Deviation vs. Variation: Pilot Navigation
Learn the difference between magnetic variation and compass deviation, how pilots use each one, and how they affect heading calculations.
Variation and deviation are easy to mix up because both affect compass navigation. They are not the same problem.
Variation is about the Earth. Deviation is about the airplane.
Once you keep that distinction clear, heading calculations become much easier to understand.
Magnetic Variation
Magnetic variation, also called magnetic declination, is the angle between true north and magnetic north at a particular location.
True north points to the geographic North Pole. Magnetic north points toward the Earth's magnetic field. Those directions are not identical, and the difference changes depending on where you are.
Variation also changes over time because the Earth's magnetic field shifts. That is why aeronautical charts and navigation databases must stay current.
How Pilots Use Variation
Flight planning often starts with a true course or true track. To use a magnetic compass or magnetic heading reference, you need to correct for variation.
A common memory aid is:
East is least. West is best.
That means easterly variation is subtracted from true direction to get magnetic direction. Westerly variation is added.
For example, if your true course is 180 degrees and the local variation is 8 degrees west, you add 8 degrees. Your magnetic course becomes 188 degrees before applying wind correction and other adjustments.
Magnetic Deviation
Deviation is the error in the aircraft's magnetic compass caused by magnetic fields in or near the airplane.
Electrical equipment, avionics, metal structures, wiring, speakers, headsets, and even items placed near the compass can influence it. The compass may not point exactly where it should, even after variation has been applied.
This error is not the same on every heading. The airplane may have one deviation value when pointed north and another when pointed east, south, or west.
The Compass Correction Card
Aircraft with a magnetic compass normally have a compass correction card near the compass. It tells the pilot what compass indication to steer for selected magnetic headings.
For example, the card might indicate that to fly a magnetic heading of 180 degrees, you should steer 176 degrees on the compass. That four-degree difference is compass deviation for that heading.
The card exists because the installed compass has been checked and adjusted, but small residual errors remain.
Why the Terms Matter in Training
These definitions show up early because they connect chart work to cockpit reality. If you are planning on paper or in an electronic flight bag, you may see true courses, magnetic courses, wind correction, and compass headings all in the same planning flow.
Mixing up variation and deviation can lead to a heading that is wrong for two different reasons. On a short local flight, that may only be annoying. On a cross-country with haze, wind, and unfamiliar checkpoints, it can create real navigation confusion.
This is also why instructors still teach compass basics even in GPS-equipped airplanes. GPS is excellent, but a pilot should understand what the magnetic compass is showing and why it may not exactly match another heading reference.
A Simple Order to Remember
A common planning flow is:
True direction, correct for variation, then correct for deviation.
In plain language:
- True is based on the chart and geography.
- Magnetic accounts for the Earth's magnetic field.
- Compass accounts for the particular airplane's compass error.
Wind correction is another step, but do not let it blur the definitions. Variation moves you between true and magnetic. Deviation moves you between magnetic and compass.
Practical Cockpit Habits
Keep magnetic items away from the compass. A headset, phone mount, tablet case, flashlight, or speaker placed near the compass can create an error.
Use current charts and databases. Variation changes over time, and old data can create avoidable navigation errors.
Look at the compass correction card before relying on the magnetic compass. Many pilots forget it exists until electrical failure or instrument training forces them back to basic navigation.
During preflight, glance at the compass area. If someone mounted equipment nearby or placed a bag on the glareshield, move it and compare the indication again. Small cockpit habits prevent small errors from growing.
Student-Pilot Takeaway
Variation is the difference between true north and magnetic north. Deviation is the difference between the aircraft's magnetic heading and what its compass indicates.
Variation belongs to the location. Deviation belongs to the airplane. Keep those two sentences in your head, and the rest of compass navigation starts to make more sense.
Related Reading
Official References
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