The turn-and-slip indicator gives the pilot a visual indication of coordinated flight. It consists of a turn needle and a ball suspended in fluid. If the ball is centered, the aircraft is in coordinated flight.If the ball is displaced in the same direction as the turn, the aircraft is in a slip. If the ball is displaced in the opposite direction as the turn, the aircraft is in a skid.
Whenever the aircraft becomes uncoordinated during
flight, the corrective action is to alter the amount of
rudder being used. This simply means to apply rudder in
the direction the ball is displaced. Therefore, if the ball is
displaced to the right, apply right rudder. A useful
mnemonic device for proper rudder correction is “step on
the ball.” The amount of rudder necessary will depend on the amount of adverse yaw.
A skid is caused by using too much rudder in the desired direction of turn. The yawing 
movement is toward the inside of the turn and the balance ball is deflected toward the outside due to centrifugal force. In a skid, turn radius will decrease and turn rate will increase. Skids are dangerous because the airplane will roll inverted if stall occurs (a skidded turn stall).
A slip is caused by insufficient rudder in the desired direction of turn. The yawing movement is toward the outside of the turn, and the balance ball is deflected toward the inside, due to gravitational pull. In a slip, turn radius will increase and turn rate will decrease. Slips are useful for crosswind landings (commonly described as “wing down, top rudder”), or when trying to increase the airplane rate of descent without increasing airspeed. A stall while in a slip will cause the airplane to roll toward wings level (a safer reaction than in a skid).
