Magnetic Compass

Magnetic Compass

The simplest of aeronautic navigation instruments that is most often used for basic orientiation is the magnetic compass.

Operation

The principle of the compass’s magnetised needle aligning itself to the field lines of the earth magnetic field allows the pilot to determine the heading of the aircraft in relation to magnetic north. The readings will usually show N, S, E and W with the intervals between marked for each 30º. Further sub-divisions between the 30º marks are shown for every 5º

There are a number of magnetic compass designs used in aircrafts. One that is present in most aircraft is the floating magnet type. Here the magnetic needle is integrated into a floating disk that carries the markings of the compass rose on its circumference. A lubber line indicates the current magnetic heading of the aircraft. Note, that when the lubber line indicates North the part of the disk that points North is actually on the opposite side of the disk. Consequently, the markings appear backwards – East is on the West side and vice versa. This could be confusing at first.

Another design that avoids this confusion is the vertical card magnetic compass. Here the nose of a symbolic aircraft points to the magnetic heading on a compass rose.

The earth magnetic field has a horizontal and a vertical component. The compass needle will react to both the horizontal and the vertical force. The horizontal component is used for navigation, as it is responsible for the magnetic needle pointing to magnetic north. However, the effect that the vertical component has on the compass performance should also be understood, as it affects the accuracy of the compass reading significantly. One could imagine the magnetic needle pointing directly down when at the magnetic pole. This effect of pulling the magnetic needle down is called the "magnet dip". Make sure you read the section on turning and acceleration errors on the Navigation Basics Page to understand the effects that the magnetic dip has on compass indication.

Coverage

In principle the magnetic field is present everywhere on earth. The direction and strength is, however, influenced by a number of geographic factors. As a result the difference in direction between the North Magnetic Pole and the North Geographic Pole (the variation) sometime shows anomalies. This is reflected in bent variation lines. As the magnetic compass is ultimately used to determine the direction to Geographic North (in a process that takes into account the variation) these anomalies are of no practical importance as they are included in the variation. In other words, it is enough to know the variation in the area to find the correct direction to Geographic North.

It should also be noted that the increasing downward component of the earth magnetic field the closer one gets to the magnetic poles, makes the compass less useful at latitudes higher than 60° North or South.

Accuracy and Errors

An aeroplane is made up of metal, rotating parts of an engine, electric equipment, etc., all of which can generate their own magnetic field. Naturally, these fields affect the compass reading deflecting it from accurately indicating Magnetic North.

An engineer who has checked the compass in any particular aeroplane might have tried to minimise the deflection by placing little magnets around the compass. The remaining deflection after such corrective action is called Deviation. Each aeroplane displays a small placard, known as the Deviation Card, which shows the pilot the corrections to be made to the compass reading to obtain the magnetic direction.

Obviously the deviation card can only take into account the influences that were present when the engineer calibrated the compass. Any magnetic influences introduced after the calibration procedure can still affect the compass reading. Therefore, one should keep in mind that materials with their own magnetic field placed in the vicinity of the compass can have a significant effect. In particular large metal items or electronic devices, such as headphones or calculators, can cause large and unpredictable errors.

There are other errors that affect the compass reading when the aeroplane accelerates or turns (to do with magnet dip as mentioned above).