Refer to figure.
A Course Deviation Indicator (CDI) is an instrument that helps determine the position to the aircraft relative to the course to or from a navigation beacon. The needle indicates where the aircraft is relative to the course. As seen in the figure, if the aircraft is to the right of the course, the needle will show a deflection to the left.
A to and from indication gives the pilot information on where the aircraft is relative to the station. The CDI does not take into account the heading. When the aircraft passes the station, the arrow automatically changes from a ‘to’ indication into a ‘from’ indication, indicating that the aircraft passed the station and is on the reciprocal of the course.
When looking at the compass rose it can be divided into 4 quadrants.
First quadrant: 0-90°
Second quadrant: 90-180°
Third quadrant: 180-270°
Fourth quadrant: 270-360°
If the aircraft flies on a 90° course to the station and then changes to a 91° course, the arrow will change from a ‘to’ indication to a ‘from’ indication. The aircraft is not flying towards the station anymore but from the station. This means that everytime the aircraft changes from quadrant, the arrow will shift to the opposite indication.
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