What is the primary function of the CDI in checking VOR systems?

The primary function of the Course Deviation Indicator (CDI) in checking VOR (VHF Omnidirectional Range) systems is to center the course deviation indicator. The CDI allows pilots to determine their position relative to the selected VOR radial and provides visual feedback on whether they are on course, left or right of the desired flight path. When using the CDI, the goal is to adjust the aircraft’s heading to keep the indicator centered, which signifies that you are directly on the selected radial. This capability is crucial for navigation and is essential for ensuring that the aircraft is following the intended route accurately, especially in instrument meteorological conditions (IMC).

The other choices reflect functionalities that are not directly related to the operation of the CDI in the context of VOR systems. Fuel efficiency measurement, airspeed calculation, and altitude determination each pertain to other aspects of flight instrumentation and navigation but do not involve the CDI's function of guiding course deviations. Thus, centering the CDI is the key action for pilots to ensure they are correctly using VOR for navigational purposes.