THE COMPLETE NOVICESGUIDE TO THE HSI
© Hal Stoen
30 August, 2001
minor revision, 8/23/2005
PURP0SE OF THIS TUTORIAL
To help the reader understand this marvelous instrument, howit works, and how to use it. We'll first look at the instrumentas a whole, then take it apart, piece by piece to help make itmore understandable.
The horse and the cart,or the cart and the horse.
There's an old saying: "Don't put the horse before thecart." There are so many things in aviation and airplaneinstrumentation that are inter-related. If you find some of thediscussion below confusing when it comes to the "VOR modes",you may wish to refer to UnderstandingVOR's, VORTAC's and How To Use Them (Just click on the link.)
"HSI"...AND THAT STANDSFOR?
Horizontal Situation IndicatorThe all-in-one instrument on your panel. Let's take a look atall of the things that are displayed on this nifty little instrument.
Heading That yellowhash mark at the top, known as a "lubber line" is yourheading. In this case it is 000 (or 360, your call) degrees, dueNorth.
The compass roseThe compass rose is the 360 degree circle around the HSI. It isbroken down into 5 degree divisions. This is a marvelous orientationdevice. Given your current heading of 000 degrees, which way isEast (090 degrees)? Taking a look at the compass rose shows usthat a right turn would take us to that heading.
Heading bug Locatedon the compass rose is a movable heading bug, the orange trianglethat is pointing at 340 degrees. This bug is moved around therose by rotating the knob on the lower right. The heading bugcan be used several ways. When hand flying the aircraft turn thebug to your desired heading. This way you will have a constantvisual reminder. If Air Traffic Control gives you a new headingmove the heading bug to the new heading and you have your visualreminder.
When using the autopilot in the HEADING mode, the autopilotwill track on the heading selected with the heading bug.
Course selectorThe course selector is the yellow arrow that is pointing at 360degrees. (This is one of those rare ambiguities in aviation. Northcan be referred to as "000 degrees" or "360 degrees".You'll hear it referred to in both ways. And, in this case, justplain "N".) The course selector is moved around therose by rotating the knob on the lower left. Notice that thereis a "tail" to the course selector at the reciprocal180 degree mark. The course selector is used for navigation trackingin several modes: RNAV (aRea NAVigation, GPS (Global PositioningSatellite), VOR (Vhf Omni Range), and ILS (Instrument LandingSystem) among others.
When in the navigation mode the center "line" ofthe course selector, known as the CDI (Course Deviation Indicator)moves off of center to indicate the direction of the desired coursethat 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 fordeviations to the right.
The course selector also comes into play when flying an ILS backcourse. It allows the pilot to select the reciprocal of the backcourse and permits the CDI to give proper "left/right"indications. (I know that was confusing, and I'm sorry. It wastechnically correct and gives you a peek into the "dark worldof instrument flying"- a subject that is not covered in thistutorial. For more information on instrument flying, see the
Glide slope indicatorsThe yellow "bars" on each side (just above the centerwhite lines) are the glide slope indicators. On each side of theHSI are three small white lines. These represent glideslope deviations.When flying an ILS the "bars" will move up when youare below the glide slope and down when you are above the glideslope.
"TO" / "FROM"arrow The yellow arrow that is pointing "up".This is used in VOR navigation to indicate if the station is towardor away from the aircraft.
If you are not familiar with navigation, and how the VOR and ILSwork, it will be difficult for you to follow some of the following.If you wish to pause here, it might help to read the tutorialHow to navigate and thencome back when you are comfortable.
OK, LET'S TAKE THIS THING APARTAND LOOK AT THE VARIOUS COMPONENTS
We'll strip the HSI down to the bare basics, then progressivelybuild it back to the complete unit, explaining the function ofeach part as we go along.
THE DG (DIRECTIONAL GYRO
Here it is, a basic DG. Notice that there is no knob to setthe heading of the DG to match that of the compass. Why? Becauseit is slaved. Slaved? Back in the tailcone of the aircraft, awayfrom all ferrous metal, is a magnetic compass. The output of thiscompass is electrically "enhanced" and amplified througha "flux gate".
Sorry, there is no flux gate capacitor.
This signal is fed to the HSI to drive the DG portion of theunit. What this means is that the reading is very accurate, notsubject to the vagrancies of the regular magnetic compass, andis constantly being corrected for drift. On some aircraft, thissignal is also used to slave the ADF heading card.
