In flying, directions are specified by the angle the direction line of interest makes with a line drawn due north. So, due east corresponds to 90 degrees. Due south is 180 degrees. Due west is 270 degrees. Due north is usually stated as 360 degrees (as opposed to 0 degrees, which is in fact the same direction). A circular plot showing labeled direction lines extending out from a central point is called a compass rose.

Vertical card compass in a WWII P-51 Mustang. Reading = 139 degrees. Click to Enlarge

Aircraft Compass

The indicator of an aircraft compass is graduated in 10-degree increments, with numbered labels given every 30 degrees. However, the cardinal points are denoted by N, E, S, and W instead of numbers.

The last zero is omitted. Therefore, 30 degrees is indicated by just 3 instead of 30. And on the indicator, 30 signifies 300 degrees, not 30 degrees.

So, the labels around the indicator proceed from due north as ...

A graduation mark is provided every 10 degrees, but not all are labeled.

(See DI - Direction Indicator on this site, under Kit Building / Instruments.)

True North

Refers to the direction of the lines of longitude drawn on the earth's surface between the north and south poles. True north is opposed to magnetic north.

Magnetic North

Refers to the direction the north-seeking end of a compass needle points. Because the magnetic poles are offset considerably from the geographic north and south poles, a compass needle does not point to true north except at certain locations.

Magnetic Variation

The difference, measured in degrees, between true north and magnetic north at a given location. Variation may be . . .

Easterly: Compass needle points east of due north. (Negative)

Westerly: Compass needle points west of due north. (Positive)

The magnetic variation at a given location can be found from sectional charts or, for airports, from the web site www.airnav.com.

Heading

The direction the nose of an airplane is pointing. Headings may be . . .

True: the direction given relative to true north.

Magnetic: the direction given relative to magnetic north.

A compass indicates magnetic headings, obviously.

To Convert Headings, Magnetic to True and True to Magnetic

Easterly Variation: True Heading = Magnetic Heading + Variation

Westerly Variation: True Heading = Magnetic Heading -- Variation

Example: At Davidson County Airport (EXX) near Lexington, NC, the variation is +7W. If a pilot in the area flies a compass heading of 30 degrees, his true heading would actually be 23 degrees.

In most cases, the pilot wishes to know what magnetic heading should be flown in order to give a specified true heading. To find this ...

Easterly Variation: Magnetic Heading = True Heading -- Variation

Westerly Variation: Magnetic Heading = True Heading + Variation

Memory aid: “East is least; West is best.” That is, add westerly variation.

Example: At Davidson Country Airport, variation +7W, to fly a true heading of 60 degrees, the pilot should fly a magnetic heading of 67 degrees. That is, orient the plane so the compass reads 67 degrees and the true heading will be 60 degrees.

Variation from VOR compass roses

On sectional charts, compass roses are drawn around most VOR stations. These are relative to magnetic north. Therefore, you can estimate the variation at a particular station just by noting the offset of the VOR compass rose.