Flight Training - May 2014 - 48

THE POLES PULL
The closer to the North or South Pole you travel, the greater magnetic variation to account for when flying. In fact,
at certain extreme northern and southern latitudes, compasses stop giving accurate headings altogether.

THE DIFFERENCE
between true north
and magnetic north is
variation.

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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).

CALCULATING
MAGNETIC COURSE
There's a saying in aviation, "East is least,
west is best." What this means is magnetic
variations that are east of true north are
subtracted (least) from true course headings. While magnetic variations that are
west of true north are added (best) to true
course headings. In our examples above:
Washington, D.C., magnetic variation 10
degrees west
True course (180 degrees) plus 10 degrees
variation (since west is "best") equals
magnetic course 190 degrees
Anchorage, Alaska: magnetic variation 19
degrees east
True course (360 degrees) minus 19
degrees variation (since east is least)
equals magnetic course 341 degrees

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Flight Training - May 2014

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