3.5 Magnetic Variation and Navigation

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In the old days (before GPS), the magnetic compass was the main instrument used to navigate by pilotage.

Unlike a GPS, the magnetic compass does not point to geographical North (True North), but to Magnetic North. Pilots therefore had to translate headings from magnetic to true, and vice versa.

You still need to be able to do these conversions because they are examinable.

1. The Earth’s Magnetic Field

The Earth is a magnet with a north and south magnetic pole. Lines of force flow between these poles, creating a magnetic field that surrounds the Earth.

A compass needle is influenced by the Earth’s magnetic field and lies parallel to one of these magnetic lines of force.

The magnetic poles are not co-located with the geographical poles (True North). Therefore, the north direction indicated by a compass is different from geographical north.

2. Magnetic Variation & Chart Lines

The angle between the direction indicated by the compass and the true geographical direction is called magnetic variation. This magnetic variation is indicated on VNCs so users can apply the required corrections.

Isogonic Lines

Isogonic lines are lines joining points that have the same magnetic variation.

Check the VNC legend to see what isogonic lines look like (dashed curved magenta lines, roughly north to south).

The variation for a specific isogonic line is shown on the VNC along the line (for example, 14°W).

Agonic Line

The isogonic line with zero magnetic variation (the only line where True North is the same as Magnetic North) is called the Agonic line.

3. Converting Headings

If the magnetic pole lies west of the geographical pole from a given point, the compass needle points west of True North, so the variation is westerly.

If the compass needle points east of True North, the variation is easterly.

To convert magnetic headings to true headings, subtract westerly variation and add easterly variation.

To convert true headings to magnetic headings, do the opposite.

West Variation

True Heading = Magnetic Heading - West variation
Magnetic Heading = True Heading + West variation

East Variation

True Heading = Magnetic Heading + East variation
Magnetic Heading = True Heading - East variation

Example: A magnetic heading of 135 near the 13°W isogonic line means a true heading of 135 - 13 = 122° true.

Magnetic Variation Practice

Use this interactive component to practice entering a true heading and a variation to calculate the correct magnetic heading.

Magnetic Variation Practice Tool

Use this to convert headings quickly. Set your local variation, choose conversion direction, and apply: East is Least, West is Best.

Conversion

Magnetic variation

14°W

True Heading (TH)

Enter 0 to 359 degrees.

Result

Magnetic Heading (MH): 104°

MH = TH + Variation

MH = 90° + 14° = 104°

True North
Magnetic North (14°W)
True Heading (90°)

Flight note: A compass is most reliable in steady, level flight. Turns and acceleration can cause errors.

4. Specific Compass Errors

As mentioned earlier, compasses are only accurate in steady level flight. Errors occur when the aircraft turns or accelerates.

Turning Errors

When an aircraft makes a turn from a northerly heading (in the Northern Hemisphere), the compass briefly indicates a turn in the opposite direction (lags).

When an aircraft makes a turn from a southerly heading, the compass indicates a turn in the correct direction but at a much faster rate than actually occurs (leads).

Thus, in a 360° turn, the compass may lead or lag depending on where the aircraft is in the turn.

Acceleration Errors

When an aircraft accelerates going east-to-west or west-to-east, the compass indicates a northerly turn.

When the aircraft decelerates going east-to-west or west-to-east, the compass indicates a southerly turn.

However, acceleration and deceleration have no effect on a north-south heading.