What does the slip indicator display?
Refer to figure.
The Slip Indicator consists of a ball immersed in a dampening liquid.
The center of the window is marked by two vertical lines spaced just wide enough apart for the ball to fit in between.
When the aircraft is flying level or the turn is coordinated, the ball lies between the lines (is centered) to indicate that the apparent vertical is parallel to the aircraft's yaw axis—the sum of all forces acts through the aircraft's normal/yaw axis.
If the ball is on the side of a turn, the aircraft is slipping, indicating that the turn is not coordinated. Conversely, when the ball is on the outside of the turn, the aircraft is skidding, suggesting an over-coordinated turn.
Therefore, the correct answer is "Apparent vertical."
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Austro Control