The Earth = a Giant Magnetic Dipole

Dipole, two poles

This dipole extends far into space (the magnetosphere), but is strongest near the surface.

Now, because it’s a dipole, its field obeys:

Magnetic field strength ∝ 1 / r³

so when you're flying in a plane

You’re increasing your distance from the source of Earth’s magnetic dipole

IT goes INVERSE CUBE LAW