Definition
The navigation aid that defines a specific segment of an instrument approach procedure and provides the lateral guidance the pilot must follow on that segment. On a published approach, the primary NAVAID is the facility (such as a VOR, NDB, localizer, or a designated GPS waypoint) on which the final approach course, intermediate segment, or missed approach is built. Aircraft equipment must be able to receive and use the primary NAVAID, and the facility must be operational, for the procedure to be flown.
Plain English
It's the main navigation source that an instrument approach is built around. If that source isn't working or your aircraft can't receive it, you can't fly the approach.
Context Anchor
Seen on instrument approach charts and in instrument procedure discussions, especially when identifying which navigation source the procedure is built around.
Derivation
Primary comes from the Latin primus, meaning 'first' or 'foremost.' Here it points to the navigation aid that takes priority for that segment of the approach -- the one the procedure is actually built on, not a backup or supporting facility.
Why Pilots Care
The primary NAVAID determines the published minimums, the final approach course, and the point at which the pilot must decide to land or go around.
Intuition Check
Primary does not mean “the navigation source you prefer.” Here it means the navigation aid designated by the published procedure as the main controlling reference.
Example Sentence 1
Before starting the approach, the crew checked NOTAMs and confirmed the primary NAVAID was in service.
Example Sentence 2
When the primary NAVAID is out of service, the approach is not authorized and the pilot must use an alternate procedure.