CDI – Course Deviation Indicator
A Navigation Instrument that indicates the course and direction to a tuned VOR station
It consists of (1) omnibearing selector (OBS) sometimes referred to as the course selector, (2) a CDI needle (Left-Right Needle), and (3) a TO/FROM indicator. The course selector is an azimuth dial that can be rotated to select a desired radial or to determine the radial over which the aircraft is flying. In addition, the magnetic course “TO” or “FROM” the station can be determined.
When the course selector is rotated, it moves the CDI or needle to indicate the position of the radial relative to the aircraft. If the course selector is rotated until the deviation needle is centered, the radial (magnetic course “FROM” the station) or its reciprocal (magnetic course “TO” the station) can be determined. The course deviation needle also moves to the right or left if the aircraft is flown or drifting away from the radial which is set in the course selector.
By centering the needle, the course selector indicates either the course “FROM” the station or the course “TO” the station. If the flag displays a “TO,” the course shown on the course selector must be flown to the station. If “FROM” is displayed and the course shown is followed, the aircraft is flown away from the station.