Course deviation indicator
Overview
 
Course Deviation Indicator (CDI) is an avionics
Avionics
Avionics are electronic systems used on aircraft, artificial satellites and spacecraft.Avionic systems include communications, navigation, the display and management of multiple systems and the hundreds of systems that are fitted to aircraft to meet individual roles...

 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.
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.
 
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