What causes the greatest compass error on long easterly and westerly cross-country flights?

The greatest compass error on long easterly and westerly cross-country flights is primarily caused by variation. Variation refers to the angle between magnetic north and true north, which varies depending on your geographical location.

During these types of flights, as a pilot travels along long east-west routes, the magnetic compass may be influenced significantly by this difference, leading to potentially substantial errors in navigation if not adequately accounted for. Unlike other factors such as deviation, which occurs due to interference from electrical equipment or magnetic fields in the aircraft, variation is a natural phenomenon related to the Earth's magnetic field itself and hence has a more consistent impact over long distances traveled east or west.

Understanding the concept of variation is crucial for ensuring accurate navigation and course plotting, particularly in the east-west flight corridors where the discrepancies can lead to navigational errors.