Using the magnetic compass as the basis for in-flight direction comes with some built-in problems that have caused pilots to get off course and even lost. Variation and Deviation make sure that flying from point A to point B is not as easy as drawing a straight line. When we plan a cross-country flight, we draw a line across the chart from where we are to where we want to go. Then, we determine the true course by comparing that line with the lines of Latitude or Longitude that crisscross the sectional chart.
Problem: The chart is aligned with the True North Pole, but the compass in your airplane is aligned with the Magnetic North Pole. Unfortunately, these poles are *not* in the same place. True North is the location about which the Earth rotates. Magnetic North is the center of the Earth’s magnetic field. They currently lie approximately 750 miles apart. A pilot’s life would be easier if these two poles were in the same place. Give it another hundred billion years – maybe we’ll get lucky. Until then:
An angle is formed from True North to the location of your airplane to Magnetic North. This angle is your airplane’s variation. The degrees of this angle must either be added or subtracted from your course line to get the Magnetic Course. Sectional charts calculate the variation angle for you. Lines of variation are called Isogonic lines and are shown as magenta dashed lines running north to south across the chart. Variation is shown as either east or west variation. Use the phrase, ‘East is least and West is best‘ to remember when to add or subtract.
Why it Matters: There are variation angles in the United States that are as great as 22 degrees. How lost would you be after flying 22 degrees off course for a couple hours?
Since the magnetic compass uses magnetism to operate, it can easily be effected by other magnetic forces besides the Earth’s natural magnetism. Radios and electrical equipment inside the airplane can cause a compass deviation and error in the compass reading. This is why the magnetic compass is usually not mounted in the panel with all the other instruments, but rather hung up high and away from this interference. The amount of error is recorded on a Compass Correction Card, which is required to be in the airplane. The errors are detected when a person ‘swings’ the compass.
Why it Matters: Errors will be widely different in different airplanes. These errors must also be added or subtracted in the course planning so that you indeed fly where you think you are flying.
Two airplanes parked side by side on the airport ramp will have virtually identical Variation, but could have quite different Deviation. A lot of us don’t fly the exact same airplane every time we head for the skies. Make sure to compensate for both Variation and Deviation so that you will end up at your intended airport.