The compass is subject to acceleration/deceleration and turning errors. Therefore, you can only have a precise indication in straight and level flight.
The magnet inside the compass is a pendulous system and is attached to the aircraft at the pivot point. When the aircraft accelerates on a westerly/easterly heading, the magnet will lag behind due to its inertia as the acceleration will act on the magnet’s center of gravity. Therefore, it will momentarily indicate a turn and the heading indication will erroneously change.
There will be no error when accelerating/decelerating in a southerly/northerly direction as this will take place in the direction of the Earth’s magnetic flux lines. In the same way, the error will become increasingly higher as you change your heading from a south/north direction towards the east/west. In other words, the acceleration error will be the highest on the easterly/westerly headings.
An easy way to remember the acceleration errors is by using the acronym ‘ANDS’:
- AN – Accelerate = apparent turn to the North.
- DS – Decelerate = apparent turn to the South.
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