At what altitude does Class A airspace begin?

Class A airspace begins at an altitude of 18,000 feet mean sea level (MSL). This classification of airspace is designed for high-altitude operations and is applicable in the United States for all aircraft that are operating under instrument flight rules (IFR).

The significance of Class A airspace starting at 18,000 feet lies in its role in ensuring the safety and separation of aircraft flying at these higher altitudes. By regulating air traffic in this space, it enables air traffic controllers to provide comprehensive guidance and management for the busy airways that exist above this altitude, which are commonly traversed by commercial airliners and other aircraft operating at cruising altitudes.

Airspace classifications such as Class B, C, and D operate at lower altitudes and are structured differently to manage traffic in those respective environments. Thus, recognizing the altitude where Class A begins is crucial for pilots and those in air traffic management to understand the operational requirements and responsibilities associated with flying within these airspace boundaries.