Slip and Skid Indication

Principle of Slip and Skid Indication

The slip and skid indicator, often called the slip skid ball, is a simple yet vital part of the turn and slip indicator or turn coordinator. It consists of a ball in a liquid-filled, curved tube. The ball responds to the combined effects of gravity and centrifugal force during turns. In straight and level flight, or a properly coordinated turn, the ball remains centered.

Interpreting the Ball

• Centered Ball: Indicates a balanced or coordinated turn. The forces acting on the aircraft are aligned with the vertical axis, so no correction is needed.
• Ball to Inside of Turn (Slip): The aircraft is slipping; the bank angle is too great for the amount of rudder applied. More rudder in the direction of the turn is needed.
• Ball to Outside of Turn (Skid): The aircraft is skidding; there is too much rudder for the bank angle. Less rudder or more bank is required.

Instrumentation and Operation

The turn indicator or turn coordinator uses a gyroscope to display the rate of turn. The slip skid ball complements this by showing turn coordination. Modern electronic displays may use a bar or graphical symbol, but the principle remains the same. The slip indication can be derived from inertial reference systems in advanced aircraft.

Bank Angle, Rate of Turn, and TAS

At higher true airspeeds, a greater bank angle is needed to achieve the same rate of turn. However, at high speeds, bank angle becomes the limiting factor due to structural and comfort constraints. The slip and skid indication remains essential for ensuring turns are coordinated regardless of speed.

Practical Use

Pilots use the slip skid ball to keep the ball centered during turns, especially in instrument flight or poor visibility. This prevents uncoordinated flight, which can lead to increased drag, passenger discomfort, or even loss of control.