Controlled airspace hierarchy from surface to 18,000 feet
Airspace in the National Airspace System is classified A through G, with A being the most controlled and G being uncontrolled. Class A airspace covers altitudes above 18,000 feet and includes all major airways used by commercial jets; all aircraft here operate under instrument flight rules with continuous ATC control. Below that, Class B is the busiest controlled airspace around major airports like New York and Los Angeles, where all aircraft need explicit clearance to enter.
Class C surrounds medium-sized airports with instrument approach capability, and Class D surrounds smaller airports with control towers. Classes B, C, and D all require two-way radio contact with ATC, but Class D permits visual flight. Class E is controlled airspace (typically above 1,200 feet MSL in non-mountainous areas) where large aircraft and flights under IFR operate, and Class G is uncontrolled airspace where small aircraft can fly without any ATC contact under visual flight rules.
Equipment requirements and visibility minimums
Entry into higher classes of airspace requires specific equipment. Class A and B require a transponder (to show aircraft position to radar). Class C requires both a transponder and a Mode C altitude reporter. Class D requires radio capability, while Class E and G have no equipment requirements for VFR flight. Visibility minimums also vary: Class A allows flight in any visibility under IFR control, while Class G under VFR requires anywhere from 1 to 5 statute miles of visibility depending on altitude.
This structure allows small aircraft and hobbyist pilots to fly in uncontrolled airspace under visual flight rules with minimal equipment and training, while ensuring that high-altitude airways and airspace around busy airports are carefully managed to prevent collisions. Commercial aviation rarely ventures into Class G or uncontrolled airspace, but general aviation depends on it for training, small-town airports, and recreational flying.