ICAO splits the sky into seven classes, A through G. Each class sets who can fly in it, what ATC services they get, whether a transponder is required, and how IFR and VFR traffic are separated. Here is what each letter means, where the boundaries sit, and which class your local airliner is actually in right now.
Before the letters, there are two binary distinctions that explain almost everything:
Controlled vs uncontrolled. Controlled airspace (Classes A–E) means ATC provides separation between participating aircraft. Uncontrolled airspace (Classes F and G) means "see and avoid" — pilots are responsible for spotting each other.
IFR vs VFR. Instrument Flight Rules (IFR) = flying by reference to instruments, on a flight plan, talking to ATC. Visual Flight Rules (VFR) = flying by looking outside, in good weather, often without talking to anyone.
Class A — IFR only, the high cruise
Class A is the busiest, most-controlled airspace. VFR is prohibited. Every aircraft must be on an IFR flight plan, in two-way radio contact, with a Mode C or Mode S transponder.
United States — FL180 to FL600 (18,000 ft to 60,000 ft pressure altitude) over the contiguous 48 states.
Europe — varies by state; typically FL195 or FL245 upward, with the upper boundary near FL660.
Above FL600 — usually Class E in the US (think U-2, SR-71 range — historically uncontrolled because so few aircraft fly there).
Class A is where airline cruise lives. A typical A320neo climbs into Class A within 15 minutes of takeoff and stays there until top-of-descent.
Class B — busy terminal airspace
Class B wraps the busiest airports — Atlanta, LAX, Chicago O'Hare, JFK, Heathrow (Class A in the European scheme), Frankfurt. Shaped like an "upside-down wedding cake": narrow at the surface, widening as it climbs. Typical dimensions:
Surface to 10,000 ft AGL, with rings of increasing radius at higher altitudes.
ATC clearance required to enter — "Cleared into the Bravo".
Two-way radio + Mode C transponder + ADS-B Out mandatory.
VFR allowed but must request clearance and stay clear of IFR traffic at ATC's direction.
The "Mode C veil" extends Class B's transponder requirement to a 30-nautical-mile ring around the primary airport, even outside the Bravo proper. Anyone flying inside that ring with an engine must have a Mode C / Mode S transponder turned on.
Class C — moderately busy airports
Class C surrounds medium-traffic airports — San Francisco, Boston, Seattle, Phoenix, Manchester (UK), Düsseldorf. Smaller than Bravo, simpler shape:
Inner core — typically 5 nm radius, surface to 4,000 ft AGL.
Outer shelf — typically 10 nm radius, 1,200 to 4,000 ft AGL.
Two-way radio contact required before entry. ATC does not have to issue an explicit "cleared into Charlie" — establishing radio contact is the clearance.
Mode C transponder required. ADS-B Out required in the US since 2020.
Class D — control zone around towered airports
Class D is the airspace around an airport with a control tower but no radar approach service. Typical shape: 4 nm radius, surface to 2,500 ft AGL. Two-way radio contact required; no transponder mandate from the tower (but adjacent Class B/C rules may still apply). Tens of thousands of regional and general-aviation airports worldwide live inside Class D.
Class E — controlled airspace for IFR
Class E is the "everything else that is controlled" bucket. In the United States it covers most airspace from 1,200 ft AGL up to 18,000 ft, including the federal airways (V-routes and T-routes). VFR traffic is allowed without ATC contact; IFR traffic is fully separated from other IFR traffic. Cloud-clearance and visibility requirements tighten as you climb.
Class F — advisory only
Class F exists in a handful of Commonwealth states (Canada in places, parts of Australia, some African nations). ATC offers an advisory service to IFR flights — separation is provided where workload allows but is not guaranteed. The class is being phased out in most jurisdictions and converted to Class E or Class G.
Class G — uncontrolled
Class G is everything below the floor of controlled airspace — typically the surface up to 700 or 1,200 ft AGL outside a Class E base, or up to 14,500 ft MSL in remote mountain areas of the US. No ATC service. "See and avoid." VFR cloud-clearance minimums apply (e.g. 1 mile visibility, clear of clouds by day below 1,200 ft AGL in the US).
Most low-altitude general-aviation flying — a Cessna 172 tooling around 1,000 ft over rural fields, a Cessna 208 dropping skydivers — is happening in Class G or Class E.
Quick reference — US airspace
Class A — FL180 to FL600 · IFR only · clearance + xpdr + ADS-B
Class B — SFC to 10,000 ft AGL (busy airports) · clearance required · Mode C veil 30 nm
Class C — SFC to 4,000 ft AGL (mid-busy) · radio contact required
Class D — SFC to 2,500 ft AGL (towered airports) · radio contact required
Class E — varies, fills the gaps · IFR controlled, VFR free
Class G — uncontrolled · see-and-avoid · usually below 1,200 ft AGL
Transition altitude (US) — 18,000 ft (set altimeter to 29.92 inHg above this)
Special Use Airspace (SUA)
Outside the A–G letters there are several overlay categories. Each is plotted on the chart even though the underlying class doesn't change.
Prohibited Areas — entry not permitted at any time without specific authorisation. P-56 covers the airspace over the White House and US Capitol; P-49 over the Bush family compound in Texas.
Restricted Areas — entry permitted only when the using agency authorises it. R-2508 over China Lake / Edwards AFB is the largest in the US — a 20,000-square-mile box where the Navy and Air Force test combat aircraft like the F-22.
Military Operations Areas (MOAs) — VFR can transit them at the pilot's risk; military training is active during published hours.
Warning Areas — offshore equivalents of restricted areas (over international waters).
Air Defence Identification Zones (ADIZ) — aircraft entering must file a flight plan and be in radio contact. ADIZ rings surround the US coastline. See our squawk-code article for what happens when a transponder code triggers a QRA scramble.
Temporary Flight Restrictions (TFRs) — drawn around presidential movements, wildfires, sporting events. Published by NOTAM.
What altitudes airliners and GA actually fly
Putting the classes together gives a clear vertical picture:
FL280–FL410 (28,000–41,000 ft) — Class A · long-haul airline cruise. Boeing 787, A350, 777.
FL280–FL390 — Class A · short-haul cruise. A320, 737, E190.
FL410–FL450 — Class A · business jets at top of climb (Gulfstream G650, Global 7500).
FL600+ — above Class A in the US · U-2, weather research, balloons.
10,000 ft to FL180 — Class E or Class B/C terminal airspace · climb, descent, regional turboprops.
Below 10,000 ft — Class B/C/D/E/G mix · approach, departure, GA cruise.
ADS-B mandates by class
The US ADS-B Out mandate (effective 1 January 2020) applies in: Class A, Class B, Class C, the airspace above 10,000 ft MSL (except the 2,500 ft AGL layer in mountainous terrain), the Mode C veil around Class B airports, and offshore over the Gulf of Mexico above 3,000 ft. Outside those zones, ADS-B Out is optional. Read more in our ADS-B explainer.