In order to effectively use the compass as a directional reference for navigation, the pilot must be aware of several compass errors inherent to the instrument.
The compass works using the earth's magnetic field. Its indications are in reference to the magnetic poles of the earth, which exist in different locations from the geographic poles.
Variation
The difference between magnetic north and true north is the magnetic variation, and varies based on where you are located. In one place the variation might be zero degrees, meaning that the compass indication is the same as true north. In another place, the compass might indicate 20 degrees different from true north.
To allow pilots to compensate for variation, it is marked on sectional charts.
Deviation
The compass is not only affected by earth's magnetic field, but also by those of onboard avionics and electrical components. These components may cause the compass to indicate slightly right or left from what it should show. This difference is magnetic deviation. Deviation is normally less than a few degrees.
To allow pilots to compensate for deviation, each airplane is marked with a compass correction card. The card shows deviation at the cardinal headings.
Lead and Lag Errors
When you turn in flight, a magnetic compass will lead a turn through southerly headings and lag behind a turn through northerly headings. The amount of lead and lag is roughly the same as your latitude. For example, if you were flying at 30 degrees north latitude your compass would lead and lag by about 30 degrees. If you were flying east and made a right turn to west, the compass would lead your actual heading, incorrectly showing a 210 heading as you turned through south. As you approached a west heading, the compass would again become accurate. If you were to continue your right turn back around to east, the compass would lag behind your actual heading, incorrectly showing 330 degrees as you turned through north. As you approached an east heading, the compass would again become accurate.
If we were to stop the turn at any point, the compass lead and lag would dissipate over a few seconds and the compass would accurately indicate heading.
If the airplane is on a southerly heading and a turn is initiated, the compass will lead that turn. As a result, it will indicate a turn in the same direction as the airplane at a faster rate than the airplane is actually turning. If a turn is initiated from a northerly heading, the compass will initially indicate a turn opposite to the actual direction of the turn.
Acceleration / Deceleration Errors
The compass also exibits acceleration and deceleration errors.
If the airplane is flown on an east or west heading and it is accelerated, the compass will indicate a turn to the north. If the airplane is decelerated, it will indicate a turn to the south.
The ANDS acronym is useful for remembering this: Accelerate-North, Decelerate-South.
When flying north or south, no errors will be apparent because of acceleration or deceleration.
The magnetic compass should be read when the aircraft is flying straight and level at a constant speed. This will help reduce compass errors to a minimum.