What altitude must pilots be aware of to ensure compliance within Class A airspace?

In Class A airspace, pilots must operate at or above an altitude of 18,000 feet Mean Sea Level (MSL). This altitude is significant because it marks the beginning of controlled airspace where aircraft are required to be on an instrument flight plan. Class A airspace extends from 18,000 feet MSL up to and including 60,000 feet MSL, necessitating that all aircraft operating within this airspace maintain a continuous working radio communication with air traffic control.

Understanding the importance of this altitude is crucial for ensuring safe operations, as Class A airspace encompasses high-altitude flight where air traffic is dense and requires significant coordination to avoid collisions. Therefore, recognizing 18,000 feet MSL as the critical altitude for compliance in Class A airspace is essential for pilots to maintain safety and adhere to regulatory requirements.