Which identifier is required before the three-letter identifier for certain airports?

The three-letter identifier for certain airports in the United States is prefixed with the letter "K." This prefix is a standard convention used to denote that the airport is located in the U.S. and is important for aviation communication and navigation. For example, Los Angeles International Airport is identified as "KLAX," where "K" indicates it is a U.S. airport, and "LAX" is the specific three-letter identifier for that location.

This standardization helps pilots, air traffic controllers, and other aviation personnel quickly recognize the location's country and manage flight operations effectively. Identifiers without this prefix may belong to airports in other countries or may not be recognized within the U.S. airspace. Understanding how these identifiers work is essential for effective communication within the aviation system.