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card from its backside. When the pilot is flying north, as the compass indicates, east is to the pilot’s right. On the card, “33,” which represents 330° (west of north), is to the right of north. The reason for this apparent backward graduation is that the card remains stationary, and the compass housing and the pilot rotate around it. Because of this setup, the magnetic compass can be confusing to read. 

## Magnetic Compass Induced Errors 

The magnetic compass is the simplest instrument in the panel, but it is subject to a number of errors that must be considered. 

## Variation 

The Earth rotates about its geographic axis; maps and charts are drawn using meridians of longitude that pass through the geographic poles. Directions measured from the geographic poles are called true directions. The magnetic North Pole to which the magnetic compass points is not collocated with the geographic North Pole, but is some 1,300 miles away; directions measured from the magnetic poles are called magnetic directions. In aerial navigation, the difference between true and magnetic directions is called variation. This same angular difference in surveying and land navigation is called declination. 

Figure 8-33  shows the isogonic lines that identify the number of degrees of variation in their area. The line that passes near Chicago is called the agonic line. Anywhere along this line 

the two poles are aligned, and there is no variation. East of this line, the magnetic North Pole is to the west of the geographic North Pole and a correction must be applied to a compass indication to get a true direction. 

Flying in the Washington, D.C., area, for example, the variation is 10° west. If a pilot wants to fly a true course of south (180°), the variation must be added to this, resulting in a magnetic course of 190° to fly. Flying in the Los Angeles, California area, the variation is 14° east. To fly a true course of 180° there, the pilot would have to subtract the variation and fly a magnetic course of 166°. The variation error does not change with the heading of the aircraft; it is the same anywhere along the isogonic line. 

## Deviation 

The magnets in a compass align with any magnetic field. Some causes for magnetic fields in aircraft include flowing electrical current, magnetized parts, and conflict with the Earth’s magnetic field. These aircraft magnetic fields create a compass error called deviation. 

Deviation, unlike variation, depends on the aircraft heading. Also unlike variation, the aircraft’s geographic location does not affect deviation. While no one can reduce or change variation error, an aviation maintenance technician (AMT) can provide the means to minimize deviation error by performing the maintenance task known as “swinging the compass.” 

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