Controlled vs. Uncontrolled — the big picture
The US National Airspace System is divided into classes, each with its own rules for who can fly there, what equipment you need, and how good the weather has to be. The single most important split is between controlled and uncontrolled airspace.
In controlled airspace — Classes A, B, C, D and E — ATC provides separation or traffic services to at least some aircraft, and entry rules, equipment, and weather minimums apply. In uncontrolled airspace — Class G — ATC has neither the authority nor responsibility to separate traffic, so pilots provide their own separation and the weather minimums are the lowest in the system.
The classes are lettered A through G (there is no F in US airspace). Roughly speaking, the earlier the letter, the busier the airspace and the stricter the rules. Everything below comes from PHAK chapter 15, the AIM, and 14 CFR Part 91.
Class A — the high-altitude airways
18,000 ft MSL up to FL600 — IFR only
Class A is the airspace from 18,000 ft MSL up to and including flight level 600 (60,000 ft). There is no VFR flight in Class A — every aircraft must operate under IFR, on an ATC clearance, and be appropriately equipped (transponder and ADS-B Out). This is where the airliners and high-performance traffic cruise.
Class B — the busiest airports
Class B surrounds the nation’s busiest airports — places like Atlanta, Los Angeles, and Chicago O’Hare. It typically extends from the surface up to about 10,000 ft MSL and is shaped like an upside-down wedding cake: tiers of progressively wider rings stacked on top of each other, so traffic descending into the primary airport stays contained. The tiers and their floors are printed right on the chart — reading a sectional walks through the shorthand, and the Class B guide covers the entry rules in depth.
Clearance required
You may not enter Class B until you hear “cleared into the Bravo.” Two-way radio alone is not enough — it takes an explicit clearance.
Equipment & pilots
A Mode C transponder and ADS-B Out are required. A private pilot may fly in Class B; student pilots need a logbook endorsement, and a handful of the busiest Class B airports are closed to student solos entirely.
Class C — moderately busy airports
Surface up to ~4,000 ft AGL — two-way radio established
Class C wraps moderately busy airports that have an operating control tower, radar approach control, and a certain volume of traffic. It generally extends from the surface up to about 4,000 ft AGL within a 5 nm inner core, with a 10 nm shelf above 1,200 ft AGL. You do not need a clearance, but you must establish two-way radio communication before entering — if the controller reads back your call sign, you are in. A Mode C transponder and ADS-B Out are required. Not sure what to say on that first call? Talking to ATC has the script.
Class D — towered fields
Surface up to ~2,500 ft AGL — two-way radio established
Class D surrounds airports with an operating control tower but less traffic than a Class C. It typically extends from the surface up to about 2,500 ft AGL, within roughly a 4 nm radius. As with Class C, no clearance is needed but you must establish two-way radio communication with the tower before entering. No transponder is required by the airspace itself. When the tower closes, Class D usually reverts to Class E or G — which changes the weather minimums that apply to you, so check the minimums by airspace before a night arrival.
Class E — controlled, but no clearance for VFR
Class E is controlled airspace that is not A, B, C or D. It fills most of the sky. VFR aircraft need no clearance to fly in Class E, but the airspace’s weather minimums and cloud clearances do apply.
700 ft AGL
Shown by a fuzzy magenta vignette on a sectional — usually around airports with instrument approaches.
1,200 ft AGL
Shown by a fuzzy blue vignette — the common floor over much of the country.
Surface
Shown by a dashed magenta line around an airport — Class E reaching the ground.
Class G — uncontrolled airspace
Surface up to the overlying Class E — no ATC services
Class G is uncontrolled airspace — ATC provides no separation services here. It lies below the floor of controlled airspace, usually from the surface up to 700 or 1,200 ft AGL, though in remote areas it can extend up to 14,500 ft MSL. Class G has the lowest weather minimums of any airspace, which is why you must stay extra vigilant about other traffic and deteriorating conditions. Before you launch into marginal weather, run the numbers through the go/no-go check.
