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Magnetic Deviation vs. Magnetic Variation in Aviation

When it comes to navigation, understanding magnetic direction is essential for every pilot. Since aircraft headings and navigation instruments rely heavily on the Earth’s magnetic field, any error in compass readings can lead to significant navigation mistakes. Two of the most important factors affecting magnetic compass accuracy are magnetic deviation and magnetic variation.


While these terms sound similar, they refer to two very different types of magnetic error. Let’s explore what each means, how they differ, and how pilots correct for them.



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What Is Magnetic Variation?

Magnetic variation (also known as magnetic declination) is the difference between true north and magnetic north.

  • True north points toward the geographic North Pole—the axis around which the Earth rotates.

  • Magnetic north points toward the Earth’s magnetic north pole—a location in northern Canada that moves slightly over time due to shifts in the Earth’s magnetic field.


The angle between these two norths at any given location is called magnetic variation.


Example:

If magnetic north is 10° east of true north at your location, you have a 10° east variation.If magnetic north is 10° west of true north, you have a 10° west variation.


Why It Matters:

  • Aeronautical charts are drawn with reference to true north.

  • Magnetic compasses, however, align with magnetic north. To fly accurately, pilots must convert between true and magnetic headings using variation.


The Rule:

“East is least, west is best.” When variation is east, subtract it from true to get magnetic. When variation is west, add it to true to get magnetic.


For example:

  • True heading 090° with 10°E variation → Magnetic heading = 090° − 10° = 080°

  • True heading 090° with 10°W variation → Magnetic heading = 090° + 10° = 100°


Where to Find Variation:

Magnetic variation is shown on aeronautical sectional charts as dashed isogonic lines, each labeled with the amount and direction of variation (e.g., “10°W” or “6°E”). The agonic line, where true north equals magnetic north, runs roughly through the central United States (but shifts slowly over time).


What Is Magnetic Deviation?

Magnetic deviation is a compass error caused by local magnetic interference inside the aircraft itself.


Every aircraft contains components that produce magnetic fields—such as radios, wiring, metal structures, and electronic instruments. These nearby magnetic sources can distort the Earth’s magnetic field, causing the compass to read incorrectly.


Example:

If your aircraft’s electrical system creates a field that causes your compass to read 3° right of the actual magnetic heading, that’s a 3° deviation error.


Why It Matters:

Even if a compass points toward magnetic north, interference from onboard equipment can make it inaccurate. This deviation is unique to each aircraft and can change when equipment is added or modified.


Correcting for Deviation:

To compensate for deviation, aircraft undergo a compass swing—a maintenance procedure where the compass is calibrated. The technician aligns the aircraft to known magnetic headings and notes the compass error for each direction.


These corrections are recorded on a compass correction card, typically mounted near the compass. The card lists headings such as:


ree

Pilots use these values to apply small corrections when flying by magnetic compass.


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Practical Example in Flight

Imagine you are flying a true course of 090° (due east).

  • The sectional chart shows a variation of 10°E.

    • Using the east rule, subtract: 090° − 10° = 080° magnetic heading.

  • Your compass card indicates a deviation of +2° on the 080° heading.

    • Correct for deviation: 080° − 2° = 078° compass heading.


So, to fly a true east course, you should steer a compass heading of 078°.


Final Thoughts

Both magnetic variation and magnetic deviation affect your compass readings, but they originate from entirely different sources.

  • Variation is caused by the Earth itself—large-scale and predictable.

  • Deviation is caused by the aircraft—localized and specific.


By understanding and compensating for both, pilots ensure accurate headings and safer navigation, especially when flying without electronic aids. Remember: mastering compass corrections is a fundamental skill that connects you to the timeless roots of aviation navigation.



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