What is the purpose of the slip/skid indicator in the instrument system?

• A. To show whether the aircraft is in coordinated flight; the ball should be centered for coordinated flight.
• B. To indicate bank angle.
• C. To show vertical speed.
• D. To indicate turn rate.

Answer: (A)

The slip/skid indicator is all about keeping flight coordinated. It uses a small ball that slides in a curved tube to sense sideways (lateral) acceleration. When the airplane is in coordinated flight, the lift, weight, and centripetal forces balance in the turn, so there’s no sideways force on the airplane and the ball sits centered. If you’re not coordinating properly—perhaps you’re banked but not enough rudder, or you’re applying too much rudder—the ball moves to one side, indicating a slip (ball to the inside of the turn) or a skid (ball to the outside). The pilot uses rudder and aileron to bring the ball back to the center, restoring coordinated flight.

This instrument doesn’t show bank angle (that’s the job of the attitude indicator), vertical speed (that’s the vertical speed indicator), or turn rate (that’s shown by the turn indicator/turn coordinator). Its purpose is specifically to reveal whether the turn is coordinated by centering the ball.