Final answer:
The correction from True heading to Magnetic heading is called 'variation' or 'magnetic variation'. On the equator with aligned geographic and magnetic north poles, compass direction remains unchanged when flying due west. Otherwise, needle orientation would vary according to how magnetic latitude lines angle away from true north.
Step-by-step explanation:
The correction needed to go from a True heading to a Magnetic heading is called 'variation' or 'magnetic variation'. This adjustment compensates for the difference between the true north (geographic north pole) and the magnetic north (where the Earth's magnetic field points). In an airplane flying due west along the equator, assuming the magnetic north pole coincides with the geographic north pole, the needle of a compass will always point straight ahead if you're flying straight west, staying perpendicular to your direction of motion. This is because both poles align, and you are moving parallel to the lines of magnetic latitude that emanate from these poles.
If the Earth's magnetic north pole were not in the same location as the geographic north pole, the compass needle direction would change as you move along different magnetic latitudes, which are curved lines emanating from the magnetic poles. The compass is a device with a magnetized needle that aligns itself with the Earth's magnetic field, indicating the direction to the magnetic north pole. Due to the Earth acting like a giant bar magnet with magnetic field lines that always point from the magnetic north pole to the magnetic south pole, the north pole of the compass needle (which is actually a magnetic south pole) is attracted to align with the magnetic field of the Earth.