Magnetic Compass Errors

Compasses are limited by their design because they are allowed to pivot freely in liquid. As you can imagine, any aircraft movement—be it a climb, bank, descent, or jolt due to turbulence—can upset the fluid and ball.

True north is the North Pole. Most maps (including aviation maps) are produced in relation to true north. Magnetic north, however, is the northern pole of the earth’s magnetic, not geographic, poles. Its position is currently in Canada in the Hudson Bay region. Its position also changes slightly every year due to magnetic field changes. The difference between the true and magnetic poles is called variation. Depending on your position on the earth’s surface, you must add or subtract a set number of degrees of variation to convert your map positioning, based on true north, to your flight positioning, which is based on magnetic north.

A compass error called deviation is caused by different metals; electrical fields produced by other aircraft instruments and accessories; and disturbances generated by deposits or metals or power plants. Most compasses are placed above the instrument panel in the windshield or screen to reduce the effects of deviation, but this phenomenon can never be entirely eliminated. The airplane manufacturer or a certified mechanic will place a compass correction card in the airplane underneath the compass to note how many degrees of deviation must be considered for different headings.

Is deviation a major consideration in flight planning? Often, yes. Sometimes the differences caused by deviation may only be one or two degrees on a particular heading, but sometimes they may be as much as six or seven degrees. A quick look at the compass correction card will tell you how much change must be made and prevent you from incorrectly determining headings for navigation.

Another error is called magnetic dip. Magnetic dip is caused by the downward pull of the magnetic poles and is greatest near the poles themselves. A weight is often placed on the compass of the aircraft on the equatorial side to help negate this effect. Compass navigation near the polar regions, however, is nearly impossible due to the errors caused by this effect.

As an airplane accelerates or decelerates on an easterly or westerly heading, acceleration error is created. When an airplane accelerates easterly or westerly, the weight on the compass in the magnet causes it to indicate a turn towards the north. When an airplane decelerates on an easterly or westerly heading, the opposite occurs, and the compass will indicate an incorrect turn toward the south. A simple memory aid to remember this error is the acronym ANDS, ‘Accelerate North and Decelerate South.’

The final compass error we will discuss is turning error. Turning error is caused when turning on a heading of east or west to the north or south. In a turn toward the south, the compass heading ‘leads’ the actual heading of the aircraft. In a turn toward the north, the compass heading ‘lags’ behind the aircraft’s actual heading. When using the aircraft compass as the primary directional control for the aircraft, it is necessary to consider the lead or lag error and predict it. The memory aid used to remember this error’s effect is called UNOS, ‘Undershoot North and Overshoot South.’

If the magnetic compass has so many errors, then why do we use it? The magnetic compass is used in conjunction with other instruments in straight and level, unaccelerated flight. It is used as the information reference to the heading indicator, which in modern aviation is the primary directional indicator on board the aircraft. During your flight training, you will learn how to use the magnetic compass correctly, and you will be given the opportunity to fly with it as one of your only instruments as well so that you can learn to use it in case of other instrument failure.