the autopilot in the HEADING mode, the autopilot will track on the heading selected with the heading bug.

Course selector The course selector is the yellow arrow that is pointing at 360 degrees. (This is one of those rare ambiguities in aviation. North can be referred to as "000 degrees" or "360 degrees". You'll hear it referred to in both ways. And, in this case, just plain "N".) The course selector is moved around the rose by rotating the knob on the lower left. Notice that there is a "tail" to the course selector at the reciprocal 180 degree mark. The course selector is used for navigation tracking in several modes: RNAV (aRea NAVigation, GPS (Global Positioning Satellite), VOR (Vhf Omni Range), and ILS (Instrument Landing System) among others.

When in the navigation mode the center "line" of the course selector, known as the CDI (Course Deviation Indicator) moves off of center to indicate the direction of the desired course that has been selected by the course selector. To return to course, steer toward the "line". If it is displaced to the left, fly left until you are back on course. The opposite applies for deviations to the right.

The course selector also comes into play when flying an ILS back course. It allows the pilot to select the reciprocal of the back course and permits the CDI to give proper "left/right" indications. (I know that was confusing, and I'm sorry. It was technically correct and gives you a peek into the "dark world of instrument flying"- a subject that is not covered in this tutorial. For more information on instrument flying, see the instrument flying tutorial.

Glide slope indicators The yellow "bars" on each side (just above the center white lines) are the glide slope indicators. On each side of the HSI are three small white lines. These represent glideslope deviations. When flying an ILS the "bars" will move up when you are below the glide slope and down when you are above the glide slope.

"TO" / "FROM" arrow The yellow arrow that is pointing "up". This is used in VOR navigation to indicate if the station is toward or away from the aircraft.

If you are not familiar with navigation, and how the VOR and ILS work, it will be difficult for you to follow some of the following. If you wish to pause here, it might help to read the tutorial How to navigate and then come back when you are comfortable.


We'll strip the HSI down to the bare basics, then progressively build it back to the complete unit, explaining the function of each part as we go along.


Here it is, a basic DG. Notice that there is no knob to set the heading of the DG to match that of the compass. Why? Because it is slaved. Slaved? Back in the tailcone of the aircraft, away from all ferrous metal, is a magnetic compass. The output of this compass is electrically "enhanced" and amplified through a "flux gate".

Sorry, there is no flux gate capacitor.

This signal is fed to the HSI to drive the DG portion of the unit. What this means is that the reading is very accurate, not subject to the vagrancies of the regular magnetic compass, and is constantly being corrected for drift. On some aircraft, this signal is also used to slave the ADF heading card.

Bottom line: A very accurate, stable, compass reading that never needs adjustment.


It would be nice if you had some visual heading reminder when hand-flying. And, it would be nice if there was some way to tell the autopilot which way it should fly when using the autopilot. This can be accomplished by adding the heading mode to the unit.

We've added the "Heading Bug" to the display, along with the knob that turns it. In the illustration, the Heading Bug is at 340 degrees. Let's say that you wanted to use 050 for your heading. Turn the knob on the lower right to the right until the Bug is over 050 degrees.

Notice that outside of the fact that the Heading Bug is now at 050 degrees, nothing else has changed. The compass rose hasn't moved, it's still at due North. It will not move until you turn the airplane to a different heading. What I'm getting at here, is that you can turn the Heading Bug to whatever setting you want to- it has no effect on the on the compass rose.

There are several ways to use this heading feature. It can be used as a reminder for what heading you want to maintain when hand-flying the aircraft. If you drift off of your desired heading, there is that bug right there on the HSI reminding you of what heading you are supposed to be on. Or, say that you are navigating to a VOR, and after reaching the VOR you will be tracking outbound on a new radial. You can set the Heading Bug to the new heading as a reminder of what you want to turn to after station passage.

Or if you are operating with the autopilot on, and have it set to the "Heading Mode", you could use the Heading Bug to steer the airplane to whatever heading you desired. A lot of pilots use this "feature" as a compromise when using the autopilot enroute, or on instrument approaches. By using the Heading Bug to steer the airplane when in the autopilot mode, the pilot is relieved of the tedious chore of hand-flying, but still aware of the situation because he is using the Bug to correct for wind drift and turns.


