Definition
Controlled airspace in the United States extending from 18,000 feet MSL up to and including flight level 600 (60,000 feet), within which all flight operations must be conducted under Instrument Flight Rules (IFR), with an IFR clearance, in radio contact with Air Traffic Control, and on an assigned route or altitude.
Plain English
The block of high-altitude airspace from 18,000 feet up to 60,000 feet where every aircraft must be flying on an IFR clearance and talking to controllers. No visual flying allowed up there.
Context Anchor
Seen in airspace discussions, flight planning, instrument training, and high-altitude route planning.
Derivation
The U.S. uses a letter system (A through G) to grade airspace by how strictly it is controlled. 'A' is the most tightly controlled — the highest, busiest layer where only IFR traffic is permitted. The letters were adopted to align with international (ICAO) airspace classifications.
Why Pilots Care
Entry requires an instrument rating and continuous air traffic control clearance to maintain safe separation among high-speed aircraft.
Intuition Check
“Class A” does not mean “best” or “premium” airspace. Here, it means a specific legal category of controlled airspace with strict entry and operating rules.
Example Sentence 1
Once we climbed through 18,000 feet, we entered Class A airspace and stayed under positive ATC control for the rest of the cruise.
Example Sentence 2
All aircraft operating in Class A airspace must remain under instrument flight rules.