Direction, Magnetic Course, and Compass Points

061-01-03Pablo Asensio Martínez2026-04-014 min

Navigating an aircraft is not as simple as pointing the nose toward the destination and accelerating. In the air, there are invisible forces and changing references that force the pilot to constantly calculate the trajectory. To understand how an aircraft is guided, we must master three fundamental concepts: north references, the effect of wind, and in-flight corrections.

1. The Three Norths: Direction References

To measure a direction in degrees (from 0° to 360°), we need a starting point or reference line. In aviation, there is not just one “North,” but three different ones:

  • True Direction (T): It is measured with respect to True North (the real North Pole). This is the constant reference used by maps and navigation charts. The Earth’s meridians indicate this direction.
  • Magnetic Direction (M): It is measured with respect to Magnetic North. This is where the compass naturally points. The problem is that Magnetic North is not in the same location as True North and it moves over time.
  • Compass Direction (C): This is what the pilot actually reads on the onboard instrument. It often does not exactly match Magnetic North due to the metallic and electrical interference of the aircraft itself.

2. Errors: Variation and Deviation

Magnetic Variation Map

In order to navigate, the pilot must convert the data from the map (True) to what is seen in the cockpit (Compass). To do this, two angular corrections are applied:

  • Variation: This is the angular difference between True North and Magnetic North. It depends on the geographical area in which we are flying.

    • On maps, the lines that connect points with the same variation are called isogonic lines. If the variation is zero, it is called an agonic line.

  • Deviation: This is the specific error of the aircraft’s compass caused by the magnetic fields of the aircraft (engine, radios). Each aircraft has a deviation card that indicates this error.

Golden Rule for Conversions: To go from one reference to another, mnemonic rules are used:

  • Variation West, Magnetic Best: If the variation is West, the Magnetic heading is greater (best) than the True heading.
  • Variation East, Magnetic Least: If the variation is East, the Magnetic heading is smaller than the True heading.
  • The same applies to Deviation between Magnetic and Compass (Deviation West, Compass Best).

3. Heading vs. Track: The Effect of Wind

This is where flight physics comes into play. There is a big difference between where the aircraft is “pointing” and where it is actually “going.”

  • Heading: The direction the nose of the aircraft is pointing.
  • Track: The actual path the aircraft draws over the ground (Track Made Good).

If there were no wind, Heading and Track would be the same. But wind usually pushes the aircraft sideways.

  • Drift: The angle by which the wind deflects the aircraft from its heading.
  • Wind Correction Angle (WCA): To counteract drift, the pilot must turn the aircraft into the wind. If the wind pushes 10° to the right, the pilot must turn 10° to the left to stay on a straight line.

In addition, the wind is broken down into two components:

  • Crosswind: Pushes laterally (calculated using the sine of the angle).
  • Headwind/Tailwind: Affects the speed at which we move over the ground (calculated using the cosine).

4. In-Flight Corrections: The 1 in 60 Rule

Despite good planning, it is normal to drift off course. To return to the route, pilots use mental calculations based on geometry, known as the 1 in 60 Rule. This rule states that an error of 1 degree results in a deviation of 1 nautical mile after flying 60 miles.

When a pilot notices that they have deviated from the route, two values are calculated:

  1. Track Error Angle (TKE): How many degrees the aircraft has deviated from the original route.
  2. Closing Angle: How many additional degrees the pilot must turn to re-intercept the route before reaching the destination.

The sum of these angles indicates how much the heading must be corrected to safely reach the destination.

5. Limitations of the Compass

Finally, it is important to know that the compass is not perfect. It works by detecting the horizontal component of the Earth’s magnetic field. Near the Magnetic Poles, the field lines enter the Earth vertically (Dip), making the horizontal force very weak. For this reason, at polar latitudes the compass becomes useless, and navigation must be carried out using true or inertial references.