Navaids and magnetic variation

[AKar]

In a recent topic there was some discussion regarding FSX/P3D navdata and magnetic variation updates. In a couple of related discussions (not on the internet), it occurred to me that the concepts of magnetic and true headings and how they relate to the traditional navaids is not entirely clear subject. Just if anyone finds it hard to gasp, I post this thought-play here too, if it helps. So, let us imagine the following setup.


1. Setup

Let there be an airport with runway 36. The runway is perfectly aligned to true north, and local magnetic variation is zero, so that both magnetic and true heading of the runway is 360°. The runway is equipped with ILS, and the approach course is also 360°. Out there straight out from the departure end, precisely on extended runway center line lies an NDB beacon, and closely co-located VOR station.

Our airplane is sitting at the beginning of the runway 36, pointed down the strip, precisely on heading 360°, both magnetic and true obviously. Our equipment consists of two navigation radios, one tuned on the VOR's frequency and the other one to the localizer's frequency. Our ADF is tuned to NDB frequency, and we've got an RMI that is slaved on our HSI. Pointer one is selected to point to the VOR station and pointer two is selected to point at the NDB beacon.


2. Initial indications

As our airplane sits on the runway, what would the navigation instruments show? Let us assume, that the HSI is slaved to magnetic, and OBS/course selected to 360°. Of course, indications would be:
• indicated heading of 360° magnetic;
• VOR course pointer showing straight ahead to 360°, with perfectly centered CDI needle;
• LOC CDI perfectly centered;
• RMI pointer one, the VOR one, pointing straight ahead to 360°;
• RMI pointer two, the ADF one, pointing straight ahead to 360°.


3. Magnetic variance is introduced

Now, let us recreate that exact the same setup some time later. Now, the local magnetic variance has changed to 6° east. Nothing else has been changed. What would our indications be now?

Think about it a little bit before reading on.

They would be:
• indicated heading of 354° magnetic;
• VOR course pointer showing 6° to the right, to our selection of 360°, with perfectly centered CDI needle;
• LOC CDI perfectly centered;
• RMI pointer one, the VOR one, pointing 6° to the right, to 360°;
• RMI pointer two, the ADF one, pointing straight ahead to 354°.

The indicated heading is a simple one: our HSI was slaved to indicate magnetic heading, so it indicates just that. As our selected VOR course is 360°, the course arrow points straight to the HSI north, which is 6° to our right due to variance. But the trick is this: why is the CDI perfectly centered with course selection of 360° even though the station is now in magnetic direction of 354°?

This is because nothing else has changed but the magnetic variation! The VOR station is exactly the same, and the radial 180° FROM the station, that is, 360° TO the station, points to exactly the same direction in space as before, and it goes precisely over our runway's center line where we are sitting!

This is also why our RMI pointer one, which points to the VOR station, shows 6° to the right: only thing the RMI knows from the VOR station is that we are sitting on the radial 360° TO the station, so it assumes the station is at direction of 360°. But because our nose is pointed to true north, that is, 354° magnetic, the RMI needle shows to the right where the heading 360° lies on our RMI card!

RMI pointer two, the one that indicates direction to NDB beacon still shows straight ahead. Why is that? Because ADF determines the direction to NDB beacon relative to our airplane's nose. Because the beacon is physically straight ahead of us, right where our nose is pointing at 354° magnetic, the needle shows there correctly.

This shows an interesting consequence when flying using a VOR station that is not calibrated to the local magnetic variance: to fly station's radial 360° for instance, we actually must fly some slightly different magnetic heading to keep the needle centered! In this example, the heading would be 354°.


4. VOR station is re-calibrated

Now the guys finally arrive and figure out the VOR station is quite a bit off from the local magnetic north. That's a problem, because by convention, the VOR radials should match the local magnetic directions as closely as practicable. So, they calibrate the VOR station so that its station magnetic variance is the same as local, that is, 6° east.

While they do that, we are sitting on the runway. If we touch nothing, how would our indications change now?

You likely figured it out, the indications after the change would be:
• indicated heading of 354° magnetic;
• VOR course pointer still pointing 6° to the right, towards our selection of 360°, but now the CDI needle shows 3 dots to the left;
• LOC CDI perfectly centered;
• RMI pointer one, the VOR one, pointing straight ahead to 354°;
• RMI pointer two, the ADF one, pointing straight ahead to 354°.

What happened? What changed is that the VOR station was re-adjusted so that its radial 360° points to magnetic direction of 360° - that is, towards true direction of 006°. Because the station's signal now reflects that, our receiver detects that we are actually sitting on the radial 354° TO the station, thereby our selection of 360° shows a CDI displacement equivalent to 6°. Also, our RMI needle returned to point straight ahead, correctly towards the beacon, because it detects the airplane is sitting on the radial 354° TO the station, so it (now correctly) assumes the station is at the direction of 354°. The RMI needle pointing towards the NDB beacon is of course unaffected.

To center the VOR CDI needle again, we of course must select our course to 354° now, that is, straight ahead towards the station in this case.


5. A cool trick

Now the guys have updated our data so that we know the VOR station is correctly calibrated for magnetic variance. Now we finally take off, and fly around. While flying back towards home, navigating towards the NDB beacon, it turns out that our HSI doesn't slave correctly to the magnetic direction, and while maneuvering, it has accumulated some significant errors. How to find the correct magnetic direction again, more accurately than using the backup compass, while realizing that we seem to have a significant wind drift and everything? We note that our RMI, which has its compass card slaved to our HSI, shows the NDB straight at where it is relative to our nose, but the VOR needle is displaced some. Why is that?

Of course! It knows the radial we are on, and points to a direction according to that! But because our compass card is misaligned, that direction towards the station is misaligned by the same amount, while the ADF still points straight towards the beacon irrespective of what our compass card shows. So, to align our cards to correct magnetic heading, just rotate the heading indicated by the directional gyro until both the RMI needles match!

Neat, huh? Though it only works if the VOR and NDB are more or less co-located, or you otherwise happen to know the actual, precise direction to the station.


6. Additional notes

Did you note what remained unaffected all the time? Well, the ADF did, because it only points towards the beacon all the time. Why would it care about anything above? But the other one was the localizer. Similarly, it remained perfectly centered all the time - obviously, because it points to the same true heading, and therefore paints the same direction over ground all the time. Our approach course has changed, to 354° magnetic, and charts are to be updated accordingly, but this is only because they are referred against magnetic directions - the physical localizer and the approach itself are unaffected by the magnetic variation if you tolerate a bit of a heading indication error when your instruments do magnetic.

-Esa