Archive for category VOR

Oh Those Wonderful Rules of Thumb

There are many rules of thumb in the aviation industry.  Things that we take for granted that our flight instructor told us to use but never explained why those rules of thumb really work.  Why do we descend at 450 ft/min when on a glideslope in small airplanes?  Where did that come from.  How about this rule of thumb: For every dot deflection on the VOR it is equal to 200 FT per NM.

I want to explore these rules of thumb and if there is one you would like me to research please leave a comment below.

2 degree pie sliceFor every dot deflection of the VOR CDI needle we are 200 FT / NM off course.  On the right is a circle representing the VOR signal emanating from the station.  We have a 2° slice from the circle representing one dot deflection and we want to know how long the yellow segment is.

Notice that this is not a right triangle so we cannot use standard right triangle formulas of Sin(Angle) = Opposite / Hypotenuse.  Time to pull out the trigonometry.  We know the lengths of the white segment and they are equal: 1NM or 6076.1 feet and we know the angle of 2° for one dot deflection.  The Side-Angle-Side formula will get us our result.

VOR 2 degree Deflection

As it turns out, every dot deflection of the CDI on the VOR is exactly 212.09 FT if you were 2° or one dot deflection on the VOR CDI.  This isn’t the whole story.  If we were 180° from our intended course we would be at least 2NM off course.  The deflection must change depending on the number of degrees we are off course.

VOR Deflection Graph 1

It turns out that 212 FT works really well if you are 2° off your course.  If you were 90° off your course then the 212 feet per dot per NM miles turns into 190.95 feet per dot per NM.  Does this really matter?  Let’s explore the difference we get from our rule of thumb versus the extremes of 2° and 90° off course.  The next graph shows the dot deflection using 190 FT, 200 FT and 212 FT for each NM from the station if we are 5 NM off course.

VOR Deflection Graph 2

We can see that with such a distance off course and as we get closer to the station the lines diverge.  We may only be 7 dots deflection off while we calculate 7.5 dots deflection.  That is not that big of a deal.  As pilots we like things really easy and 200 feet is much easier to calculate than 190 or 212 feet.  In the end, that average is good enough to keep us where we want to go.


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Interpreting HSI Instrument Rating Test Questions

Since I have discussed a new method for tracking and intercepting a victor airway I thought it might be interesting to go through the instrument rating knowledge test and answer some of the questions using this technique.  Perhaps, this method will make the test questions easier than using the recommended procedure in the Instrument Flying Handbook.

Below I have presented both the aircraft position and the HSI.  You can enlarge the pictures by clicking on them.

IR HSI Pictures on VORIR Aircraft Position

The steps to answer these questions:

  1. The course deviation indicator (CDI) will shade the headings needed to fly in order to intercept the selected course.  If we need to fly east to intercept the course, we must currently be west of the course.
  2. The TO/FROM flag  will tell you that if you fly those shaded headings in the quadrature, you will either fly towards the station or farther away.
  3. The heading on top of the HSI is the current aircraft heading.

Let’s delve in and go through each of the HSI displays and then we will discuss the aircraft that don’t have an associated HSI display.  For learning purposes, try to answer which aircraft belongs to each HSI before continuing. 

  1. The first HSI show that a southerly heading will cause us to intercept the 090° or 270° radial.  The TO/FROM flag indicates that a southeasterly heading will intercept the course and proceed closer to the station.  Last we are currently heading on a 200°  If a southeasterly heading will intercept and track closer to the station then the airplane must be opposite or in the northwest quadrant.  The only airplane that is flying a southwesterly heading in the NW quadrant is aircraft 1.
  2. The second HSI shows a northerly heading will intercept the 090° or 270° radial.  The TO/FROM flag has a easterly heading to track towards the station.  Thus a northeasterly heading will intercept and track towards the station.  On a heading of 135° in the SW quadrant indicates airplane 19.
  3. The third HSI shows a NW heading will intercept the 360° or 180° and track to the station.  The aircraft is in the SE quadrant and it’s current heading of 315° indicates airplane 12.
  4. The fourth HSI shows us that a northeasterly heading will intercept the 360° or 180° radial and track towards the station.  The current heading of south shows the proper aircraft to be number 17.
  5. The fifth HSI shows a southwesterly heading will track the 360° or 180° radial and track towards the station.  On the airplane’s current heading of 360° the only aircraft position is number 6
  6. The last HSI shows us centered on the 360° or 180° radial with a northerly heading to track towards the station.  It current northeasterly heading indicates that aircraft 16 is the correct answer

How did you do?  I think following the three step method really works.  What about the other airplanes that weren’t the answer to the HSI quesitons?