Let's remove the Heading Feature for a minute, and go back to the basic DG again.

Now, let's add the "navigational feature", the "Course Selector".

Let's take a closer look at the "Course Selector".

I've taken the liberty of changing the color to red because yellow shows up so poorly in this format.

HEAD This is physically attached to the TAIL within the instrument housing. When they rotate about the instrument face, one moves right along with the other. The HEAD, CDI and the TAIL are collectively called the COURSE SELECTOR. They are moved around the face of the instrument by turning the knob on the lower left face of the HSI.

"TO" / "FROM" INDICATOR Used in VOR navigation, it indicates if the station is ahead of, or behind the aircraft. It is retracted and not displayed when in the ILS mode.

CDI Course Deviation Indicator. This moves "left or right", or "up and down" depending if you are on the selected course.

Huh? Yeah, that was a rough one. And, not coincidentally, this can be the most difficult part of the HSI to understand. Let's set up a situation and see how the CDI presentation changes as we move around.

We're going to see how the HSI display will look at each position. In all cases, the Course Selector will be at 300 degrees, that is the 180 degree reciprocal of the 120 degree radial shown on the chart above. Notice that the chart is divided into "TO Country", and "FROM Country". That is because once you select a course, one half of the VOR's area will display "TO", and one half "FROM". In this case, the dividing line is the 030 degree radial Northeast of the VOR, and the 210 degree radial Southwest of the VOR. Note that is 90 degrees on either side of the selected 120 degree radial, and that totals 180 degrees, one half of a 360 degree circle. FOR THESE EXAMPLES THERE WILL BE NO WIND.

Also, bear in mind that the HSI does not care in which direction the airplane is pointed. If it helps, think of the HSI being mounted in a flying saucer, with no front or rear. Heading does not matter to the CDI display. Only the aircraft's position relative to the VOR, and the radial selected by the Course Selector matter.

Position "A"

Note: The heading bug has been removed for clarity in these examples.

The aircraft's heading is due North, 000 degrees, and is right on the 120 degree radial. The Course Selector is set to 300 degrees, and the "TO/FROM" flag shows "TO". If you continue on the 000 degree heading, the CDI will slowly start moving to the left, indicating that your selected course is in that direction. If you turned to a heading of 300 degrees, the needle would stay in the center (no wind, remember), and the "TO/FROM" flag would show "TO" until you crossed over the VOR. At that time, if you touched nothing, and remained on the 300 degree heading, the "TO/FROM" flag would flip over to "FROM". Nothing else would change.

The question may arise "If I'm on the 120 degree radial, why is the Course Selector set to 300 degrees?" It is set there only for the purpose of this tutorial. It could just as well be to the 120 degrees. From a practical standpoint, if you were going to fly to the VOR, you would set the Course Selector to 300 degrees. If you were going to fly away from the VOR, you would use 120 degrees. From an orientation standpoint, you are on the 120 degree radial.

What would happen if you were on the 120 degree radial, tracking it toward the VOR on a heading of 300 degrees, with the CDI centered, and you started rotating the Course Selector knob? Just started rotating it around. Well, the CDI would slide from one side to the other as you went past the various radials, and the "TO/FROM" flag would flip over each time you dialed past the 210 and 030 radials that separate the "TO/FROM" sectors. If you stopped right at 120 degrees, the CDI would be centered, and the "TO/FROM" flag would indicate "FROM".

Position "B"

The aircraft is flying on a heading of 050 degrees. The CDI doesn't give a hoot, the only thing that it does care about is where the selected 120 degree radial is. And, it is showing you that IF you were on a heading of 300 degrees, it would be to your left. You have no way of knowing how far to the left it is though.

Position "C"

The CDI shows that your selected radial, 120 degrees, is off to your right. How far? You don't know. At your current heading, 320 degrees, you may intercept the radial before you cross the VOR, but then again, you may not. In this position, you are roughly paralleling the radial. If true, and you continue on this heading, the "TO/FROM" flag will slowly flip over from "TO" to "FROM" as you cross over the 210 degree radial.

