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TURN-AND-SLIP INDICATOR
The turn-and-slip indicator (right) provides the only information of either wing's level or bank attitude if the other gyroscopic instruments should fail. This indicator is sometimes called the "needle and ball". This instrument, along with the airspeed indicator, magnetic compass and altimeter, can assist the pilot in flying through instrument weather conditions, even when it is the only gyro instrument operating.
The turn needle of the turn-and-bank indicator gives an indirect indication of the bank attitude of the aircraft. When the turn needle is exactly centered, the aircraft is in straight flight. When the needle is displaced from center, the aircraft is turning in the direction of the displacement. Thus, if the ball is centered, a left displacement of the turn needle means the left wing is low and the aircraft is in a left turn. Return to straight flight is accomplished by coordinating aileron and rudder pressures.
The ball of the turn-and-bank indicator is actually a separate instrument, conveniently located under the turn needle so the two instruments can be used together. This instrument is best used as an indication of attitude. When the ball is centered within its glass tube the maneuver is being executed in a coordinated manner. However, if the ball is out of its center location, the aircraft is either slipping or skidding . The side to which the ball has rolled indicates the direction of the slip or skid.
In a slip, the rate of turn is too slow for the angle of bank, and the lack of centrifugal force causes the ball to be displaced to the inside of the turn. (To correct, decrease the angle of bank, or use rudder to increase the rate of turn, or both). In a skid the rate of turn is too fast for the angle of bank, and excessive centrifugal force causes the ball to be displaced to the outside of the turn. (To correct, increase the bank angle, or use rudder to decrease the rate of turn, or both).
In coordinated flight, the needle may be used to measure the rate of turn; in a "standard rate turn", the needle is aligned with the left or right marker (dog-house) and the aircraft will turn at the rate of 3° per second or 180° in one minute. Hence, in these conditions, the needle indicates both direction and rate of turn.
The answer to controlling and trimming an aircraft in straight and level flight by means of the turn-and-bank indicator requires a return to basic control principles - i.e., control yaw with the rudder and keep the wings level with aileron. Therefore, when flying straight and level through the use of the turn-and-bank indicator, prevent yawing with appropriate rudder pressure, and keep the wings level with appropriate aileron pressure. The needle will not deflect while heading is constantly maintained, since no turn exists.
In other words, control the ball with rudder since the ball moves parallel to a plane passing through the rudder pedals, and control the needle with aileron since the ailerons affect bank angle, a primary requirement for a normal turn.
It is important that both the needle and ball are used together. The problem associated with using these instruments separately is that although the ball will positively indicate that the aircraft is slipping or skidding, just which one of these the aircraft is doing can only be determined by reference to the needle. Furthermore, the needle will not positively indicate a bank attitude. An aircraft could be in a bank attitude and yet the needle could remain centered or indicate a turn in the opposite direction, if controls are not coordinated.
