Figure 3-21. The effects of acceleration error. Figure 8-37.  The effects of acceleration error.  indication toward north; deceleration causes an indication  toward south.  Oscillation is a combination of all of the errors previously  mentioned and results in fluctuation of the compass card in  relation to the actual heading direction of the aircraft. When  setting the gyroscopic heading indicator to agree with the  magnetic compass, use the average indication between the  swings.  The vertical card magnetic compass eliminates some of the  errors and confusion encountered with the magnetic compass.  The dial of this compass is graduated with letters representing  the cardinal directions, numbers every 30°, and tick marks  every 5°. The dial is rotated by a set of gears from the shaft- mounted magnet, and the nose of the symbolic aircraft on  the instrument glass represents the lubber line for reading the  heading of the aircraft from the dial.  [Figure 8-38]  When starting a turn from a northerly heading, the compass  lags behind the turn. When starting a turn from a southerly  heading, the compass leads the turn.  In the case of a vertical card magnetic compass, flux from  the oscillating permanent magnet produces eddy currents in  a damping disk or cup. The magnetic flux produced by the  eddy currents opposes the flux from the permanent magnet  and decreases the oscillations.  Oscillation Error  The Vertical Card Magnetic Compass  Lags or Leads  Eddy Current Damping  Figure 8-38.  Vertical card magnetic compass.  N  E  S  W  3  33  24  21  12  6  NAV GS  NORTH  S o u t h  OBS  15  30  View is from the pilot’s  perspective, and the  movable card is reset  after each turn.  N  W  S  E  33 30  24 21 15 12 6  3  8-27 