Class A airspace extends from 18,000 feet (5,500 m) mean sea level MSL to FL600 (approximately 60,000 feet (18,000 m) MSL) throughout the continental United States and Alaska.[2] Unlike the altitude measurements used in other airspace classes, the FLnnn flight levels used in class A airspace are pressure altitudes referenced to a standardized altimeter setting of 29.92" Hg and thus the true altitudes depend on local atmospheric pressure variations. Unless otherwise authorized by ATC, all flight operations in class A airspace must be under ATC control, and must be operating IFR, under a clearance received prior to entry. An example of an exception to IFR-only flight in this airspace is the use of wave windows. These specific airspace blocks may be opened by ATC to allow sailplanes to fly in the lee waves of mountains.[3] Since class A airspace is normally restricted to instrument flight only, there are no minimum visibility requirements.

Class B airspace is defined around key airport traffic areas, usually airspace surrounding the busiest airports in the US according to the number of IFR operations and passengers served. The exact shape of the airspace varies from one class B area to another, but in most cases it has the shape of an inverted wedding cake, with a series of circular "shelves" of airspace of several thousand feet in thickness centered on a specific airport. Each shelf is larger than the one beneath it. Class B airspace normally begins at the surface in the immediate area of the airport, and successive shelves of greater and greater radius begin at higher and higher altitudes at greater distances from the airport. Many class B airspaces diverge from this model to accommodate traffic patterns or local topological or other features. The upper limit of class B airspace is normally 10,000 feet (3,000 m) MSL.[4] All aircraft entering class B airspace must obtain ATC clearance prior to entry and must be prepared for denial of clearance. Aircraft must be equipped with a two-way radio for communications with ATC and an operating Mode C transponder, furthermore aircraft overflying the upper limit of any class B airspace must have an operating Mode C transponder. Visual flight rules (VFR) flights may proceed under their own navigation after obtaining clearance but must obey any explicit instructions given by ATC. Some class B airspaces include special transition routes for VFR flight that require communication with ATC but may not require an explicit clearance. Other class B airspaces include VFR corridors through which VFR flights may pass without clearance (and without technically entering the class B airspace).[4] VFR flights operating in class B airspace must have three miles (5 km) of visibility and must remain clear of clouds (no minimum distance).[4][5] Class B airspace has the most stringent rules of all the airspaces in the United States. Class B has strict rules on pilot certification. Pilots operating in class B airspace must have a private pilot's certificate, or have met the requirement of 14 CFR 61.95. These are often interpreted to mean "have an instructor's endorsement for having been properly trained in that specific class B space". However, it does not apply to student pilots seeking sport or recreational certificates. Some class B airports (within class B airspaces) prohibit student pilots from taking off and landing there.[4] In addition to this, some class B airspaces prohibit special VFR flights. Certain class B airports have a mode C veil, which encompasses airspace within thirty nautical miles of the airport. Aircraft operating within the Mode C veil must have an operating Mode C transponder (up to 10,000 feet (3,000 m) MSL) unless the aircraft is certified without an engine-driven electrical system and it operates outside the class B and below the ceiling of the class B and below 10,000 feet (3,000 m) MSL.

