Which type of airspace requires ATC clearance for all aircraft?

Class A airspace is the type that requires ATC clearance for all aircraft. Class A airspace extends from 18,000 feet mean sea level (MSL) to 60,000 feet MSL, covering the airspace where the majority of high-altitude, high-speed flight occurs. This airspace is designed for instrument flight rules (IFR) operations, and all aircraft operating within it must be on an IFR flight plan and maintain communication with air traffic control (ATC). The requirement for ATC clearance ensures that all aircraft are properly sequenced and managed, maintaining safe separation and efficiency within the busy skies.

In contrast, Class D airspace requires ATC clearance only when an aircraft is operating within the airspace around an airport with an operational control tower, but does not apply to all flights in the vicinity. Class E airspace has variable requirements that may not necessitate ATC clearance for certain operations, especially below 10,000 feet. Class G airspace is uncontrolled airspace, meaning it does not require ATC clearance for operations, allowing pilots more freedom to conduct visual flight rules (VFR) operations without constant ATC communication.