Position "D"

You have to pay close attention here. It may appear that the "TO/FROM" indicator is showing "TO", but the top of the card is where the head (the arrow) is. If you look at it in that respect, the indication is correctly shown as "FROM". Remember that the CDI does not care what the heading of the airplane is. So, in this situation, if the airplane was on a heading of 300 degrees, your selected radial would be to your right, and the VOR would be behind you.

Position "E"

You are on the 300 degree radial. The VOR is behind you. If you continue on this heading (no wind), the CDI will slowly drift over to the right side, indicating that you are moving away from your selected radial.

The glideslope indicators do not function in the VOR mode.


Let's bring all of the parts together for a complete HSI and see how it looks on an ILS approach. The Heading Bug is left out for greater clarity, however it normally would be set to the heading used for the Missed Approach Procedure.

Here's a view of an ILS. Let's call it the ILS for runway 9. You would set your navigation receiver to the correct frequency, and set your Course Selector to 090 degrees.

Position "A"


Our ace is high, and to the left. His display shows that the Glideslope is below him (fly down), and the Localizer is to the right (fly right).

Position "B"

The aircraft is above the glideslope and to the right of the localizer. The HSI shows that to get back on the approach he must fly left and down.

Position "C"

Below, and to the right of the desired approach course. And, the HSI dutifully displays that our intrepid pilot must fly up and to the left to get back on "course".

Position "D"

This one is you. On glideslope, on the center of the localizer. Nice flying.

Position "E"

Below glideslope, to the left of the localizer.


For more information on flying the Back-Course approach, see the tutorial How to fly the Back Course approach.

One of the bugaboos of flying instrument approaches is the Back Course approach, where left is right and right is left. An old flying joke goes "You're flying a Back Course approach. All is going well until just at the end of the approach your aircraft is caught up in a downburst and slammed into the approach end of the runway. This tears off the landing gear, and the aircraft flips over inverted so that you are now sliding down the runway upside down and backwards. You notice that the CDI is drifting to the left. Which rudder do you push to correct?"

OK, here's an ILS with a Back Course approach. For this example, there will be no usable glideslope on the Back Course, which is the normal situation. Approach plates will have the notation "IGNORE GLIDESLOPE SIGNALS."

You have the navigation receiver set for the proper frequency, and the Course Selector set at 090 degrees. For the following examples your heading will remain at 090 degrees as we fly across the runway at a constant altitude. We're curious as to how the CDI display on the HSI shows the localizer on the Front Course and the Back Course. Don't worry about any glideslope readings on either course.

First off, let's fly from Position "2" to Position "5"

Position "2 "Position "5" Nothing changed.

In fact, on an ILS approach, except for the glideslope descending you have no way of knowing where you are on the approach without the Marker Beacons, a LOM, or DME.

Let's fly from Position "1" to Position "4"

Position "1" Position "4" Again, nothing changed

How about from Position "3" to Position "6"?

Position "3" Position "6" Do you see a trend developing here?

OK, so what's up with the Back Course approach. How do you set the HSI for that?

First off, lets do the "natural thing" and set the Course Selector to the heading for the Back Course approach, which in this case would be 270 degrees.

With the Course Selector set at 270 degrees, here is how the HSI would look on the Back Course side of the runway.

Position 4 Position 5 Position 6

Well, that's no good. With this setup, if you are right of course, the CDI shows the wrong fly command, the same if you are left of course. Ah, but here is the final goodie from the HSI- making the Back Course display make sense.

Now we'll reset the Heading Selector to the Front Course, 090 degrees.

Position 4 Position 5 Position 6

Now, when you are right of course (Position 5), the fly command is correct- the course is off to your left. And, if you are left of course (Position 6), the CDI correctly shows the localizer is to your right.

And that is how you set up an HSI to fly a Back Course the easy way. Quite frankly, if the instrument did nothing else, this feature alone would be worth the investment.


Hopefully, this tutorial has helped to make the HSI easier for you to understand. If you have any comments, corrections, or something was just not explained enough for your satisfaction, please email me.

5/8/2003: Visual on "position 5" on the orientation map was in error. Thanks to Bernard Piret for pointing this out.

8/23/2005: Notation added about referring to VOR understanding at the top of the page. Thanks to Ivan Stankovic for making this suggestion.


For CD information click here. For eBook information click here.

© Hal Stoen

30 August, 2001

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