Course Deviation Indicator

Course Deviation Indicator (CDI) is an avionics instrument used in aircraft navigation to determine aircraft position relative to a course or navigation aid.

When used with a GPS it shows actual distance left or right of the programmed courseline. When used with a VOR or VORTAC the courseline is selected by turning an "OMNI Bearing Selector" or "OBS" knob usually located in the lower left of the instrument. It then shows the number of degrees deviation from the desired course to the Navigational aid (navaid), and is used to intercept and fly TO or FROM any of the 360 compass "radials" that emanate from the navaid. (See Using a VOR for usage during flight.) When used for instrument approaches using a LDA or ILS the OBS knob does not function, since the courseline is usually the runway heading, and is determined by the ground transmitter. Many CDI's also incorporate a second, horizontal, needle. This is used to provide vertical guidance when used with a precision ILS approach, and the decent courseline (usually 3 degrees) is also determined by the transmitter located on the ground. A CDI is normally not used with an ADF, which receives information from a normal AM radio station or an NDB. An ADF indicator or Radio Magnetic Indicator (RMI) is used instead, both of which provide direction or heading information. Older CDI's were designed to receive a signal from a VOR, LDA or ILS receiver. These receivers outputted a signal composed of two AC voltages. An internal converter converted the signal to drive the needle left or right. These CDI units are not compatible with GPS units. Many modern VOR/LDA/ILS receivers include the converter, and output a differential voltage to drive a "converterless" CDI. The CDI's without a converter are compatible with aviation GPS units.