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.

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.”

To swing the compass, an AMT positions the aircraft on a series of known headings, usually at a compass rose. [Figure 8-34] A compass rose consists of a series of lines marked every 30° on an airport ramp, oriented to magnetic north. There is minimal magnetic interference at the compass rose. The pilot or the AMT, if authorized, can taxi the aircraft to the compass rose and maneuver the aircraft to the headings prescribed by the AMT.

As the aircraft is “swung” or aligned to each compass rose heading, the AMT adjusts the compensator assembly located on the top or bottom of the compass. The compensator assembly has two shafts whose ends have screwdriver slots accessible from the front of the compass. Each shaft rotates one or two small compensating magnets. The end of one shaft is marked E-W, and its magnets affect the compass when the aircraft is pointed east or west. The other shaft is marked N-S and its magnets affect the compass when the aircraft is pointed north or south.

The adjustments position the compensating magnets to minimize the difference between the compass indication and the actual aircraft magnetic heading. The AMT records any remaining error on a compass correction card like the one in Figure 8-35 and places it in a holder near the compass. Only AMTs can adjust the compass or complete the compass correction card. Pilots determine and fly compass headings using the deviation errors noted on the card. Pilots must also note the use of any equipment causing operational magnetic interference such as radios, deicing equipment, pitot heat, radar, or magnetic cargo.

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 the true course that is being flown when the compass course is known: Compass Course ± Deviation = Magnetic Course ± Variation= True Course

The Earth's magnetic field runs parallel to its surface only at the Magnetic Equator, which is the point halfway between the Magnetic North and South Poles. As you move away from the Magnetic Equator towards the magnetic poles, the angle created by the vertical pull of the Earth's magnetic field in relation to the Earth’s surface increases gradually. This angle is known as the dip angle. The dip angle increases in a downward direction as you move towards the Magnetic North Pole and increases in an upward direction as you move towards the Magnetic South Pole.

If the compass needle were mounted so that it could pivot freely in three dimensions, it would align itself with the magnetic field, pointing up or down at the dip angle in the direction of local Magnetic North. Because the dip angle is of no navigational interest, the compass is made so that it can rotate only in the horizontal plane. This is done by lowering the center of gravity below the pivot point and making the assembly heavy enough that the vertical component of the magnetic force is too weak to tilt it significantly out of the horizontal plane. The compass can then work effectively at all latitudes without specific compensation for dip. However, close to the magnetic poles, the horizontal component of the Earth’s field is too small to align the compass which makes the compass unusable for navigation. Because of this constraint, the compass only indicates correctly if the card is horizontal. Once tilted out of the horizontal plane, it will be affected by the vertical component of the Earth’s field which leads to the following discussions on northerly and southerly turning errors.