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 unuseable 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.  The center of gravity of the float assembly is located lower  than the pivotal point. As the aircraft turns, the force that  results from the magnetic dip causes the float assembly to  swing in the same direction that the float turns. The result is  a false northerly turn indication. Because of this lead of the  compass card, or float assembly, a northerly turn should be  stopped prior to arrival at the desired heading. This compass  error is amplified with the proximity to either magnetic pole.  One rule of thumb to correct for this leading error is to stop  the turn 15 degrees plus half of the latitude (i.e., if the aircraft  is being operated in a position near 40 degrees latitude, the  turn should be stopped 15+20=35 degrees prior to the desired  heading).  [Figure 8-36A]  The magnetic dip and the forces of inertia cause magnetic  compass errors when accelerating and decelerating on  easterly and westerly headings. Because of the pendulous- type mounting, the aft end of the compass card is tilted  upward when accelerating and downward when decelerating  during changes of airspeed. When accelerating on either  an easterly or westerly heading, the error appears as a  turn indication toward north. When decelerating on either  of these headings, the compass indicates a turn toward  south. A mnemonic, or memory jogger, for the effect of  acceleration error is the word “ANDS” (Acceleration- North/Deceleration-South) may help you to remember the  acceleration error.  [Figure 8-37]  Acceleration causes an  When turning in a southerly direction, the forces are such that  the compass float assembly lags rather than leads. The result  is a false southerly turn indication. The compass card, or float  assembly, should be allowed to pass the desired heading prior  to stopping the turn. As with the northerly error, this error  is amplified with the proximity to either magnetic pole. To  correct this lagging error, the aircraft should be allowed to  pass the desired heading prior to stopping the turn. The same  rule of 15 degrees plus half of the latitude applies here (i.e.,  if the aircraft is being operated in a position near 30 degrees  latitude, the turn should be stopped 15+15+30 degrees after  passing the desired heading).  [Figure 8-36B]  Southerly Turning Errors  Acceleration Error  Northerly Turning Errors  Figure 8-36.  Northerly and southerly turning errors.  8-26  21  S  15  12  21  S  15  12 21  S  15  12  D $_{i}$p e f $_{f}$e c t C A R D  C A R D  Left turn  No error  Right turn  D i p e f e c t DIP  DIP  DIP  B  3  N  33  30 3  N  33  30  3  N  33  30 C A R D  D $_{i}$p e f $_{f}$e c t C A R D  D i p e f e c t t  Left turn  No error  Right turn A  DIP  DIP  DIP  t  t  t 