Course deviation indicator
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A course deviation indicator (CDI) is a vertical needle, generally referred to as "the needle". It is one of two indicators that make up the Omni Bearing Indicator (OBI), which is more commonly referred to as the "VOR Indicator", "Omni Indicator", or most preferably, "VOR Head". The other indicator is the "TO/FROM indicator". The VOR head also has an Omni Bearing Selector (OBS) which indicates the direction of radial the pilot has selected. The VOR Head, in turn, is the display component of the VHF Aircraft Navigation Radio Receiver, usually separate from the tuner component. The VHF Aircraft Navigation receiver is called "the NAV radio" and receives VORs and Localizers. It outputs sound: the identifier, and in some cases, recorded or live voice. It also selects channels for UHF receivers (Glide Slope and Distance Measuring Equipment or DME) if the aircraft is equipped with them.
Use
The indicator shows the direction to steer to correct for course deviations. Correction is made until the vertical needle centers, meaning the aircraft has intercepted the given courseline. The pilot then steers to stay on that line. Only the receiver's current position determines the reading: the aircraft's heading is not indicated; heading change without position change does not affect the indication. When "reverse sensing", the indicator deflects in the opposite direction (right or left) for zero course deviation. For example,a left deflected needle when forward sensing indicates the selected radial is to the left, and when reverse sensing, the selected radial is to the right.
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, which should be called a "Radial" but has also been called a "Bearing" or "Course", is selected by turning an "omni bearing selector" or "OBS" knob usually located in the lower left of the indicator. The CDI then shows the number of degrees deviation between the aircraft's current position and the "radial" line emanating from the signal source. This can be used to find and follow the desired radial. Deflection is 10° deviation 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 localizer approach, localizer type directional aid (LDA)approach, or instrument landing system (ILS)approach, the OBS knob has no function because the courseline is the runway heading (unless it is an LDA approach), and is determined by the ground transmitter. A VOR head might include a Glide Slope Indicator (GSI), which is a horizontal needle to provide vertical guidance when used with a precision ILS approach where the glideslope is broadcast by another transmitter located on the ground.
A CDI is not used with an automatic direction finder (ADF), which receives information from a normal AM radio station or an NDB. A Radio Magnetic Indicator (RMI), however, can serve as two VOR heads, two ADF indicators, or one of each. It also gives aircraft heading information obtained from a gyrocompass or flux gate compass.
Operation
The CDI was designed to interpret a signal from a VOR, Localizer, LDA, or ILS transmitter. These transmitters output a signal composed of two AC voltages. A converter decodes this signal, and, by determining the desired heading or radial from a resolver connected to the OBS knob, provides a 150mV control signal to drive the CDI needle left or right. Most older units and some newer ones integrate a converter with the CDI. CDI units with an internal converter are not compatible with GPS units. More modern units are driven by a converter that is standalone or integrated with the radio. The resolver position is sent to the converter which outputs the control signal to drive the CDI. For digital units, the desired position of the needle is transmitted via a serial ARINC 429 signal from the radio or GPS unit, allowing the CDI design to be independent of the receiver and by multiple receiver types.