CDI stands for which instrument?

• A. Course Direction Indicator
• B. Course Deviation Instrument
• C. Course Display Indicator
• D. Course Deviation Indicator

Answer: (D)

The main idea here is understanding what the navigation instrument tells you about your track. The CDI, or Course Deviation Indicator, shows how far off you are from the selected navigation course. You set a course with the OBS, and the CDI needle deflects left or right to indicate deviation from that course. When the needle centers, you’re on course, and you use that feedback to steer back onto the intended path.

This is used with VOR, localizer (LOC), or ILS approaches—the CDI visually guides you toward the correct bearing by indicating cross-track error. The center position is your target; the further the deflection, the more off course you are, so you adjust heading to bring the needle back toward center.

The other listed phrases aren’t the standard term for this instrument. It’s called a Course Deviation Indicator because it specifically indicates deviation from the selected course, whereas terms like Course Direction Indicator or Course Display Indicator don’t convey that deviation feedback in the aviation navigation context. The wording with Instrument instead of Indicator isn’t the conventional name, even though it might sound similar.