Magnetism and Compass – Ibex Aviation Academy

Earth Magnetism and Magnetic Compass

Earth behaves as a large magnet with a Magnetic North and South
Magnetic North is the direction indicated by the blue end of a freely suspended magnet
Earth’s magnetic field is not exactly aligned to its polar axis since the Magnetic North is not co located with True north
Magnetic north is approximately 400 NM away from True north and moves at at a slow rate of 22 NM per year
Magnetic North and South poles are not diametrically opposite to each other
Magnetic compass is the primary direction-finding instrument in an aircraft
Compass uses only the Magnetic North seeking end for direction finding
Magnetic south does not play any role in direction finding

Magnetic Variation is the angular difference between the True and Magnetic north
Variation are measured in degrees depending on Magnetic North being east or west of True North
Variation is dependent on the relative position of Magnetic North and Aircraft
Maximum value of variation can be 180 deg east or west

Isogonal are lines joining places of equal variation
Isogonal lines converge on the True North and South as well as Magnetic North and South poles
Agonal are lines joining places zero variation
Agonal would be a great circle connecting true and Magnetic North poles

Magnetic dip is the angle between the direction Earth’s Total Magnetic Field from the Horizontal to Earth Surface
Earth’s Magnetic field can be resolved into horizontal and vertical components
Horizontal component is the directive force of the magnet and helps in sensing the Magnetic North
Dip angle is zero at magnetic equator where the magnet stays horizontal to surface of earth
Dip angle is 90 degrees at poles where the magnet would become vertical to earth’s surface
Field strength of a magnet is the measure of directive force or horizontal component of magnet measured in micro tesla
Due to high dip angle, compass is ineffective near the poles since it requires a minimum of 6 micro tesla to function properly

Lines of Inclination join places of equal magnetic dip
Aclinical line joins places of zero dip angle which would coincide with the magnetic equator

Compass deviation is difference between the magnetic north and the north indicated by aircraft’s compass called compass north
Electric and metallic objects near aircraft’s compass cause called Deviation
Deviation is expressed in degrees East or West based on compass north being east or west of Magnetic North
Compass swing is the procedure to detect, correct and prepare a compass deviation card for residual deviation

Compass Deviation has to to be applied to Magnetic Directions to get Compass Directions
Plus and Minus signs are applied to compass heading to get magnetic heading
West is given a value of (-) and East is given a value of (+)
Compass direction can be calculated using the value of deviation
Deviation west (-) compass best
Deviation east (+) compass least