The 4000C/D is an air-driven directional gyro with A.C. autopilot pickoffs. Left turns produce clockwise dial rotation and right turns produce counter-clockwise rotation. Two knobs control the gyro. The left knob is used to cage the unit, and the right knob is used to select the instrument’s heading. To change course, the right selector knob is pressed and turned to disengage the dial shaft and set the pointer to a new heading. Releasing the selector knob re-engages the pointer with the dial shaft. As the aircraft turns onto a new course, the pointer returns to its original position. The on-course position of the pointer is always at the top of the azimuth circle (0° on the dial with the aircraft headed north).