Plain English Meaning

It is the piece of equipment that sends out the signal a pilot follows to line up with the runway and descend safely when flying on instruments. If that equipment is not working, the approach that depends on it cannot be flown.

Full Definition

An approach NAVAID is a ground-based or space-based facility that provides the guidance signal used to fly a published instrument approach procedure to a specific runway or airport. Examples include an ILS, localizer, VOR, NDB, or GPS/WAAS signal that defines the final approach course and, where applicable, vertical guidance. In the IFR alternate-requirements context, the term refers to the specific NAVAID that supports the a pilot intends to use at the , and that NAVAID must be forecast to be and usable at the .