Which class of airspace has non-VFR clearance requirements?

Class A airspace is characterized by its specific operational requirements, one of which includes the necessity for all aircraft to operate under Instrument Flight Rules (IFR). This means that pilots flying in Class A airspace must receive clearance from air traffic control (ATC) before entering and must adhere to IFR protocols. This airspace is located from 18,000 feet mean sea level (MSL) up to 60,000 feet, where weather conditions are typically not suitable for Visual Flight Rules (VFR) flight, making ATC clearance essential for maintaining safety and organized traffic flow.

In contrast, other classes of airspace, such as Class B, Class E, and Class G, have varying levels of clearance requirements primarily aimed at VFR operations. Class B requires ATC clearance for VFR flights but is specifically tailored for congested areas around major airports. Class E does have certain operational requirements but does not mandate non-VFR clearance in many circumstances. Class G is uncontrolled airspace and does not require ATC clearance for VFR flights at any altitude. Therefore, the unique requirement for non-VFR clearance specifically for IFR operations makes Class A airspace distinct in this regard.