What is the maximum allowable CDI error when checking the VOR or TACAN at a designated ground checkpoint?

The maximum allowable course deviation indicator (CDI) error when checking a VOR or TACAN at a designated ground checkpoint is ± 4°. This standard ensures that the navigation instruments are accurate enough for safe flight operations. A CDI error within this range indicates that the system is functioning correctly and that the pilot can rely on the navigation information being presented.

The significance of this specific error margin pertains to the required precision for navigating safely and reliably within controlled airspace and during instrument flight rules (IFR) operations. Such accuracy is critical for maintaining situational awareness and ensuring that aircraft are on the correct flight path, particularly during approaches and during operation in congested airspace.

Ensuring that the CDI error does not exceed this limit is crucial for both pilot safety and compliance with regulatory standards, which is why this error threshold is established.