Bottom line: A very accurate, stable, compass reading thatnever needs adjustment.
THE HEADING BUG
It would be nice if you had some visual heading reminder whenhand-flying. And, it would be nice if there was some way to tellthe 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, alongwith the knob that turns it. In the illustration, the HeadingBug is at 340 degrees. Let's say that you wanted to use 050 foryour heading. Turn the knob on the lower right to the right untilthe Bug is over 050 degrees.
Notice that outside of the fact that the Heading Bug is nowat 050 degrees, nothing else has changed. The compass rose hasn'tmoved, it's still at due North. It will not move until you turnthe airplane to a different heading. What I'm getting at here,is that you can turn the Heading Bug to whatever setting you wantto- it has no effect on the on the compass rose.
There are several ways to use this heading feature. It canbe used as a reminder for what heading you want to maintain whenhand-flying the aircraft. If you drift off of your desired heading,there is that bug right there on the HSI reminding you of whatheading you are supposed to be on. Or, say that you are navigatingto a VOR, and after reaching the VOR you will be tracking outboundon a new radial. You can set the Heading Bug to the new headingas a reminder of what you want to turn to after station passage.
Or if you are operating with the autopilot on, and have itset to the "Heading Mode", you could use the HeadingBug to steer the airplane to whatever heading you desired. A lotof pilots use this "feature" as a compromise when usingthe autopilot enroute, or on instrument approaches. By using theHeading Bug to steer the airplane when in the autopilot mode,the pilot is relieved of the tedious chore of hand-flying, butstill aware of the situation because he is using the Bug to correctfor wind drift and turns.
THE COURSE DEVIATION INDICATOR(CDI)
Let's remove the Heading Feature for a minute, and go backto the basic DG again.
Now, let's add the "navigational feature", the "CourseSelector".
Let's take a closer look at the "Course Selector".
I've taken the liberty of changing the color to red becauseyellow shows up so poorly in this format.
HEAD This is physicallyattached to the TAIL within the instrument housing. When theyrotate about the instrument face, one moves right along withthe other. The HEAD, CDI and the TAIL are collectively calledthe COURSE SELECTOR. They are moved around the face of the instrumentby turning the knob on the lower left face of the HSI.
"TO" / "FROM"INDICATOR Used in VOR navigation, it indicatesif the station is ahead of, or behind the aircraft. It is retractedand not displayed when in the ILS mode.
CDI Course DeviationIndicator. This moves "left or right", or "up anddown" depending if you are on the selected course.
Huh? Yeah, that was a rough one. And, not coincidentally, thiscan be the most difficult part of the HSI to understand. Let'sset up a situation and see how the CDI presentation changes aswe 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, thatis the 180 degree reciprocal of the 120 degree radial shown onthe chart above. Notice that the chart is divided into "TOCountry", and "FROM Country". That is because onceyou select a course, one half of the VOR's area will display "TO",and one half "FROM". In this case, the dividing lineis the 030 degree radial Northeast of the VOR, and the 210 degreeradial Southwest of the VOR. Note that is 90 degrees on eitherside of the selected 120 degree radial, and that totals 180 degrees,one half of a 360 degree circle. FOR THESE EXAMPLES THERE WILLBE NO WIND.
Also, bear in mind that the HSI does not care in which directionthe airplane is pointed. If it helps, think of the HSI being mountedin a flying saucer, with no front or rear. Headingdoes not matter to the CDI display. Only the aircraft's positionrelative to the VOR, and the radial selected by the Course Selectormatter.
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 righton the 120 degree radial. The Course Selector is set to 300 degrees,and the "TO/FROM" flag shows "TO". If youcontinue on the 000 degree heading, the CDI will slowly startmoving to the left, indicating that your selected course is inthat direction. If you turned to a heading of 300 degrees, theneedle 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 300degree 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 setthere only for the purpose of this tutorial. It could just aswell be to the 120 degrees. From a practical standpoint, if youwere going to fly to the VOR, you would set the Course Selectorto 300 degrees. If you were going to fly away from the VOR, youwould use 120 degrees. From an orientation standpoint, you areon the 120 degree radial.
