What must pilots maintain when operating in Class A airspace?

When operating in Class A airspace, pilots are required to maintain active communication with Air Traffic Control (ATC). This is crucial as Class A airspace is defined from 18,000 feet MSL to Flight Level 600, where all aircraft must operate under Instrument Flight Rules (IFR). Active communication ensures that pilots receive necessary instructions from ATC, which is essential in managing the high volume of traffic and ensuring safe separation between aircraft in this controlled environment.

Class A airspace does not allow for visual flight rules (VFR) operations, so options related to VFR visibility and cloud clearance, as well as operating under visual flight rules, do not apply. Furthermore, there is no altitude requirement specific to Class A that is higher than that of Class B airspace, as both have distinct altitude ceilings and regulations that pertain primarily to communication and IFR usage. Thus, the requirement for continuous communication with ATC is fundamentally what differentiates Class A airspace operation.