COMPASS DEVIATION
Besides the magnetic fields generated by the earth, other magnetic fields are produced by metal and electrical accessories within the helicopter. These magnetic fields distort the earth’s magnet force and cause the compass to swing away from the correct heading. Manufacturers often install compensating magnets within the compass housing to reduce the effects of deviation. These magnets are usually adjusted while the engine is running and all electrical equipment is operating. Deviation error, however, cannot be completely eliminated; therefore, a compass correction card is mounted near the compass. The compass correction card corrects for deviation that occurs from one heading to the next as the lines of force interact at different angles.
MAGNETIC DIP
Magnetic dip is the result of the vertical component of the earth’s magnetic field. This dip is virtually nonexistent at the magnetic equator, since the lines of force are parallel to the earth’s surface and the vertical component is minimal. As you move a compass toward the poles, the vertical component increases, and magnetic dip becomes more apparent at these higher latitudes.
Magnetic dip is responsible for compass errors during acceleration, deceleration, and turns.
Acceleration and deceleration errors are fluctuations in the compass during changes in speed. In the northern hemisphere, the compass swings toward the north during acceleration and toward the south during deceleration. When the speed stabilizes, the compass returns to an accurate indication. This error is most pronounced when you are flying on a heading of east or west, and decreases gradually as you fly closer to a north or south heading. The error does not occur when you are flying directly north or south. The memory aid, ANDS (Accelerate North, Decelerate South) may help you recall this error. In the southern hemisphere, this error occurs in the opposite direction.