What would happen if you were on the 120 degree radial, trackingit toward the VOR on a heading of 300 degrees, with the CDI centered,and you started rotating the Course Selector knob? Just startedrotating it around. Well, the CDI would slide from one side tothe other as you went past the various radials, and the "TO/FROM"flag would flip over each time you dialed past the 210 and 030radials that separate the "TO/FROM" sectors. If youstopped 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 CDIdoesn't give a hoot, the only thing that it does care about iswhere the selected 120 degree radial is. And, it is showing youthat IF you were on a heading of 300 degrees, itwould be to your left. You have no way of knowing how far to theleft it is though.
Position "C"
The CDI shows that your selected radial, 120 degrees, is offto your right. How far? You don't know. At your current heading,320 degrees, you may intercept the radial before you crossthe VOR, but then again, you may not. In this position, you areroughly paralleling the radial. If true, and you continue on thisheading, the "TO/FROM" flag will slowly flip over from"TO" to "FROM" as you cross over the 210 degreeradial.
Position "D"
You have to pay close attention here. It may appear that the"TO/FROM" indicator is showing "TO", but thetop of the card is where the head (the arrow) is. If you lookat it in that respect, the indication is correctly shown as "FROM".Remember that the CDI does not care what the heading of the airplaneis. So, in this situation, if the airplane was on a headingof 300 degrees, your selected radial would be to your right, andthe VOR would be behind you.
Position "E"
You are on the 300 degree radial. The VOR is behind you. Ifyou continue on this heading (no wind), the CDI will slowly driftover to the right side, indicating that you are moving away fromyour selected radial.
The glideslope indicators do not function in the VOR mode.
HOW THE HSI WORKS ON AN ILS
Let's bring all of the parts together for a complete HSI andsee how it looks on an ILS approach. The Heading Bug is left outfor greater clarity, however it normally would be set to the headingused 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"
Note:The heading bug has been removed for clarity in these examples.
Our ace is high, and to the left. His display shows that theGlideslope is below him (fly down), and the Localizer is to theright (fly right).
Position "B"
The aircraft is above the glideslope and to the right of thelocalizer. The HSI shows that to get back on the approach he mustfly 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 upand 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.
THE HSI's ULTIMATE FEAT, MAKINGTHE BACK COURSE APPROACH A PIECE OF CAKE
For more information on flying the Back-Course approach, seethe tutorial Howto fly the Back Course approach.
One of the bugaboos of flying instrument approaches is theBack Course approach, where left is right and right is left. Anold flying joke goes "You're flying a Back Course approach.All is going well until just at the end of the approach your aircraftis caught up in a downburst and slammed into the approach endof the runway. This tears off the landing gear, and the aircraftflips over inverted so that you are now sliding down the runwayupside down and backwards. You notice that the CDI is driftingto 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 isthe normal situation. Approach plates will have the notation "IGNOREGLIDESLOPE SIGNALS."
You have the navigation receiver set for the proper frequency,and the Course Selector set at 090 degrees. For the followingexamples your heading will remain at 090 degrees as we fly acrossthe runway at a constant altitude. We're curious as to how theCDI display on the HSI shows the localizer on the Front Courseand the Back Course. Don't worry about any glideslope readingson 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 descendingyou have no way of knowing where you are on the approach withoutthe 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 BackCourse approach. How do you set the HSI for that?
First off, lets do the "natural thing" and set theCourse Selector to the heading for the Back Course approach, whichin this case would be 270 degrees.
With the Course Selector set at 270 degrees, here is how theHSI would look on the Back Course side of the runway.
Position 4 
Position5 
Position 6 
Well, that's no good. With this setup, if you are right ofcourse, the CDI shows the wrong fly command, the same if you areleft 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 Selectorto the Front Course, 090 degrees.
Position 4 
Position5 
Position6 
Now, when you are right of course (Position 5), the fly commandis correct- the course is off to your left. And, if you are leftof course (Position 6), the CDI correctly shows the localizeris to your right.
And that is how you set up an HSI to fly a Back Course theeasy way. Quite frankly, if the instrument did nothing else, thisfeature alone would be worth the investment.
SUMMATION
Hopefully, this tutorial has helped to make the HSI easierfor 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 orientationmap was in error. Thanks to Bernard Piret for pointing this out.
8/23/2005: Notation added about referring to VOR understandingat the top of the page. Thanks to Ivan Stankovic for making thissuggestion.
This tutorial is available on aCD
This tutorial, along with additional content, is availableon a CD. Click here formore information.
© Hal Stoen
30 August, 2001
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