During a VOR ground check, how close must the CDI be centered within the radial?

The ground check for a VOR (VHF Omnidirectional Range) is a critical procedure for ensuring the accuracy of the VOR navigational system before flight. The standard requirement is that the Course Deviation Indicator (CDI) should be centered within 4 degrees of the selected radial. This means that during the ground check, when a VOR receiver is tuned to a known VOR station, the CDI must show a centered indication when the aircraft is precisely on that radial.

Maintaining alignment within this tolerance is crucial because any deviation beyond this range could lead to navigation errors during flight. A centered CDI within 4 degrees indicates that the VOR system is functioning correctly and the pilot can trust the navigational information provided.

The other options suggest larger tolerances, which do not meet the FAA requirements for VOR accuracy. A standard of 2 degrees or less might be suitable for some applications, but the FAA specifically calls for a 4-degree check for VORs to ensure navigational safety. Therefore, the choice of 4 degrees is established as the correct and accepted standard in aviation practice.