VFR weather minimums by class
Each class carries its own visibility and cloud-clearance requirements. Two memory aids cover most of it: 3-152 (3 statute miles visibility; 500 ft below, 1,000 ft above, 2,000 ft horizontal from clouds) for Class E below 10,000 MSL and the controlled-airspace default, and 5-111 (5 SM; 1,000 ft below, 1,000 ft above, 1 SM horizontal) at and above 10,000 ft MSL.
Class B simplifies things to 3 SM visibility and “clear of clouds.” Class G is the loosest — as little as 1 SM and clear of clouds by day at low altitude. The full table, with the day/night and altitude splits, lives in our dedicated guide.
Equipment & ADS-B Out
Under 14 CFR 91.225, ADS-B Out is required in Class A, B and C airspace, above the ceiling of Class B and C up to 10,000 ft MSL, generally at and above 10,000 ft MSL (except below 2,500 ft AGL), and within the Mode C veil — the 30 nm ring around the primary Class B airport. The full rule text is in 14 CFR 91.225.
In short: wherever a Mode C transponder is required, ADS-B Out generally is too. Confirm your aircraft’s equipage before you plan a route through any of this airspace.
Airspace at a glance
Altitudes and dimensions are typical values; the actual limits for any specific airspace are published on the sectional chart — look up any field on the airport pages to see what applies.
Class-by-Class Deep Dives
A focused guide for each class — dimensions, entry and equipment, VFR minimums, and speed limits.
See airspace the way pilots do
FlightKit’s Airspace guide breaks down every class with the dimensions, entry rules, and weather minimums — and ties them to what you actually see on the sectional.
Keep learning
Frequently Asked Questions
What is the difference between controlled and uncontrolled airspace?
Controlled airspace (Classes A, B, C, D and E) is where ATC provides separation or traffic services to at least some aircraft, and where specific entry rules, equipment, and weather minimums apply. Uncontrolled airspace (Class G) is where ATC neither has the authority nor the responsibility to control air traffic — pilots are responsible for their own separation, and the weather minimums are the lowest in the system.
Do I need a clearance to enter Class C or D airspace?
No clearance is required, but you must establish two-way radio communication before entering. The rule of thumb: if the controller reads back your call sign, you are cleared to enter; if they say “aircraft calling, stand by,” you must stay clear until two-way communication is established. Class B is different — there you need an explicit clearance: you may not enter until you hear “cleared into the Bravo.”
What is Class B airspace?
Class B surrounds the nation’s busiest airports. It is shaped like an upside-down wedding cake — tiers of progressively wider rings stacked from the surface up to roughly 10,000 ft MSL. Entry requires an explicit ATC clearance (“cleared into the Bravo”), a Mode C transponder, and ADS-B Out. A private pilot may fly in Class B; student pilots need a logbook endorsement, and a few of the busiest Class B airports are off-limits to student solos entirely.
What does Class E airspace look like on a sectional?
A fuzzy magenta vignette marks where controlled Class E starts at 700 ft AGL; a fuzzy blue vignette marks where it starts at 1,200 ft AGL. A dashed magenta line around an airport shows Class E down to the surface. Where you see none of these markings over the countryside, Class E typically begins at 1,200 ft AGL with Class G below it.
Where is Class G airspace?
Class G (uncontrolled) is the airspace below the floor of controlled airspace. In most of the country that means from the surface up to either 700 or 1,200 ft AGL, where Class E takes over. In remote areas it can extend much higher — up to 14,500 ft MSL. Class G has the lowest weather minimums of any airspace.
Do I need ADS-B Out?
You need ADS-B Out (under 14 CFR 91.225) in Class A, B and C airspace, above the ceiling of Class B and C up to 10,000 ft MSL, generally at and above 10,000 ft MSL (except below 2,500 ft AGL), and within the Mode C veil — the 30 nm ring around the primary Class B airport. The same airspace that requires a Mode C transponder generally requires ADS-B Out.