AOPA Pilot Magazine - March 1958 - (Page 65) "Compass Punchy?" (Coutimted from page SS) der these conditions is called "compass north." Since directions, especially heading', are frequently measured with a compass, compass north ie a neceasary reference direction. The amoont that a particular compass deviates f o magnetic north, rm either i an easterly or a westerly dln rection, is called compass deviation, or simply "deviation." The amount of this deviation Is dependent on the particular aircraft and compass, the location of the compass i the aircraft, and the n amount of electronic equipment turned om Adjustable magnets in the compass are ~rovidedto counteract the effect of extrkeous magnetic fields, and,in same cases, deviation can lie reduced t o a negligible amount. If this cannot be done, it is necessary to make appropriate corrections fir deviation when changing measurements from a compass to a magnetic reference or vice versa. This procedure is similar t o the corrections for variation. First =tabliah whether c&mpas~ north is east or west of magnetic north. Then apply deviation corrections so that measurements w t respect to the westernmost ih reference are larger. The fourth commonly used reference di~ectionis that o f the aircraft heading itself. The term "rdative" i s used to describe this reference because the direction i measured relative to the s aircraft heading. This reference i used s most often for measuring the bearing or direction of some object from the airplane as shown in figure ( ) The 6. difference between a. relative bearing and the same bearing measured with respect to true north, magnetic north, or compass north is the aircraft heading measured with respect t o true north, magnetic north, or compass north, respectively. This i illustrated s in figure ( 6 ) . (See figures 6 and 6 on next; page.) In sunun&ry, let us consolidate the four commonly used reference dtrect i m a and their definitions, as follows: (1) True north-the direction of the earth's north pole from the point where a measurement i made. s (2) Magnetic n o r t h 4 9 direction of the earth's magnetic lhm of f o r e at the point where a measurement i made. s (3) Compass north-the direction in which a particular compass actually points. ( 4 ) Relative-an adjective mdicating measurement of direction relative to, or w t respect to, aircraft heading. ih Having defined the four' basic dtrectiens used in aerial navigation and the four fundamental reference directions used to measure them, we can comprehend clearly the terminology for apeeifyhg directions. In specifying any given direction we muit give both the direction and the reference from whi& it ifi measured. This is done with two words, the first an adjective denoting the reference used and the second a noun specifying (he direction measured. The adjectives used are "true," " m g - is not affected by an aircraft or its heading:. As ia shown in figure (S), if the variation is westerly, vz, if magi. netic north i s west of true north, then the measure of any direction w t reih spect to magnetic north will be greater than the same measurement with reapect to true north. Conversely, as shown in figure (41, if variation is easterly, then measurements with respect to true north will be .greater. Of course, the direction being measured is the same regardless of the reference used. T i illustrates precisely the reasonhs ing which should be employed in eonvtfrting a direction measurement from a magnetic reference to & true reference or vice VCTBR. First determine which is more -westerly, magnetic north or true north. Then apply the variation correction, adding or subtracting so that measurements with respect to the westm o s t reference are larger and those with respect to the eaatemmo~treference are smaller. Unfortunately, a compass will respond to other magnetic influences besides that of the earth's magnetic field, Frequently, metal parts of an aircraft's structure become partially maguetiaed and will cause a cornpaas to deviate from the earth's field w magnetic north. Also, magnetic fields set up by electronic equipment in the aircraft will cause deviation. The direction that 3 particular compass actually points imMARCH, 1958 0 - E i 2 3 C H A N N E L GH PERFORMANCE m C UffTERmpEfEIUm COMPLETE In, facsaw tnttelfwl MEETS FCC AND CAA UEQUIXEMENTS I airrgtnry 1111.51 WRITE FOR FREE LITERATURE OR ORDER FROM THIS CONVENIENT COUPON
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