Which types of airspace require an ATC clearance prior to entry?

Class A and B airspace require an ATC clearance prior to entry due to the density and complexity of air traffic within these classifications. Class A airspace is located from 18,000 feet MSL up to and including FL 600, and it is all under positive control, meaning that all aircraft operating in this airspace must be in communication with air traffic control. Similarly, Class B airspace surrounds major airports, extending from the surface to a designated altitude, and serves to manage traffic in areas with high volumes of both commercial and general aviation. Pilots must obtain a clearance from ATC before entering this airspace to ensure safe separation between aircraft.

This requirement is crucial for maintaining safety and efficiency in areas with potentially congested air traffic. In contrast, while Class C and D airspace also requires communication with ATC, a formal clearance is not always necessary; Class C requires establishing and maintaining two-way communication rather than a specific clearance. Meanwhile, Class E airspace typically does not require ATC clearance unless specified by the conditions of flight operations. Understanding the distinctions between these airspace classifications, especially regarding the need for ATC clearance, is essential for pilots navigating through the national airspace system.