What is an airport called that is approved for use by an air carrier certificate holder when the regular airport is unavailable?

The term for an airport that is approved for use by an air carrier certificate holder when the primary or regular airport is unavailable is defined as an "alternative airport." This designation is crucial for ensuring that aircraft have a predetermined location to divert to in case of emergencies, operational difficulties, or other unforeseen circumstances at the main airport intended for landing.

Alternative airports are included in flight planning as a standard operational procedure. They must meet specific regulatory and safety requirements to be deemed suitable for use during unforeseen situations, ensuring adequate services and facilities for the arriving aircraft and its passengers.

In contrast, the other options do not accurately represent this concept. An "emergency airport" may imply an unscheduled landing due to unforeseen circumstances but does not specifically denote an approved alternate location for regular operations. A "provisional airport" is not a recognized term in the context of air carrier operations and may lead to confusion regarding its official status. A "designated airport" typically refers to airports recognized for particular purposes but is not specifically tied to the operational flexibility required when a regular airport is unavailable. Thus, alternative airport is the correct terminology used in aviation regulations that align with operational protocols for air carriers.