In VOR mode, what does the CDI left/right needle indicate?

The Course Deviation Indicator (CDI) in VOR mode is designed specifically to indicate the aircraft's deviation from a selected radial. When the CDI needle points to the center, it means the aircraft is exactly on the chosen radial. If the needle is deflected to the left, it signifies that the aircraft is off course to the left of that radial, and conversely, if the needle is deflected to the right, the aircraft is off course to the right of the radial.

Understanding this concept is crucial for navigating effectively using VOR aids. Pilots rely on the CDI to make timely adjustments to their heading in order to intercept and fly along the selected radial. This aspect of navigation helps ensure that the aircraft remains on course during flight, particularly under instrument flight rules where visual references may be limited.

The other options do not relate to the function of the CDI in VOR mode. Airspeed, heading, and vertical speed are monitored using different instruments that do not provide information about the aircraft's position relative to a navigational radial. Thus, the indication of radial alignment is the key function of the CDI needle.