IFH 5-13

030 060 E 120 150 S 210 240 W 300 330 N Figure 3-18. A Compass rose upon which deviation error is compensated for. True north Figure 5-18. Utilization of a compass rose aids compensation for deviation errors. Figure 3-19. A compass correction card shows the deviation correction for any heading. Figure 5-19. A compass correction card shows the deviation correction for any heading. is no magnetic interference. Lines, oriented to magnetic north, are painted every 30°, as shown in Figure 5-18. The pilot or AMT aligns the aircraft on each magnetic heading and adjusts the compensating magnets to minimize the difference between the compass indication and the actual magnetic heading of the aircraft. Any error that cannot be removed is recorded on a compass correction card, like the one in Figure 5-19, and placed in a cardholder near the compass. If the pilot wants to fly a magnetic heading of 120° and the aircraft is operating with the radios on, the pilot should fly a compass heading of 123°. The corrections for variation and deviation must be applied in the correct sequence as shown below starting from the true course desired. Step 1: Determine the Magnetic Course True Course (180°) ± Variation (+10°) = Magnetic Course (190°) The Magnetic Course (190°) is steered if there is no deviation error to be applied. The compass card must now be considered for the compass course of 190°. Step 2: Determine the Compass Course Magnetic Course (190°, from step 1) ± Deviation (–2°, from correction card) = Compass Course (188°) NOTE: Intermediate magnetic courses between those listed on the compass card need to be interpreted. Therefore, to steer a true course of 180°, the pilot would follow a compass course of 188°. To find true course when the compass course is known, remove the variation and deviation corrections previously applied: Compass Course ± Deviation = Magnetic Course ± Variation = True Course 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. 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° plus half of the latitude (i.e., if the airplane is being operated in a position around the 40° of latitude, the turn should be stopped 15° + 20° = 35° prior to the desired heading). [Figure 5-20A] Southerly Turning Errors When turning in a southerly direction, the forces are such that the compass float assembly lags rather than leads. 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° plus half of the latitude applies here (i.e., if the airplane is being operated in a position around the 30° of latitude, the turn should be stopped 15° + 15° + 30° after passing the desired heading). [Figure 5-20B] Acceleration Error 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