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.  The magnetic compass is the simplest instrument in  the panel, but it is subject to a number of errors that must  be considered.  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.  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.  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  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.”  Figure 8-33.  Isogonic lines are lines of equal variation.  8-24  Deviation  Variation  Magnetic Compass Induced Errors  6 0  5 0  40  30 2 0  10  1 0  20  0 0  1 0  0  2 0  $_{1}$0 0  0  $_{2}$0  1 0 1 3 0 1 0  1 0 $_{0 }$9 0  8 0  7 0  6 0  50  4 0  30 8 0  7 0  2 0 10  -40  -9 0  - 1 0 0  $_{-}$1 $_{1}$0  $_{-}$1 2 0 - 1 $_{3}$0  -5 0  - 40  -3 0 - 20 - 1 0  - 3 0 - 2 0  - 1 0  $_{-}$20  $_{-}$30  - 1 0  - 1 0  $_{$_{-}$8}$0  - $_{7}$0  - 6 0  - 2 0  - 1 0  - 1 0  180˚W 165 ˚ W  150 ˚ W  135 ˚ W  120 ˚ W  105 ˚ W  90 ˚ W  75 ˚ W  60 ˚ W  45 ˚ W  30 ˚ W  15 ˚ W  0 ˚ 15 ˚ E  30 ˚ E  45 ˚ E  60 ˚ E  75 ˚ E  90 ˚ E  105 ˚ E  120 ˚ E  135 ˚ E  150 ˚ E  165 ˚ E  180˚W 70˚N 70˚N 60 ˚ N  60 ˚ N  45 ˚ N  45 ˚ N  30 ˚ N  30 ˚ N  15 ˚ N  15 ˚ N  0 ˚ 0 ˚ 15 ˚ S  15 ˚ S  30 ˚ S  30 ˚ S  Main field dec lination (D) Contour interval:  degrees 45 ˚ S  45 ˚ S  red contours positi v (east)  blue negative (w est) pink  (agonic) zero lin e.  Mercator Projection. 60 ˚ N  60 ˚ N  P osition of dip poles  70˚N 70˚N 180˚W 165 ˚ W  150 ˚ W  135 ˚ W  120 ˚ W  105 ˚ W  90 ˚ W  75 ˚ W  60 ˚ W  45 ˚ W  30 ˚ W  15 ˚ W 0 ˚ 15 ˚ E  30 ˚ E  45 ˚ E  60 ˚ E  75 ˚ E  90 ˚ E  105 ˚ E  120 ˚ E  135 ˚ E  150 ˚ E  165 ˚ E  180˚W