Generally, that airspace from the surface to 10,000 feet MSL surrounding the nation's busiest airports in terms of IFR operations or passenger enplanements.

In the United States, "controlled airspace" generally extends from the surface or a designated floor up to and including 60,000 feet (Flight Level 600). The most common upper limit for standard "Class A" controlled airspace—where all pilots must fly under Instrument Flight Rules (IFR)—is 18,000 feet MSL up to 60,000 feet. Above Flight Level 600, the airspace becomes "Class E" again, but it is effectively uncontrolled for most civilian operations because very few aircraft are capable of sustaining flight at that altitude. In 2026, with the rise of commercial spaceflight and high-altitude balloons, the FAA and global aviation bodies are increasingly monitoring the "stratospheric" layers above FL600, but for standard general and commercial aviation, the "ceiling" of strictly controlled, high-density traffic ends at the 60,000-foot mark. Below the Class A layer, controlled airspace exists in various tiers (Class B, C, D, and E) with varying floors, but they all eventually merge into the Class A structure that blankets the country once you climb above 18,000 feet.