What causes the northerly turning error in a magnetic compass?

The northerly turning error in a magnetic compass is primarily caused by the magnetic dip characteristic. This phenomenon occurs because the magnetic compass relies on the Earth's magnetic field for orientation, which becomes more pronounced as the compass approaches the magnetic poles.

When an aircraft is turning from a northerly heading, the magnetic compass tends to lag behind due to the vertical component of the magnetic field that pulls the compass needle downward toward the Earth's surface. This results in an initial indication that is behind the actual turn, causing a momentary error where the compass shows a heading that is less than the actual heading. As the turn continues and the compass stabilizes, it eventually catches up and indicates the correct heading, but this delay is what gives rise to the northerly turning error.

Understanding this characteristic of the magnetic compass is crucial for pilots, especially when navigating in areas with strong magnetic influences or when making sharp turns, as it helps mitigate the risks associated with navigating based on inaccurate compass readings.