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, the "top" of controlled airspace is officially 60,000 feet Mean Sea Level (MSL), also known as Flight Level 600. Controlled airspace is a layered system categorized into Classes A through E. Class A is the highest layer of standard controlled airspace, extending from 18,000 feet up to 60,000 feet; all aircraft in this zone must operate under Instrument Flight Rules (IFR). Above 60,000 feet, the airspace transitions back into Class E (Controlled), but it is rarely utilized by civilian traffic and is primarily the domain of high-altitude research balloons, specialized military aircraft, and commercial space ventures. This 60,000-foot ceiling is a critical boundary because it encompasses the "cruise" altitudes of almost all commercial airliners. Beyond this limit, the air is too thin for conventional wing lift and standard engine performance. Understanding these vertical boundaries is essential for pilots and air traffic controllers to ensure the safe separation of thousands of high-speed jets hurtling across the country at various altitudes.