When decelerating on a westerly heading in the Northern Hemisphere, the compass card of a direct reading magnetic compass turns…
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.
An easy way to remember the acceleration errors is by using the acronym ‘ANDS’:
In order to visualize in which direction is the compass card turning, try to imagine a simple turn without considering the acceleration error. The magnet inside the compass case is always ‘North seeking’, so it will always remain in a steady position. The compass heading indications will change because the airplane is turning around the compass card and not the opposite. If you are on a westerly heading, and you turn to the South - the airplane will turn anticlockwise around the compass card. As a consequence, the opposite is true => we can say that the compass card has turned clockwise as the airplane made a turn towards the South.
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.
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.
In order to visualize in which direction is the compass card turning, try to imagine a simple turn without considering the acceleration error. The magnet inside the compass case is always ‘North seeking’, so it will always remain in a steady position. The compass heading indications will change because the airplane is turning around the compass card and not the opposite. If you are on a westerly heading, and you turn to the South - the airplane will turn anticlockwise around the compass card. As a consequence, the opposite is true => we can say that the compass card has turned clockwise as the airplane made a turn towards the South.
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