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Compass

The compass uses the magnetic poles of the earth to indicate direction. Variation is the angle between the Magnetic Meridians and the True Meridians, because the magnetic poles and the true poles are not the same. Isogonic Lines are lines joining points of equal variation, and an agonic line is the line for points of zero variation. Deviation is the amount of error the compass indicates due to structures near the compass. Adjustments made to minimize these errors are made when swinging the compass. Any uncorrected errors are displayed near the compass.

When heading North, turns in either direction will briefly indicate a turn in the opposite direction and then the compass reading will lag behind the actual heading. When heading South, turns in either direction will show a faster turn in the correct direction than is actually happening, and the compass will lead the turn. (Note to instructors: the best aid to understanding these phenomena is to use a styrofoam cup through which two pencils are pushed at right angles to each other to indicate NEWS. This “compass” may then be suspended on a third pencil and used to explain the above turning errors.)

The lag or lead of the compass in turns due to these ‘heading’ effects will be roughly equal to the
latitude the aircraft is at. That is at 30 degrees latitude the compass reading will lag by approximately 30 degrees on turns to and from northerly headings and lead the turn by 30 degrees in turns to and from southerly headings.

When accelerating on an Easterly or Westerly heading the compass will initially indicate a turn to the north. When decelerating on these headings the compass will initially swing to the south. A convenient way to remember this is by the mnemonic ANDS - Accelerate North, Decelerate South.