Navigation

this course off the magnetic compass would not provide an accurate course between the two points due to three elements Easterly variation Westerly variation 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. Agonic line Magnetic Deviation Because each aircraft has its own internal effect upon the onboard compass systems from its own localized magnetic Figure 16-9. Note the agonic line where magnetic variation is zero. influencers, the pilot must add or subtract these influencers based upon the direction he or she is flying. The application of Zero degree variation exists on the agonic line where deviation (taken from a compass deviation card) compensates MN and TN coincide. This line runs roughly west of the the magnetic course unique to that aircraft’s compass system Great Lakes, south through Wisconsin, Illinois, western (as affected by localized magnetic influencers) and it now Tennessee, and along the border of Mississippi and Alabama. becomes the compass course. Therefore, the compass course, Compare Figures 16-9 and 16-10. when followed (in a no wind condition), takes the aircraft from point A to point B even though the aircraft heading Because courses are measured in reference to geographical may not match the original course line drawn on the chart. meridians that point toward TN, and these courses are maintained by reference to the compass that points along a If the variation is shown as “9° E,” this means that MN is magnetic meridian in the general direction of MN, the true 9° east of TN. If a TC of 360° is to be flown, 9° must be direction must be converted into magnetic direction for the subtracted from 360°, which results in a magnetic heading purpose of flight. This conversion is made by adding or of 351°. To fly east, a magnetic course of 081° (090° – 9°) subtracting the variation indicated by the nearest isogonic would be flown. To fly south, the magnetic course would be line on the chart. 171° (180° – 9°). To fly west, it would be 261° (270° – 9°). To fly a TH of 060°, a magnetic course of 051° (060° – 9°) For example, a line drawn between two points on a chart would be flown. is called a TC as it is measured from TN. However, flying NP NP NP v a r i a E tio as nt MP MP MP W v e a s ri t ation Zero variation 33 N 3 33 N 3 33 N 3 03 6 03 6 03 6 W E W E W E 24 12 24 12 24 12 21 S 15 21 S 15 21 S 15 SP SP SP Figure 16-10. Effect of variation on the compass. 16-7