Magnetic Variation & Deviation

Magnetic variation and deviation are the two corrections that stand between the true course a pilot measures on a chart and the compass heading actually flown, and the FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25) explains both in its navigation chapter. The distinction matters because aeronautical charts are drawn in reference to true north, the geographic north pole, while the aircraft's magnetic compass points toward the magnetic pole and is further disturbed by the metal and electrical systems around it.

Variation is the angular difference between true north and magnetic north at a particular point on the earth's surface. Because the magnetic pole is not co-located with the geographic pole, this angle changes with geographic position. On sectional charts, lines connecting points of equal variation are called isogonic lines and are drawn as dashed magenta lines labeled with the number of degrees east or west. The single line along which variation is zero, where a compass points to true north with no correction, is the agonic line; in the United States it runs roughly from the Great Lakes region down toward Florida. To convert a true course to a magnetic course, the pilot adds westerly variation and subtracts easterly variation, captured by the memory aid "east is least, west is best."

Deviation is the error introduced by magnetic fields generated within the aircraft itself: engine components, radios, wiring, and other ferrous or electrically active hardware pull the compass needle away from magnetic north. Deviation is unique to each airframe and even changes with the aircraft's heading, which is why every aircraft carries a compass correction card mounted near the instrument. The card lists the compass heading to steer for a range of magnetic headings, and it is periodically re-verified through a swing of the compass.

The two corrections combine in a fixed sequence known by the mnemonic TVMDC: True course, apply Variation to get Magnetic course, apply Deviation to get Compass course, with the reminder to add westerly corrections. Working the chain in order, a pilot who has measured a true course on the chart and computed a true heading in the wind triangle first applies variation to obtain magnetic heading and then applies deviation from the correction card to obtain the compass heading to fly.

The concept is essentially universal, because every magnetic compass everywhere is subject to both effects, but the specific values are local: variation depends on where the flight takes place and drifts slowly over years as the magnetic pole moves, while deviation is a property of the individual aircraft. Understanding both keeps navigation accurate, prevents the compounding heading errors that lead to being off course, and underlies the compass work that also matters when other heading references, such as gyroscopic instruments, are lost.

For a flight school, magnetic variation and deviation are core ground-school and cross-country teaching points, and they surface directly on the private pilot knowledge test and practical test. Instructors have to make sure each student can measure a true course, apply variation from the chart, and use the aircraft's compass correction card correctly before signing a cross-country solo endorsement. Getting this wrong is a common early error that shows up as students drifting off their planned track, so it is a recurring focus of dual navigation lessons.

Deviation also intersects with maintenance and airworthiness. A current, legible compass correction card is required equipment, and the periodic compass swing that produces it is part of keeping the aircraft properly equipped. A school operating a fleet needs to know that each aircraft's card is present and current, which ties the training concept to the maintenance and records side of the operation rather than leaving it purely in the classroom.

Aviatize helps a school keep the training and airworthiness sides of this topic organized. The Training Management module tracks each learner's progress through the navigation syllabus, including the ground-school lessons where variation and deviation are taught, and records the cross-country endorsements that depend on the student mastering them. Ground Training & Checking supports the knowledge-test preparation where these conversions are examined.

On the aircraft side, Maintenance Control and Digital Data & Records help the school keep documentation such as the compass correction card and its periodic verification associated with the right airframe, so that a fleet manager can confirm each aircraft is properly equipped without hunting through paper folders. Keeping the training record and the aircraft record in one system means the concept a student learns in the classroom is backed by an aircraft that is documented and inspection-ready.