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Magnetism and the Magnetic Compass

The Earth's Magnetic Poles

The phenomenon of magnetism was well known since ancient times. The first documented compass was recorded in China around 1090 AD. The Earth's magnetic field passes through the North and South magnetic poles. The magnetic polarity of the Earth is unstable and is influenced by many factors including magnetic disturbances originating from the sun magnetic storms. Geologists discovered that the magnetic fields of minerals around the globe suggest a remarkable phenomenon; the North and South poles change places! It is believed that the Earth magnetic field has reversed polarity some 25 times in the last 5 million years. For unknown reason, the unstable interior of the Earth weakens its magnetic field, which loses its bipolar character and regenerates with reversed polarity. North Pole Movement The North Magnetic Pole is constantly moving. The last determination of the North Magnetic Pole's average position was made in 1994. At that time the North Magnetic Pole was located on the Noice Peninsula, southwest Ellef Ringnes Island, at 78.3 N, 104.0 W. The average movement of the North Magnetic Pole is now 15 km (9.3 miles) per year. The magnetic pole wanders daily in an elliptical path around the average pole's position. When the magnetic field of the earth is disturbed, it may move as much as 50 miles from the average position.

North Magnetic Pole movements since 1600

Magnetic Compass
Earth's Magnetic Field

A magnet is a piece of iron, steel or steel alloy that was induced by a magnetic field. A magnet will attract similar metals. Some steel alloys have the quality to retain magnetism forever, thus forming permanent magnets. When a magnetized needle is suspended on a pivot, the North Magnetic Pole attracts one end of the needle while the other end is attracted to the South Magnetic Pole. A magnetic compass is an instrument that measures the direction relative to the magnetic north pole.


Isogonic Line

If a person is to follow the magnetic compass needle, that person will arrive at the Magnetic North Pole. Because the magnetic and geographic north poles are not co-located, a correction must be made to compensate for this discrepancy. The angle between the north geographic (true) and the north magnetic poles in a given location is called variation. An imaginary line that connects points with equal (same) variation is an isogonic line.


Magnetic Dip

Magnetic Dip

At the magnetic equator, the attraction of the compass needle towards the north and south poles is equal and the needle remains unbiased. As the compass is moved either north or south of the magnetic equator, the attraction to the nearest pole is increased, thus the needle will be biased towards the nearest pole. This phenomenon is called magnetic dip.

Aeronautical Magnetic Compass

Magnetic Compass

The magnetic compass is constructed from a float with two magnetic needles mounted on it. A compass card marked with letters for cardinal headings (N, E, S, W) and numbers at 30 degrees increments is mounted on the float. The last zero of the numbers is omitted. Magnetic Compass Between the numbers, the card is graduated for each 5 degrees.
The float assembly is balanced on a pivot in a sealed chamber filled with acid free white kerosene. The fluid serves three purposes. It reduces the weight that the float imposes on the pivot; it lubricates the pivot assembly and decreases the float oscillations.
The glass face of the compass is marked with a lubber, or reference line, by which compass indications are read. To correct magnetic influence of the aircraft and its electronic equipment, two adjustable compensating magnets are mounted on top of the compass. Two sets of screws labeled N-S and E-W are used to calibrate the compass.


Magnetic Compass Errors


Variation - The difference between the locations of the magnetic and true north must be corrected according to the variation at a given location. If the variation is westerly the degrees of variation are added to the true heading to obtain a Magnetic Course(Track). When the variation is easterly, the variation is subtracted from the true heading to obtain Magnetic Course(Track). A common mnemonic device that helps to remember this rule is:

Deviation Correction Card

Deviation - The discrepancy caused by the electromagnetic field of the aircraft including its electronic devices. The deviation can be controlled to a certain extent by the adjustable magnet compensators. A deviation correction card is mounted adjacent to compass. The card indicates which compass heading should be flown to achieve a desired magnetic heading.  
Magnetic Dip - The magnetic dip is contributing to the following compass errors:

The same forces that act on the airplane, act also on the compass's float assembly. If no other forces are introduced, the float assembly will remain balanced. For example, when the aircraft is banked the float assembly will bank with it. The attraction of the compass poles to the earth magnetic poles, introduces forces that affect the stability of the float assembly. These forces will act in opposite direction depending on the proximity of nearest magnetic poles. The discussion here is limited to the Northern Hemisphere, that is north of the Magnetic Equator. In the Southern Hemisphere the error are reversed.

Northerly Turning Errors  

The center of gravity of the float assembly is located lower than the pivotal point. As the airplane turns, the force that results from the magnetic dip causes the float assembly to swing in the same direction that the float turns. Northerly turn errors The result is a false northerly turn indication. Because of this lead of the compass card, or float assembly, a northerly turn should be stopped prior to arrival at the desired heading. This compass error is amplified with the proximity to either pole. One rule of thumb to correct for this leading error is to stop the turn 15 degrees plus half of the latitude, i.e. if the airplane is being operated in a position around the 40 degrees of latitude, the turn should be stopped 15+20=35 degrees prior to the desired heading.

Southerly Turning Errors  
When turning in southerly direction, the forces are such that the compass float assembly lags rather than leads. Southerly turn errors The result is a false southerly turn indication. The compass card, or float assembly, should be allowed to pass the desired heading prior to stopping the turn. As with the northerly error, this error is amplified with the proximity to either pole. To correct this lagging error, the aircraft should be allowed to pass the desired heading prior to stopping the turn. The same rule of 15 degrees plus half of the latitude applies here, i.e. if the airplane is being operated in a position around the 30 degrees of latitude, the turn should be stopped 15+15=30 degrees after passing the desired heading. Si-No Rule

The SI/ NO rule - is a very helpful device for determining the compensation for the turning errors. South is symbolized by the Spanish word SI, meaning yes while North is symbolized by NO, meaning no.
In the northern hemisphere, when turning to northerly headings, the NO rule applies. Because the compass card is leading, the turn must be stopped prior to the compass indicating the desired heading. It is a NO for passing this heading. On the other hand, when turning to southerly headings, the compass card is lagging; therefore, the turn should be stopped after passing the desired heading. SI here is used to say YES to pass the desired heading before stopping the turn.  
Acceleration error

Acceleration errors

The magnetic dip and the forces of inertia cause magnetic compass errors when accelerating and decelerating on Easterly and westerly headings. Because of the pendulous-type mounting, the aft end of the compass card is tilted upward when accelerating, and downward when decelerating during changes of airspeed. When accelerating on either an east or west heading , the error appears as a turn indication toward north. When decelerating on either of these headings, the compass indicates a turn toward south. The word "ANDS" (Acceleration-North/Deceleration-South) may help you to remember the acceleration error.

Oscillation Error.

Oscillation error results from erratic movement of the compass card which may be caused by turbulence or rough control technique. During oscillation, the compass is affected by all of the factors discussed. With proper training, the instrument can be effectively used despite the errors.  
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Last update May 17, 2005
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