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Pilot's Handbook of Aeronautical Knowledge

Latitude and Longitude (Meridians and

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Effect of variation on the compass.
Figure 15-10. Effect of variation on the compass.

the agonic line
Figure 15-9. Note the agonic line where magnetic variation is

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

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 aircraft’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.