Flight Instruments

card from its backside. When the pilot is flying north, as the the two poles are aligned, and there is no variation. East compass indicates, east is to the pilot’s right. On the card, of this line, the magnetic North Pole is to the west of the “33,” which represents 330° (west of north), is to the right of geographic North Pole and a correction must be applied to north. The reason for this apparent backward graduation is a compass indication to get a true direction. that the card remains stationary, and the compass housing and the pilot rotate around it. Because of this setup, the magnetic Flying in the Washington, D.C., area, for example, the compass can be confusing to read. 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 Magnetic Compass Induced Errors a magnetic course of 190° to fly. Flying in the Los Angeles, The magnetic compass is the simplest instrument in California area, the variation is 14° east. To fly a true course the panel, but it is subject to a number of errors that must of 180° there, the pilot would have to subtract the variation be considered. and fly a magnetic course of 166°. The variation error does not change with the heading of the aircraft; it is the same Variation anywhere along the isogonic line. The Earth rotates about its geographic axis; maps and charts Deviation are drawn using meridians of longitude that pass through the geographic poles. Directions measured from the geographic The magnets in a compass align with any magnetic field. poles are called true directions. The magnetic North Pole to Some causes for magnetic fields in aircraft include flowing which the magnetic compass points is not collocated with electrical current, magnetized parts, and conflict with the the geographic North Pole, but is some 1,300 miles away; Earth’s magnetic field. These aircraft magnetic fields create directions measured from the magnetic poles are called a compass error called deviation. magnetic directions. In aerial navigation, the difference between true and magnetic directions is called variation. This Deviation, unlike variation, depends on the aircraft heading. same angular difference in surveying and land navigation is Also unlike variation, the aircraft’s geographic location called declination. does not affect deviation. While no one can reduce or change variation error, an aviation maintenance technician Figure 8-33 shows the isogonic lines that identify the number (AMT) can provide the means to minimize deviation error of degrees of variation in their area. The line that passes near by performing the maintenance task known as “swinging Chicago is called the agonic line. Anywhere along this line the compass.” 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 -10 -40 -10 10 70˚N -30 20 0 10 60˚N 60˚N -20 -10 20 45˚N 45˚N 0 30˚N 0 30˚N 10 15˚N 0 15˚N 0˚ 10 -10 -20 -10 0˚ 0 -10 15˚S -20 15˚S -30 10 20 30˚S 30˚S Main field declination (D) 20 Contour interval: 30 2 degrees 30 45˚S red contours positive (east) 10 -60 45˚S 40 b p l i u nk e ( n a e g g o a n t i i c ve ) z ( e w r e o s l t i ) ne. 20 -70 50 40 6 7 0 0 ˚ ˚ N N 80 70 60 50 M e P r o c s a i t t o io r n P o ro f j d e i c p t i p o o n l . es -20 -30 -40 -50 -80 -110 -1 - 0 12 0 0 -9 - 0 130 130 100 1 7 1 9 0 0 0 80 60 7 6 0 0 ˚ ˚ N 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 Figure 8-33. Isogonic lines are lines of equal variation. 8-24