A compass needle always points north—but which one? To successfully navigate your aircraft, it’s important to understand the science of magnetic variation. There are two norths you’ll need to consider. Which north you choose to use for navigating has critical ramifications to your success.
So just what are these two norths? First, there’s true north, which is the geographic location of the North Pole, marked by the Earth’s axis, in relation to where you are. This north is the northern location of Washington state in relation to California or Maine in relation to Florida. Aviation sectional charts use true north for their orientation.
The second north—magnetic north—is the location of the molten core of the Earth, which is constantly (and quite rapidly) moving. Currently located in the Arctic Sea, since its “discovery” in the 1800s the magnetic north pole has been observed shifting increasingly farther (geographically) north and west. Magnetic north is the north that compasses point toward. It’s less commonly referenced or understood than true north.
Two norths would be more of an interesting bit of trivia than a crucial navigation factor if the two poles resided in the same location; however they do not align. Actually they’re quite far from one another. Since the magnetic north doesn’t align with the geographic one, the difference between them must be taken into account for proper navigation. This difference is an angle known as declination on land, or variation in the skies.
And if the two norths weren’t enough of a nuisance for pilots in training, there is another compounding issue: The Earth’s magnetic field drifts, which causes the location of magnetic north to change over time—which in turn means the angle of variation shifts over time, too.
So what does all this mean to you?
The magnetic poles are shifting rapidly enough that magnetic drift requires changes when the FAA updates sectional charts every six months. And every five years, federal agencies tabulate and publish the updated magnetic variation figures based on local areas. Similar to declination on the ground, these figures correlate the true north direction to the magnetic north compass readings that are required for navigation.
Since we fly by the compass, which points to magnetic north, we have to adjust the aircraft heading to be on the correct true heading. Magnetic variations, mapped as isogonic lines, vary across the hemispheres. Sectional charts show isogonic lines for every one degree of magnetic variation. An isogonic line is simply a line drawn through points of equal magnetic variation. Unlike longitude, however, these lines are not straight lines. For example, southwest Oregon and northern Montana are on the same isogonic variation line (20 degrees of magnetic variation). Which makes sense when you consider longitude is a geographical location measurement, whereas isogonic lines are a magnetic median measurement.
The degree of magnetic variation plays a crucial role in air navigation. And the closer to both the true and magnetic poles you get, the greater this variation becomes. Let’s do a case study. If you were flying in the Washington, D.C., area, the variation is 10 degrees west. So if you wanted to fly a true south (180-degree) heading, you’d need to take the magnetic variation into account by adding it to calculate the true course heading for south, or 190 degrees.
When you head geographically farther north, to Anchorage, Alaska, for example, that magnetic variation becomes even more pronounced. Let’s say you leave Anchorage’s Lake Hood Airstrip (Z41) and decide you’d like to visit Talkeetna (TKA). You depart on a north heading of 360 degrees for Talkeetna without looking up the magnetic variation. Forty-five minutes later, when you should be approaching Talkeetna, there is nothing but wilderness beneath you. Why?
If you followed your compass’s magnetic north heading, you’d actually be dramatically off course for true north. Anchorage and Talkeetna both have 19 degrees of eastern magnetic variation. To calculate a true north course heading you would travel with a magnetic compass heading of 341 degrees, the 360 degrees true north heading minus the 19 degrees of magnetic variation (see “Calculating Magnetic Course,” below).
You may have heard that several airports across the United States have been renaming their runways. This isn’t done as a means to make you stock your flight bag with new supplements. It’s necessary for accuracy as magnetic poles shift to the point of new compass orientations. Because runways are designated according to the points on a compass and magnetic North is in a constant state of drift, runway numbers will periodically need to be changed in order to be accurate. Since runway numbers are used to update navigational aids such as instrument landing systems and beacons, the FAA requires that a magnetic variation change of more than five degrees requires that airport’s runways to be renumbered.
Take for example Tampa International Airport (TPA), which in 2011 was required to make just such an update. Its west parallel runway was 36L-18R, to designate the compass points of 360 degrees and 180 degrees respectively. But with the magnetic drift constantly taking place, those runways were starting to line up more accurately to 10 degrees and 190 degrees. As a result, if you fly into TPA today, you’ll find its west parallel runway designated 1L-19R, to accurately reflect the shifted magnetic north.
Magnetic versus true north has many implications to the success of your flying. Knowing tricks of the trade, like runways lining up to magnetic azimuths and magnetic variation calculations, will help you to be successful on your checkride, and also in your future aviation endeavors. Remember to take magnetic variation into account, and you’ll end up at your intended destination each time.
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