Not that long ago, navigation was done only with a map, a pencil, an E6B, a watch, and a wet compass. The “wet” compass, which gets its name because of the fluid it contains (similar to kerosene) to dampen any oscillations, doesn’t require electricity or any other mechanical support, but it does have its drawbacks. It is prone to a number of errors, many of which are a result of the magnetic fields it relies on to provide reliable heading information. But with a little knowledge, the compass is still the most dependable instrument in the airplane.
Vertical card compass
Many aircraft owners choose to replace the wet compass with a vertical card compass. This instrument displays the compass card vertically, and looks somewhat like a heading indicator or directional gyro. Although the vertical card compass indications are truer than the wet compass, it is still prone to a few errors, including a tendency to oscillate, and some turning error.
Variation
The Earth rotates around the geographic poles, which also serve as its physical axes. Lines of longitude and latitude are based on the poles, and the charts are oriented to these points as a result. The magnetic north and south poles in comparison aren’t collocated at the geographic poles, but rather some 1,300 miles away. The difference between the geographic poles and the magnetic poles is called variation. To properly navigate with a chart, magnetic variation must be calculated. Isogonic lines on the charts indicate the local variation. If east, the degree difference is subtracted from your true heading, and if west the difference is added.
Deviation
Because the compass relies on fixed magnets within its housing to indicate direction, metal within the airplane can skew the true number. So can radios and other electrical equipment in the aircraft. This difference is deviation, and the numbers are different for each airplane. A mechanic will test the airplane by pointing it at a few defined headings, and then note the compass reading. The deviation will then be noted on a card, which is mounted on or near the compass, indicating the difference between magnetic heading and compass heading.
Turning error
Because the Earth’s magnetic fields (or flux) leave the planet at the magnetic North Pole and enter at the magnetic South Pole, the compass is drawn down toward the surface. A weight is placed inside the compass to help combat this dip, but it’s not as effective in turns. If turning to or from a northerly heading, the compass will lag the turn. So if you want to turn to a heading of 360 degrees, stop the turn when the compass indicates some number of degrees before the target. The latitude at your current location determines the number by which you should undershoot. So if you’re flying in the northern part of the country, lag the turn by 40 to 45 degrees; in the southern United States, lag by about 30 degrees. If turning to or from a southerly heading, the compass leads the turn. That means the pilot must overshoot the decision heading, and roll out a number of degrees of latitude beyond the target. If you want to fly to that same 360-degree heading, roll out instead on a heading of between 030 and 045. This turning error doesn’t exist between easterly or westerly headings. One way to remember this is OSUN—overshoot south, undershoot north.
Acceleration error
The dip-correction weight on the compass causes indication errors when accelerating on an easterly or westerly heading. When accelerating on a heading of 090 or 270 degrees, the inertia of the weight holds back the compass and causes it to rotate toward north. When the aircraft slows down on these headings, the compass will indicate a turn toward the south. An easy way to remember this is ANDS—accelerate north, decelerate south.
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