Course deviation indicator

Correction is made until the vertical needle centres, meaning the aircraft has intercepted the given course line.

Only the receiver's current position determines the reading: the aircraft's heading, orientation, and track are not indicated.

The deflection of the needle is proportional to the course deviation, but sensitivity and deflection vary depending on the system being used: A CDI is not used with an automatic direction finder (ADF), which receives information from a normal AM radio station or an NDB.

The CDI was designed to interpret a signal from a VOR, LDA, or ILS receiver.

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.