Variation as Applied to the VOR, the NDB and the Direction Indicator (DI)

CPL Assist
Most of us know from PPL studies that variation is the difference between the direction to true north and the direction to Magnetic north and that variation differs depending on you position on the earth. Applying variation for a normal cross-country flight involves measuring the true bearing and adding or subtracting the average variation along the track to calculate the magnetic bearing of the required track.

There are certain questions in the Commercial exams that require you to think a little more specifically about how variation affects VOR radials, NDB directions and Direction Indicators (DIs). The VOR is at the centre of most of the exam questions which test your knowledge of the differences.

Lets look at the VOR and variation:

The VOR defines 360 radial magnetic tracks which radiate away from the VOR beacon. The VOR station is a permanent fixture at a specific geographical position on the earth. The direction of each radial is determined at the station, which uses the local (station) variation to convert the true direction to Magnetic. The radial then travels in a great circle track for as far as the circumstances allow (max range). To draw the radial on a chart we would have to convert the magnetic bearing to a true bearing. The main thing to remember here is that the variation used to create the radial (the station variation) has to be used to change the radial back to true.

Imagine, then, if you were to fly the true track of the radial without reference to the VOR. You would use a chart to examine the changing variation along the tack and apply the average variation to calculate the Magnetic bearing. This would be different from the radial’s magnetic name which was calculated based on the variation at one end of the track, that being the station end.

What does this tell us?

If you think about it, both magnetic results are incorrect. Most true tracks cut through a constantly changing variation. The magnetic name of the radial was wrong immediately after leaving the station because of the changing variation, and the magnetic track calculated for the cross-country was only correct in one place because it was based on an average of a changing variation. To determine the magnetic direction to fly to stay on the great circle track, you would have to continually calculate the magnetic direction from the prevailing variation. The result would be flying a continually changing magnetic heading.

This is in fact what you will be doing if you use the VOR indicator to stay on the radial.

How does the Exam test your knowledge?

Look at the following question:

Q. An aircraft (Variation 14W) is inbound to a VOR (Variation 16W), tacking the 120 Radial, with 8 degrees of left drift. What will its current magnetic heading be?

If you were not aware that the VOR radial looses its magnetic correctness immediately after leaving the station, you may calculate the reciprocal of the radial and turn the nose of the aircraft 8 degrees to the right (adjusting for drift) and get an answer of 308 Magnetic heading

If however, you are aware of the VOR variation problem, you would convert the VOR radial to true before proceeding. You would also remember that to do this you will apply the variation at the station (where the radial was created): 120 – 16W = 104 degrees true. Having done that you can now calculate the true inbound track by calculating the reciprocal: 104 + 180 = 284 degrees true. From hear we can calculate the true heading of the aircraft by turning the nose 8 degrees to the right: 284 + 8 = 292 degrees true heading. Only now do we apply variation to convert the true heading to a magnetic heading, using the variation at the aircraft: 292 + 14 = 306 degrees Magnetic heading. You will see a two degree difference from the first wrong answer; equal to the difference in variations.

A variation on the last question would be:

Q. An aircraft (Variation 14W) is inbound to a VOR (Variation 16W), tacking the 120 Radial, with 8 degrees of left drift. The aircraft is equipped with an two pointer RMI, one pointer connected to the VOR and the second pointer connected to the ADF tuned to an NDB collocated with the VOR. What indications will the two pointers be indicating?

This question requires you to understand that the Radio Magnetic Indicator (RMI) pointers, point to the QDM (which is the magnetic bearing from the Aircraft to the beacon.

In the case of the ADF, this is achieved by rotating the compass card to match the DI, thereby indicating the Magnetic heading under the heading marker, and then moving the pointer around the dial from the heading by an amount equal to the Relative baring (e.g., the ADF/RMI pointer will be parallel to the RBI pointer). The effect of this is to add the magnetic heading to the relative bearing, the addition of which equals the QDM to the NDB.

In the case of the VOR, the needle simply points to the reciprocal of the radial, making an assumption that the radial is the correct magnetic direction to the aircraft (the QDR) and therefore its reciprocal will be the QDM. This would be the case if the variation at the aircraft was the same as at the Station, but in most cases, as with this question, there is a two degree difference.

Below are the calculations for each pointer:

ADF pointer

Magnetic heading (from the last question) + the RBI

The aircraft is inbound with its nose 8 degrees to the right (compensating for left drift), therefore it Relative bearing will be 352 degrees Relative (RBI)

306 degrees Magnetic Heading + 352 RBI = 658 (-360) = 298 QDM

VOR pointer

Reciprocal of the radial = 120 + 180 = 300 QDM

The difference of 2 degrees is equal to the difference between the two variations. The needles will show a two degree separation even thought they are both assumed to be indicating the QDM.