15.magnetic-variation-and-compass-deviation. Magnetic Variation and Compass Deviation
The magnetic compass is the only direction-seeking instrument required by 14 CFR § 91.205 for both VFR and IFR flight, yet its indications are rarely identical to the true direction an aircraft is traveling. Two systematic errors — variation and deviation — must be applied to convert between true, magnetic, and compass courses. Understanding these errors is fundamental to dead-reckoning navigation and to flying any heading published on a sectional, IFR chart, or instrument procedure.
True North vs. Magnetic North
Charts are constructed using true north — the geographic axis of the Earth — as the reference for meridians of longitude. The magnetic compass, however, aligns itself with the Earth's magnetic field, whose north pole lies in the Canadian Arctic, hundreds of miles from the geographic pole and slowly drifting. The angular difference between the direction to true north and the direction to magnetic north at a given location is called magnetic variation.
Variation is depicted on sectional and IFR charts by dashed magenta lines called isogonic lines, each labeled with the variation in degrees east or west. A line of zero variation, where true and magnetic north align from the observer's position, is called the agonic line, and it currently runs roughly through the central United States. East of the agonic line, variation is generally west; west of it, variation is generally east.
To convert a true course (TC) measured on a chart into a magnetic course (MC), apply the rule:
- East is least, West is best — subtract easterly variation, add westerly variation.
Examples:
- TC 090° at a location with 10° W variation → MC = 090° + 10° = 100°.
- TC 270° at a location with 14° E variation → MC = 270° − 14° = 256°.
Because variation depends only on geographic location (and slowly on time), every pilot operating in or near an isogonic line must read the chart carefully and apply the appropriate value for the route segment being flown.
Compass Deviation
Even after correcting for variation, the compass card may not point at magnetic north. The aircraft itself contains ferrous metal, electrical wiring, radios, and other equipment that generate small local magnetic fields. The error these fields induce in the compass is called compass deviation. Unlike variation, deviation is unique to each aircraft, varies with heading, and even changes when electrical equipment (landing light, pitot heat, avionics) is switched on.
Deviation is determined by a process called swinging the compass, performed by a certificated technician on a compass rose painted on the ramp. With the engine running and normal avionics on, the aircraft is aligned with each cardinal and intercardinal heading, and small compensating magnets in the compass housing are adjusted to minimize error. The residual error is recorded on a compass correction card mounted in the cockpit adjacent to the compass. A typical card lists:
- For (magnetic heading): N 30 60 E 120 150 S 210 240 W 300 330
- Steer (compass heading): 0 28 57 86 117 148 180 212 243 274 303 332
The card tells the pilot what compass heading to fly to track a desired magnetic heading. In the example above, to fly a magnetic heading of 060°, the pilot would steer compass heading 057°, a deviation of −3°.
The TVMDC Conversion
The complete chain of corrections from chart to cockpit is remembered with the mnemonic TVMDC + Wind — "True Virgins Make Dull Companions, Add Whiskey":
- True Course (from chart)
- ± Variation = Magnetic Course
- ± Wind Correction Angle = Magnetic Heading
- ± Deviation = Compass Heading
Example: A flight planned from a chart shows TC 135°. Local variation is 8° W. The wind correction angle is +6° (right crosswind). The compass correction card shows that on a magnetic heading of 149°, the pilot should steer 151°.
- TC 135° + 8° W = MC 143°
- MC 143° + 6° WCA = MH 149°
- MH 149° + 2° deviation = CH 151°
Practical Considerations
- Variation values printed on sectionals are updated each chart cycle; over decades, the magnetic poles drift enough to change variation by several degrees.
- Deviation is small (rarely more than 10°) but cumulative with variation; a 10° W variation combined with 4° E deviation still leaves a 6° W net error.
- Avoid placing ferrous objects — flashlights, headsets, kneeboards with metal clips, electronic devices — near the compass, as they can induce additional, uncalibrated deviation in flight.
- The compass correction card must be legible and present for the compass to be considered an airworthy required instrument.
- Modern slaved gyro and AHRS-based heading systems still derive their reference from a magnetic flux detector, so variation and deviation principles continue to apply.