Definition
The block of U.S. airspace from 18,000 feet MSL up to and including flight level 600 (60,000 feet), plus the airspace overlying the waters within 12 nautical miles of the U.S. coast. All flight in Class A airspace must be conducted under Instrument Flight Rules (IFR), and pilots must hold an instrument rating, file an IFR flight plan, and be in contact with Air Traffic Control.
Plain English
The high-altitude airspace where every flight has to be on instruments and talking to controllers. You cannot fly through it under visual rules, no matter how clear the weather is.
Context Anchor
Seen in airspace rules, flight planning, altitude assignments, and instrument flying discussions.
Derivation
U.S. airspace is divided into letter classes (A, B, C, D, E, G) by the FAA. Class A is the highest and most restrictive in terms of pilot requirements. The lettering follows the ICAO airspace classification system adopted by the U.S. in 1993; it does not stand for anything — A is simply the top of the list.
Why Pilots Care
Entry requires an ATC clearance, and all operations must be conducted under instrument flight rules with specific aircraft equipment.
Intuition Check
Class A does not mean the best or highest-quality airspace. It means a specific regulatory category of airspace with specific entry and operating rules.
Example Sentence 1
Because their planned cruise altitude was 22,000 feet, the crew filed an IFR flight plan to operate legally in Class A airspace.
Example Sentence 2
All aircraft flying at 39,000 feet must be in Class A airspace and on an IFR clearance.