Remember, if variation is west, add; if east, subtract. One  method for remembering whether to add or subtract variation  is the phrase “east is least (subtract) and west is best (add).”  Determining the magnetic heading is an intermediate step  necessary to obtain the correct compass heading for the flight.  To determine compass heading, a correction for deviation  must be made. Because of magnetic influences within an  aircraft, such as electrical circuits, radio, lights, tools, engine,  and magnetized metal parts, the compass needle is frequently  deflected from its normal reading. This deflection is called  deviation. The deviation is different for each aircraft, and  it also may vary for different headings in the same aircraft.  For instance, if magnetism in the engine attracts the north  end of the compass, there would be no effect when the plane  is on a heading of MN. On easterly or westerly headings,  however, the compass indications would be in error, as shown  in  Figure 16-11 . Magnetic attraction can come from many  other parts of the aircraft; the assumption of attraction in the  engine is merely used for the purpose of illustration.  Some adjustment of the compass, referred to as compensation,  can be made to reduce this error, but the remaining correction  must be applied by the pilot.  Proper compensation of the compass is best performed by  a competent technician. Since the magnetic forces within  the aircraft change because of landing shocks, vibration,  mechanical work, or changes in equipment, the pilot should  occasionally have the deviation of the compass checked. The  procedure used to check the deviation is called “swinging the  compass” and is briefly outlined as follows.  The aircraft is placed on a magnetic compass rose, the engine  started, and electrical devices normally used (such as radio)  are turned on. Tailwheel-type aircraft should be jacked up into  flying position. The aircraft is aligned with MN indicated on  the compass rose and the reading shown on the compass is  recorded on a deviation card. The aircraft is then aligned at  30° intervals and each reading is recorded. If the aircraft is to  be flown at night, the lights are turned on and any significant  changes in the readings are noted. If so, additional entries  are made for use at night. The accuracy of the compass can  also be checked by comparing the compass reading with the  known runway headings.  A deviation card, similar to  Figure 16-12,  is mounted near  the compass showing the addition or subtraction required to  correct for deviation on various headings, usually at intervals  of 30°. For intermediate readings, the pilot should be able to  interpolate mentally with sufficient accuracy. For example,  if the pilot needed the correction for 195° and noted the  correction for 180° to be 0° and for 210° to be +2°, it could  be assumed that the correction for 195° would be +1°. The  magnetic heading, when corrected for deviation, is known  as compass heading.  The preceding discussion explained how to measure a TC  on the aeronautical chart and how to make corrections for  variation and deviation, but one important factor has not  been considered—wind. As discussed in the study of the  atmosphere, wind is a mass of air moving over the surface of  the Earth in a definite direction. When the wind is blowing  from the north at 25 knots, it simply means that air is moving  southward over the Earth’s surface at the rate of 25 NM in  1 hour.  Under these conditions, any inert object free from contact  with the Earth is carried 25 NM southward in 1 hour. This  effect becomes apparent when such things as clouds, dust,  and toy balloons are observed being blown along by the wind.  Obviously, an aircraft flying within the moving mass of air  is similarly affected. Even though the aircraft does not float  freely with the wind, it moves through the air at the same  time the air is moving over the ground, and thus is affected  by wind. Consequently, at the end of 1 hour of flight, the  aircraft is in a position that results from a combination of  the following two motions:  Figure 16-11.  Magnetized portions of the airplane cause the  compass to deviate from its normal indications.  Figure 16-12.  Compass deviation card.  16-8  Deviation  Effect of Wind  N  3 3  3 0  W  2 4  2 1  S  $_{1}$5  $_{1}$2 $^{E }$ 6  3  N  3 3 3 0  W  2 4  2 1 S  $_{1}$5  1 2$^{E}$ 6  3 N  3 3  3 0  W  2 4  2 1  S  $_{1}$5 1 2$^{E}$ 6  3 Magnetic North  Magnetic North  Magnetic North  Magnetized engine  For (Magnetic)  N  30  60  E  120  150  Steer (Compass)  0  28  57  86  117  148  For (Magnetic  S  210  240  W  300  330  Steer (Compass)  180  212  243  274  303  332 