turn and bank indicator, aircraft instrument containing one indicator to show turning, or rotation about the vertical axis, and another to show banking, or rotation about the longitudinal axis. The two indicators are essentially separate instruments, but they are customarily placed together. The bank indicator is the simpler of the two and consists of a curved glass tube filled with a damping liquid in which a small steel ball rolls. When the craft is horizontal, the ball is located in the lowest part of the tube; as the craft banks, gravity holds the ball at the lowest point as the tube rotates from side to side. The tube can be calibrated to show the angle of banking. The turn indicator contains a gyroscope that develops a torque when the craft rotates. This torque controls a pointer that indicates to the pilot in degrees per unit of time the rate at which the craft is turning.