Course deviation indicator

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CDI, or Course Deviation Indicator, is a cockpit display that consists of an OBS ring, an OBS knob, and a left/right needle, along with a TO/FROM flag.

The OBS ring is a movable ring around the CDI instrument calibrated in 360 degrees for the full circle, and is rotated by the pilot twisting the OBS knob on the instrument case. The receiver then compares the OBS setting to the received VOR signal to adjust the left/right needle and TO/FROM flag to indicate to the pilot what radial the aircraft is currently on. It reads the radial directly on the OBS when the needle is centered and the flag reads FROM, which is also the course to go directly away from the VOR station. With a TO flag and centered needle, the OBS reads 180ยบ from the radial, corresponding to the course to go directly to the VOR station.

See also the course deviation indicator page on Wikipedia.

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