Course Deviation Indicator (CDI) is an avionics instrument used in aircraft navigation to determine an aircraft's lateral position in relation to a track. If the location of the aircraft is to the left of course, the needle deflects to the right, and vice versa.
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The instrument shows the direction to steer to correct for course deviations. Correction is made until the needle centers, and the aircraft is on course. The pilot then resumes the course heading. The deflection of the needle is proportional to the course deviation, but sensitivity and deflection vary depending on the system being used.
When used with a GPS it shows actual distance left or right of the programmed courseline. Sensitivity is usually programmable or automatically switched, but 5 nautical miles (9.3 km) deviation at full scale is typical for en route operations. Approach and terminal operations have a higher sensitivity up to frequently .3 nautical miles (0.56 km) at full scale.
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. Deflection is 10° at full scale, with each dot on the CDI representing 2°. (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 descent courseline or glideslope (usually 3 degrees) is also determined by a transmitter located on the ground.
A CDI is normally not used with an Automatic Direction Finder (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.
CDI's were originally designed to receive a signal from a VOR, LDA or ILS receiver. These receivers outputted a signal composed of two AC voltages. A converter decoded this signal, and by determining the desired heading or radial from a resolver connected to the OBS knob, provided a control voltage to drive the needle left or right. Most of the older units, and many newer ones, contain the converter in the CDI. Generally, CDI units with an internal converter are not compatible with GPS units.
More modern units generally provide the converter within the radio, although it occasionally is a separate unit. In either case, the resolver position is sent to the converter, and the converter outputs a 150mv control signal to the CDI to drive the needle left or right. Most recently, the desired position of the needle is transmitted via a serial ARINC 429 signal from the radio or GPS unit, making the CDI design independent of the radio or GPS type.
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