  1. Aircraft 1 to 4 in the northwest quadrant.  What would the HSI look like if the OBS was set to 180° radial?  The 270° radial?  For each aircraft imagine rotating the HSI so the current heading is always on top.HSI NW Quadrant
  2. Aircraft 6 to 10 in the NE quadrant.  Same question and imagine rotating the HSI’s so the aircraft’s current heading is on top.HSI NE Quadrant
  3. Aircraft 11 thru 14 are in the SE quadrant.  Same question as before.HSI SE Quadrant
  4. Aircraft 17 thru 20 are in the SQ quadrant.  Same question as before.HSI SW Quadrant

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Travelling along Victor Airways

Loss of GPS NavigationImagine flying along on an IFR flight plan in IMC conditions and all of a sudden your trusty GPS displays a message stating it is unusable.  What do you do?  Well, the very first thing to do is to stay calm and to let ATC know per §61.187.  The second thing is to follow ATC instructions.  They could give you radar vectors to your destination, an alternate, or a new clearance along victor airways.  You do remember how to fly those right?


KTUL LoEnroute 1The clearance you receive is “N123AB fly heading 320 intercept the 179 radial inbound to Tulsa (TUL) V506 SGF Direct LANEZ Direct KLBO.”  You think no problem and proceed to turn to the new heading and dial in the VOR indicator.  Tune, Identify and Twist in the radial.  You place 359 on top of the VOR needle and do you expect to see the needle deflected left or right?

How can we figure this out really quickly?  If we take a look at the VOR needle and place our finger at the top where 359 is and move LEFT to 340 we know the needle should be displaced left if ATC gave us the correct instructions.  You will realized if you had read my previous posts on the VOR that the headings to intercept the radial are always shaded by the VOR needle.

VOR Indicator359

If the needle is deflected left, where are you in relation to the airway?  Are you east or west?  To answer this question pick the exact opposite of the shaded headings and notice that you are S and E of the VOR or by Haskell airport.  You have intercepted the radial and are tracking inbound, on the second VOR you put the 036 radial to fly outbound on V506.  What indication do you expect to see on the VOR needle?  Left or Right and TO or FROM?  Let our finger do the walking again.  The current course of 359 is LEFT of 036 so the needle should be left with a TO indication because we have not passed the station yet.

KTUL LoEnroute 2Outbound from TUL on V506 coming up to VINTA intersection we need to turn right to intercept the 248 radial from EOS VOR.  What are the needle indications?

Again, we will let the fingers do the walking and after setting in the reciprocal of 248 which is 068 we will see that our current heading / course is LEFT of 068 on the VOR indicator.  The needle should be deflected LEFT once again with a TO indication.

KTUL LoEnroute 3After we passed the EOS VOR and on our way (067 heading) we are presented again with another course turn at an BILIE intersection.  What are the VOR indications after dialing in the reciprocal 026 course?  If you said a RIGHT deflection with a TO indication I would be proud of you.

Leaving SGF on a 054 heading to LANEZ, ATC calls up and clears you for the SDF 36 into KLBO.  You dial in the SDF course of 359 and decide to figure out the needles make sense.  What should they be?

KTUL LoEnroute 4

If you let your fingers do the walking again from 082 heading to 359, the needle should be deflected LEFT. 

Are you starting to get the picture yet?  If you take your finger and place it on your course and move the shortest direction to your current heading whatever direction you went, the needle will be deflected that way.  Typically, if the heading value is less than the course value, the needle will be deflected left and if the heading value is greater than the course value, the needle will be deflected to the right.  The only issue with that statement is when your finger passes through North.  Hopefully, now you can trust but verify the information ATC gives you because one day it just may save your life.   Not yet a believer, take the course I laid out here and fly it in a VOR simulator to verify this method.

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