Course Deviation Indicator
A course deviation indicator (CDI) is the instrument (or display element) that shows the aircraft’s lateral displacement from a selected VOR radial or localizer course. It consists of a vertical needle (the course deviation bar) that deflects left or right to indicate which side of the selected course the aircraft is on, and a TO/FROM indicator showing whether the course will take the aircraft toward or away from the navaid.
How to Interpret the CDI
- Needle deflected to the left: the selected course is to the left — turn left to intercept
- Needle deflected to the right: the selected course is to the right — turn right to intercept
- Full-scale deflection on a VOR: typically 10° off course; on a localizer, approximately 2.5° off course
- TO flag: flying the selected course will take the aircraft toward the station
- FROM flag: flying the selected course will take the aircraft away from the station
VOR vs. Localizer Sensitivity
The CDI is considerably more sensitive on a localizer approach than on a VOR. Full-scale deflection on a localizer represents only about 2.5° off course, compared to 10° on a VOR. For this reason, localizer approaches require more precise tracking than en route VOR navigation.