October 2003Departments

Flying Smart : What It Looks Like

Turn coordinator, turn indicator

Do you fly with a turn coordinator, or a turn indicator? If you fly different airplanes, you may well fly with each. It's easy to tell which is which. Most turn coordinators have a head-on view of an airplane to indicate rate of turn (below left). A turn indicator, also known as a turn and bank, slip and skid, or needle and ball, usually has a simple vertical bar or needle to indicate rate of turn (below right). Though an older design, turn indicators still are found in many light aircraft.

Both instruments show rate of turn (based on the standard rate of turn index marks, found at either side of wings-level), but only the turn coordinator shows roll. That's because of the way the internal gyro is mounted in each. In the older turn indicator, the gyro is set in a gimbal, which is aligned with-and pivots about-the airplane's longitudinal axis. When the airplane turns about the yaw axis, gyroscopic precession causes the gyro and gimbal to tilt. That tilt is displayed as a displacement of the needle in the direction of the turn.

In a newer turn coordinator, the gyro/gimbal mechanism is canted up at about 30 degrees with respect to the airplane's longitudinal axis. That makes it sensitive to changes in both yaw and roll.

The difference can be subtle in flight-unless the turn coordinator or indicator becomes the primary attitude instrument because the attitude indicator has failed in instrument meteorological conditions. In that case, the turn coordinator is preferable because its sensitivity to both roll and yaw makes it easier for the pilot to maintain a wings-level attitude.

It's important to remember that neither instrument shows angle of bank or pitch attitude.

The names of the instruments are misleading. Both can be used to check control coordination in a turn, because both have an inclinometer-a small metal ball encased in a curved, fluid-filled tube in the lower part of the instrument face. The inclinometer shows direction of G-forces. In a nicely coordinated turn, the ball will stay planted between the center hash marks on the tube. Too much rudder (yaw) results in a skidding turn, displacing the ball to the outside, while too little yaw-a slipping turn-displaces the ball to the inside of the turn.