Class C space is structured in much the same way as class B airspace, but on a smaller scale. Class C airspace is defined around airports of moderate importance; airports with regular commercial passenger jet service of 100 passengers per flight or more are typically Class C. The FAA requirements for Class C airspace status are an operational control tower, a radar-controlled approach system, and a minimum number of IFR approaches conducted per year. The airspace class designation is in effect only during the hours of tower and approach operation at the primary airport; the airspace reverts to Class D if approach control is not operating, and to class E if the tower is closed. The vertical boundary is usually 4,000 feet (1,200 m) above the airport surface. The core surface area has a radius of five nautical miles (9 km), and goes from the surface to the ceiling of the class C airspace. The upper "shelf" area has a radius of ten nautical miles, and extends from as low as 1,200 feet (370 m) up to the ceiling of the airspace. A procedural "outer area" (not to be confused with the shelf area) has a radius of 20 nautical miles.[6] All aircraft entering class C airspace must establish two-way radio communication with ATC prior to entry; explicit clearance to enter is not required, however the controller of Class C space may instruct aircraft initiating communication to "remain outside" the airspace. The aircraft must be equipped with a two-way radio and an operating Mode C (altitude reporting) radar transponder, furthermore aircraft overflying above the upper limit of class C airspace upward to 10,000 feet MSL must have an operating Mode C transponder. VFR flights in class C airspace must have three miles (5 km) of visibility, and fly an altitude at least 500 feet (150 m) below, 1,000 feet (300 m) above, and 2,000 feet (600 m) laterally from clouds.[6] There is no specific pilot certification required. Aircraft speeds must be below 200 knots (230 mph) at or below 2,500 feet (760 m) above the ground, and within 4 nautical miles (7 km) of the class C airport.[6]

Class D airspace is typically established around any airport with a functioning control tower, but that does not see significant IFR approaches which would make Class B or C more appropriate (usually because there is no scheduled commercial passenger service). Class D airspace is generally cylindrical in form and normally extends from the surface to 2,500 feet (760 m) above the ground. The outer radius of the airspace is variable, but is generally 4 nautical miles. Airspace within the given radius, but in surrounding class C or class B airspace, is excluded. Class D airspace reverts to class E or G during hours when the tower is closed, or under other special conditions.[7] Two-way communication with ATC must be established before entering class D airspace, but no transponder is required. VFR cloud clearance and visibility requirements are the same as class C.

Controlled airspace which is neither class A, B, C nor D.[8] In most areas of the United States, class E airspace extends from 1,200 feet (370 m) AGL up to but not including 18,000 feet (5,500 m) MSL, the lower limit of class A airspace. There are areas where class E airspace begins at either the surface or 700 AGL, these areas are used to transition between the terminal and en-route environments (around non-towered airports). These areas are designated on sectional charts. Most airspace in the United States is class E. The airspace above FL600 is also class E.[8] No ATC clearance or radio communication is required for VFR flight in class E airspace. VFR visibility and cloud clearance requirements are the same as for class C and D airspaces when below 10,000 feet (3,000 m) MSL. Above 10,000 ft MSL, the visibility requirement is extended to 5 miles (8 km) and the cloud clearance requirement is extended to 1,000 feet (300 m) below clouds, 1,000 feet (300 m) above, and 1 mile (1.6 km) laterally.

Class G airspace includes all airspace below 14,500 feet (4,400 m) MSL not otherwise classified as controlled.[10] There are no entry or clearance requirements for class G airspace, even for IFR operations. Class G airspace is typically the airspace very near the ground (1,200 feet or less), beneath class E airspace and between class B-D cylinders around towered airstrips. Radio communication is not required in class G airspace, even for IFR operations. Class G is completely uncontrolled. VFR visibility requirements in class G airspace are 1 mile (1.6 km) by day, and 3 miles (5 km) by night, for altitudes below 10,000 feet (3,050 m) MSL but above 1,200 ft AGL. Beginning at 10,000 feet MSL, 5 miles (8 km) of visibility are required, day and night. Cloud clearance requirements are to maintain an altitude that is 500 ft below, 1,000 ft above, 2,000 ft horizontal; at or above 10,000 ft MSL, they are 1,000 ft below, 1,000 ft above, and 1 mile laterally. By day at 1,200 feet (370 m) AGL and below, aircraft must remain clear of clouds, and there is no minimum lateral distance.[5] It should be noted that there are certain exceptions where class G extends above 1,200 feet AGL. This is usually either over mountainous terrain (e.g., some areas in the Rocky Mountains), or over very sparsely populated areas (e.g., some parts of Montana and Alaska).