Figure 4-19. Magnetic Compass

TM 1-1500-204-23-4 (1) Principles of operation. A magnetic compass operates on the principle of magnetic attraction. The magnets, which are free to pivot in a horizontal plane, will assume a position with the north- seeking ends pointing toward the earth's north magnetic pole. The magnetic heading is read by reading a compass card attached to the float. (2) Compass errors. Magnetic variation and magnetic deviation are explained in the following paragraphs. (a) Magnetic variation. Magnetic variation is the angular difference in degrees between the geographic north pole and the magnetic north pole. This variation is caused by the magnetic field, which is constantly changing. Since variation differs according to geographic location, its effect on the compass cannot be removed by any type of compensation. Variation is called west variation when the earth's magnetic field draws the compass needle to the left of the geographic north pole and east variation when the needle is drawn to the right of the geographic north pole. (b) Magnetic deviation. Magnetic deviation is the magnetic disturbances within the aircraft that deflect the compass needle from alignment with magnetic north. Compasses must be checked and periodically compensated by adjustment. Deviations remaining after adjustment are recorded on a compass correction card, as shown in figure 4-20, mounted near the compass. (3) Compass compensation. Compass compensation frequency requirements, pre- compensation instructions, compass rose method of compensating, and the B-16 master sighting compass method of compensating are explained in the following paragraphs. (a) Compensation frequency requirements. Compass will be checked for accuracy, compensated when necessary, and readings recorded on DD Form 1613 (Pilot Compass Correction Card) at the following times: · At least once during each 12 month period Figure 4-19. Magnetic Compass 4-25