Image: VOR Course Deviation Indicator explanation

Description: CDI is an air navigational device which indicates deviation of a selected VOR radial. The reading of this information needs explanation. On the gauge you see radial 252 as selected, this is the direction from the VOR station, always based on magnetic North. The deviation of the magnetic North at this place is 15 degrees to the East, for the geographic North. You select the number of the radial with turning OBS knob, the position of the needle and FR/TO indication is made by the gauge. The scale if usually marked by 2 degrees. On the position (1) you are on the radial 252, and the direction flag marks FR. This means if you are flying on this radial, with heading 252 then you are exactly on the course away FRom the VOR station. If you don't then you are just crossing this radial. This indication says nothing more, especially about your current heading and path. Some combined CDIs look like a compass, don't confuse the real informations. In situation (2) and (3) if you fly away FRom the station then the CDI's needle shows the direction and distance of the 252 radial for you. If you fly towards the station then the needle "lies", indicates direction and distance of you for the radial. If you want to find the proper heading to fly over a VOR station then turn "OBS" knob until you see TO mark and the needle moves into the center. Read the radial's number and turn your plane into that direction, go ahead. Air routes are often constructed of VOR to VOR segments. Airplanes ussually have two NAV receivers, two CDIs. If you find the current radial of you from another VOR station, then you can point your position on the map, at the crossing of the radial lines. If you are wide away from the selected radial then the striped flag (5) warns you that you are out of the segment where meaningful indication can be done. Turn OBS if you want to move the segment around the station towards you. On the position (6) you are on the back course of radial 252. You see that if you fly with heading 252 degrees then you fly exactly TO the station. At (7) and (8) you are close to the back course radial, and if you fly towards the station then the needle indicates the direction and distance of the back course, the radial 072, which is 180 degrees subtracted from 252. Summerized: if you fly roughly by the direction indicated by TO/FR mark the the needle shows you the place of the selected radial, at the other case the needle shows contrary situation.
Title: VOR Course Deviation Indicator explanation
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Author: Orion 8
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