To navigate properly it is necessary to understand how aircraft compasses work. On this page we talk about the earths magnetic field, where true and magnetic north are and the effect of this variation on the compass onboard the aircraft. Navigating the aircraft by using only the magnetic compass, clock and sectional is not done by every pilot these days, it is almost a lost art with all these GPS systems available. So we expand on that subject a little as I believe that basic flying by compass, map and clock should be the primary navigation method and be complemented by radio- and satellite navigation as these could even fail or be unserviceable at the time of your flight.

Magnetic and True North

The rotating earth contains a lot of iron and as such it can be seen as a very large magnet. This magnetic field is not constant all over the earth as iron concentrations are not evenly distributed. Even the magnetic north pole is not located were the true north pole is, called variation or declination.
Making things complicated is navigating with a magnetic compass if the variation is not know for your location. To add to this situation: the magnetic lines (which run from north to south) are parallel near the equator but tend to dive to the ground near the poles, called inclination or dip. Which causes a compass dip error in the higher latitudes, ie near the north and south pole.

Adding up to this is the fact that the magnetic north pole is not even fixed in one location. According to the National Geophysical Data Center it is moving northwest with a speed of 40Km per year.

Aircraft Compasses

A compass aligns itself with the magnetic lines around the earth. This doesn't mean it is pointing to the true north. To find the true north this compass indication must be adjusted for magnetic declination or variation. The National Geophysical Data Center has a calculator online where you can enter your location (lat long) and obtain the magnetic declination/variation.

Correcting for true bearing
To get the true bearing from a compass (magnetic) bearing you must add the variation (for your location) to the compass bearing. Just remember that when calculating from magnetic to true: west variations are negative and east variation are positive (it exactly opposite when calculating for your navigation plan). Here you will find an example for an east variation and a west variation (all pictures courtesy from NGDC).

Variation and Deviation
Variation (declination) is caused by the fact that the magnetic north pole is not located at the same spot as the true north pole. Nothing we can do about that.. Some definitions: lines of equal variation are called isogonic and were the variation is zero it is called an agonic line.

Deviation are compass errors caused by influences of the vehicle where the compass is installed. This can be caused by ferrous materials, wiring with running currents from, for example, radio installations. Compass deviation is written on a little chart located on or near the compass, it could show: to fly heading 090 stear 093.

Compass Errors

As nothing is perfect and so is the magnetic compass, it suffers from a number of peculiar effects which influence the way it operates and the pilot needs to be aware off these effects.

Turning and Acceleration errors
Lets establish this compass mechanical fact first: The compass card is mounted in such a way that its center of gravity is lower than the pivot point. And this causes some problems when turning and accelerating.

Acceleration: When accelerating or decelerating on an easterly or westerly heading the compass will make a turn. Through inertia in the compass and because the acceleration/decelerating vector is more or less perpendicular to the earth magnetic lines this will result in a turning of the compass. On a northerly or southerly heading the compass will usually not turn but dip. Remember ANDS: Acceleration North and Deceleration South.

Turning: When the aircraft is in a banked turn, the card also banks because of centrifugal force. And in this attitude, the vertical component of the earth's magnetic field causes the compass to dip to the low side of the turn.

This compass turning error is most apparent when turning through headings close to north and south. When the aircraft makes a turn from a heading of north, the compass briefly indicates a turn in the opposite direction. When the aircraft makes a turn from a heading of south, the compass indicates a turn in the correct direction but at a considerably faster rate than is actually occurring. Thus, when making a 360° right turn beginning at north, the compass card initially turns in the wrong direction; then, as the aircraft passes through east, the compass "catches up" with the actual heading. Passing through south, the compass leads the turn considerably. As the aircraft heading passes through west, the compass should approximate the correct heading. Then, as the aircraft heading approaches north again, the compass lags.

Turbulence
Turbulence is not a real compass error. It makes the compass unstable and therefore more or less unreliable indication. Gyrocompasses give a much stabler reading in turbulence, problem with these gyro's is that their indication changes due to the rotation of the earth, 15°/hour (as they are fixed to one point in space).