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Eastern Standard Time..........Add 5 hours
Central Standard Time..........Add 6 hours
Mountain Standard Time...... Add 7 hours
Pacific Standard Time.......... Add 8 hours
For Daylight Saving Time, 1 hour should be subtracted from the calculated times.Measurement of Direction
By using the meridians, direction from one point to another can be measured in degrees, in a clockwise direction from true north. To indicate a course to be followed in flight, draw a line on the chart from the point of departure to the destination and measure the angle which this line forms with a meridian. Direction is expressed in degrees, as shown by the compass rose in Figure 15-6.
15-6
Figure 15-7. Courses are determined by reference to meridians on aeronautical charts.
Course A to B 065°
BACourse B to A 245°065°245°
Figure 15-8. Magnetic meridians are in red while the lines of longitude and latitude are in blue. From these lines of variation (magnetic meridians), one can determine the effect of local magnetic variations on a magnetic compass.
MN
TN
Figure 15-6. Compass rose.
36
33302724211815121563NNW NNE NEN ESE SSE SSW WSW WNW N NE E SE S SW W NW
Because meridians converge toward the poles, course measurement should be taken at a meridian near the midpoint of the course rather than at the point of departure. The course measured on the chart is known as the true course (TC). This is the direction measured by reference to a meridian or true north. It is the direction of intended flight as measured in degrees clockwise from true north.
As shown in Figure 15-7, the direction from A to B would be a true course of 065°, whereas the return trip (called the reciprocal) would be a true course of 245°.
The true heading (TH) is the direction in which the nose of the aircraft points during a flight when measured in degrees clockwise from true north. Usually, it is necessary to head the aircraft in a direction slightly different from the true course to offset the effect of wind. Consequently, numerical value of the true heading may not correspond with that of the true course. This is discussed more fully in subsequent sections in this chapter. For the purpose of this discussion, assume a no-wind condition exists under which heading and course would coincide. Thus, for a true course of 065°, the true heading would be 065°. To use the compass accurately, however, corrections must be made for magnetic variation and compass deviation.Variation
Variation is the angle between true north and magnetic north. It is expressed as east variation or west variation depending upon whether magnetic north (MN) is to the east or west of true north (TN).
The north magnetic pole is located close to 71° N latitude, 96° W longitude and is about 1,300 miles from the geographic or true north pole, as indicated in Figure 15-8. If the Earth were uniformly magnetized, the compass needle would point toward the magnetic pole, in which case the variation between true north (as shown by the geographical meridians) and magnetic north (as shown by the magnetic meridians) could be measured at any intersection of the meridians.
Actually, the Earth is not uniformly magnetized. In the United States, the needle usually points in the general direction of the magnetic pole, but it may vary in certain geographical localities by many degrees. Consequently, the exact amount of variation at thousands of selected locations in the United States has been carefully determined. The amount and the
15-7
Figure 15-9. Note the agonic line where magnetic variation is zero.
Easterly Variation
Westerly VariationAgonic Line
Figure 15-10. Effect of variation on the compass.
Zero
variationN3330W2421S1512E63N3330W2421S1512E63N3330W2421S1512E63Westvariation Eest variationNPMPSPSPNPMPNPMPSP
direction of variation, which change slightly from time to time, are shown on most aeronautical charts as broken magenta lines, called isogonic lines, which connect points of equal magnetic variation. (The line connecting points at which there is no variation between true north and magnetic north is the agonic line.) An isogonic chart is shown in Figure 15-9. Minor bends and turns in the isogonic and agonic lines are caused by unusual geological conditions affecting magnetic forces in these areas.
On the west coast of the United States, the compass needle points to the east of true north; on the east coast, the compass needle points to the west of true north.
Zero degree variation exists on the agonic line, where magnetic north and true north coincide. This line runs roughly west of the Great Lakes, south through Wisconsin, Illinois, western Tennessee, and along the border of Mississippi and Alabama. [Compare Figures 15-9 and 15-10.]
Because courses are measured in reference to geographical meridians which point toward true north, and these courses are maintained by reference to the compass which points along a magnetic meridian in the general direction of magnetic north, the true direction must be converted into magnetic direction for the purpose of flight. This conversion is made by adding or subtracting the variation which is indicated by the nearest isogonic line on the chart.
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Pilot's Handbook of Aeronautical Knowledge航空知识手册3(19)
