Pilot's Handbook of Aeronautical Knowledge Navigation Latitude and Longitude (Meridians andParallels)

Pilot's Handbook of Aeronautical Knowledge

Preface

Acknowledgements

Appendix

Glossary

Index

 Figure 15-10. Effect of variation on the compass.
 Figure 15-9. Note the agonic line where magnetic variation is zero. 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. For example, a line drawn between two points on a chart is called a true course as it is measured from true north. However, flying this course off the magnetic compass would not provide an accurate course between the two points due to three elements that must be considered. The first is magnetic variation, the second is compass deviation, and the third is wind correction. All three must be considered for accurate navigation. Magnetic Variation As mentioned in the paragraph discussing variation, the appropriate variation for the geographical location of the flight must be considered and added or subtracted as appropriate. If flying across an area where the variation changes, then the values must be applied along the route of flight appropriately. Once applied, this new course is called the magnetic course. Magnetic Deviation Because each aircraft has its own internal effect upon the onboard compass systems from its own localized magnetic influences, the pilot must add or subtract these influences based upon the direction he or she is flying. The application of deviation (taken from a compass deviation card) compensates the magnetic course unique to that aircraftâ€™s compass system (as affected by localized magnetic influences) and it now becomes the compass course. Therefore, the compass course when followed (in a no wind condition) takes the aircraft from point A to point B even though the aircraft heading may not match the original course line drawn on the chart.

15-7