Navigation

Remember, if variation is west, add; if east, subtract. One The aircraft is placed on a magnetic compass rose, the engine method for remembering whether to add or subtract variation started, and electrical devices normally used (such as radio) is the phrase “east is least (subtract) and west is best (add).” are turned on. Tailwheel-type aircraft should be jacked up into flying position. The aircraft is aligned with MN indicated on Deviation the compass rose and the reading shown on the compass is Determining the magnetic heading is an intermediate step recorded on a deviation card. The aircraft is then aligned at necessary to obtain the correct compass heading for the flight. 30° intervals and each reading is recorded. If the aircraft is to To determine compass heading, a correction for deviation be flown at night, the lights are turned on and any significant must be made. Because of magnetic influences within an changes in the readings are noted. If so, additional entries aircraft, such as electrical circuits, radio, lights, tools, engine, are made for use at night. The accuracy of the compass can and magnetized metal parts, the compass needle is frequently also be checked by comparing the compass reading with the deflected from its normal reading. This deflection is called known runway headings. deviation. The deviation is different for each aircraft, and it also may vary for different headings in the same aircraft. A deviation card, similar to Figure 16-12, is mounted near For instance, if magnetism in the engine attracts the north the compass showing the addition or subtraction required to end of the compass, there would be no effect when the plane correct for deviation on various headings, usually at intervals is on a heading of MN. On easterly or westerly headings, of 30°. For intermediate readings, the pilot should be able to however, the compass indications would be in error, as shown interpolate mentally with sufficient accuracy. For example, in Figure 16-11. Magnetic attraction can come from many if the pilot needed the correction for 195° and noted the other parts of the aircraft; the assumption of attraction in the correction for 180° to be 0° and for 210° to be +2°, it could engine is merely used for the purpose of illustration. be assumed that the correction for 195° would be +1°. The magnetic heading, when corrected for deviation, is known Some adjustment of the compass, referred to as compensation, as compass heading. can be made to reduce this error, but the remaining correction must be applied by the pilot. Effect of Wind The preceding discussion explained how to measure a TC Proper compensation of the compass is best performed by on the aeronautical chart and how to make corrections for a competent technician. Since the magnetic forces within variation and deviation, but one important factor has not the aircraft change because of landing shocks, vibration, been considered—wind. As discussed in the study of the mechanical work, or changes in equipment, the pilot should atmosphere, wind is a mass of air moving over the surface of occasionally have the deviation of the compass checked. The the Earth in a definite direction. When the wind is blowing procedure used to check the deviation is called “swinging the from the north at 25 knots, it simply means that air is moving compass” and is briefly outlined as follows. southward over the Earth’s surface at the rate of 25 NM in 1 hour. Magnetic North 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, Magnetic North Magnetic North and toy balloons are observed being blown along by the wind. Obviously, an aircraft flying within the moving mass of air 33 N 3 03 6 is similarly affected. Even though the aircraft does not float W E freely with the wind, it moves through the air at the same 03 33 N 3 6 24 21 S 15 12 03 33 N 3 6 t b i y m e w t i h n e d . a C ir o i n s s m eq o u v e in n g tl y o , v a e t r t t h h e e g en ro d u o n f d , 1 a n h d o u t r h u o s f i f s l i a g f h fe t, c t t h ed e W E W E aircraft is in a position that results from a combination of 24 12 24 12 21 S 15 21 S 15 the following two motions: For (Magnetic) N 30 60 E 120 150 Steer (Compass) 0 28 57 86 117 148 Magnetized engine For (Magnetic S 210 240 W 300 330 Steer (Compass) 180 212 243 274 303 332 Figure 16-11. Magnetized portions of the airplane cause the Figure 16-12. Compass deviation card. compass to deviate from its normal indications. 16-8