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When I'm reading about aviation, it's very common to see references to Class B or Class C airspace and airports.

What are the different classes and what are the differences between them?

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Although Lnafziger's answer is correct, I'd like to elaborate on the purpose of the airspace classes.

  • Class A: This airspace is intended for high-speed, point to point travel. That is why pilots flying in Class A must be instrument rated and in contact with air traffic control (ATC); aircraft above 18,000 feet are likely to travel quickly and may not have time to avoid each other visually. This is the upper layer of the airspace system.*
  • Class B: This airspace is intended for large airports with lots of jet traffic. The large number and size of aircraft require space for ATC to get them efficiently in and out of the airports. Approach control services are provided. As a result, class B areas are physically large. It is possible for small aircraft flying under visual flight rules to operate here with explict ATC permission, though permission to enter may be denied during periods of high traffic. Small piston-powered aircraft land at Class B airports infrequently, typically due to high landing fees and fuel prices.
  • Class C: Smaller airports with a mix of piston-powered aircraft and jets. Most Class C airports are home to flight schools and small aircraft hangars as well as scheduled service by airlines. While still busy, Class C airports have less traffic than Class B, so ATC doesn't need as much space to keep them organized. Like Class B, approach control services are provided. Pilots are only required to establish two-way communication to enter the airspace.
  • Class D: The smallest airspace type for tower-controlled airports, traffic primarily consists of general aviation aircraft. Often these airports handle cargo, business jets, charter service, maintenance or assembly facilities, etc. as well as private aircraft and flight schools. They also host firefighting operations, airshows, or anything else that needs facilities but would interfere with normal traffic if put at a larger airport. Although Class D airports are small, some are very busy; others handle only a few flights an hour. Approach control services are only provided if the airport is near Class B or C airspace.
  • Class E: This is the lowest level of "controlled airspace." "Controlled" doesn't mean you have to talk to ATC, but that ATC services are available (within the capabilities of radar and radio equipment). This is general-purpose airspace. Aircraft flying under visual flight rules can fly more-or-less whatever they want (weather permitting), and instrument flight rules traffic operate under positive control by ATC. There are regulations in this airspace, but they aren't onerous, and they're designed to accommodate the wide variety of aircraft and activities that are found here. Some small airports have Class E airspace surrounding them at the ground. Although Class E is a single class of airspace, there are tighter regulations above 10,000 feet, where there are no speed restrictions (other than the prohibition on supersonic flight over land), compared to at lower altitudes where speeds are limited to 250 knots.
  • Class F: See What is Class F airspace, and why is it not used in the US?
  • Class G: This is found near the ground everywhere except controlled airspace around airports, and occasionally at higher altitudes in remote areas (Alaska or the Rockies, usually), but never above 14,500 feet. Most untowered airports have Class G airspace surrounding them. This is "uncontrolled" airspace, so there is no ATC service available at all. Radio and radar work better the higher you go, so at low altitudes you just can't expect good results. Class G serves as a notice that ATC services are unavailable, typically due to a lack of radar coverage. In remote areas, the same problems occur at higher altitudes (radar won't see you if you go behind a mountain, and sometimes you're just too far) so Class G is found there as well. Most of the time, the only reason anybody goes into Class G airspace is because they're taking off or landing at an uncontrolled airport, or transiting certain areas that lack radar coverage.
  • Other: There are many kinds of special-use airspace, all of which serve as keep-out signs of varying levels from "please be considerate" up to "will shoot on sight." In the US, these don't really interact with airspace classification.

* Technically, airspace above 60,000 feet is Class E, but nobody goes up there except scientific and spy/military flights.

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    $\begingroup$ The decision to make an airport class B, C, or D is based on the number of operations, not the size it speed of the airplanes. It usually ends up that way, but it isn't the reason why. Also, I've flown into Class B airspace lots of times in small airplanes and never been denied like you say is likely. In fact, they have always been very accommodating and if IFR, they can't deny any properly equipped aircraft from entering Class B. Again, in Class B it isn't about the speed, it's about knowing where all aircraft are and keeping them separated. $\endgroup$ – Lnafziger Apr 1 '14 at 14:07
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    $\begingroup$ You don't need permission to enter class C; it's only necessary to have established contact with ATC. Of course, if ATC doesn't want you to enter, they just pretend they didn't hear your call sign. $\endgroup$ – Edward Falk Apr 2 '14 at 21:43
  • $\begingroup$ Or they say "Remain outside the class C". I've actually heard - only once - a controller direct an aircraft to turn around and leave the class C and not come back that day. (the airspace was extremely busy and the pilot was not only talking.....very.....slowly... but was well known to get clearances wrong) $\endgroup$ – Steve V. Jul 8 '14 at 2:54
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    $\begingroup$ The descriptions in this answer are correct, but the editorial comments are way off base. For example, anyone can fly in Class B airspace and permission is routinely given to small planes to transition the airspace and even land at the big airport. Likewise, I wouldn’t categorize 'Class E as mostly Cessna’s puttering around'. The majority of piston aircraft fly spend most of their time in Class E airspace. You’ll find everything from local sightseeing flights to twin engine charters in this airspace. As well as the jets trnsitioning to Class A airspace. $\endgroup$ – JScarry Jul 11 '14 at 16:20
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The short version, for non-aviation people is as follows:

  • Class A: All Airspace above 18,000 ft. Anybody flying here must receive a clearance from, be talking to, and be controlled by ATC.
  • Class B: Airspace within approximately 30 miles and 10,000 feet of the ground around the busiest airports in the US. Again, anybody flying here must receive a clearance from, be taking to, and be controlled by ATC.
  • Class C: Airspace within approximately 10 miles and 4,000 feet of airports that are less busy than Class B airports. The equipment requirements are less restrictive to fly in this airspace and pilots must be talking to ATC.
  • Class D: The airspace around the least busy airports that still require an ATC control tower. It is approximately 4 miles and 2,500 feet of airspace and they usually don't have radar so just coordinate with airplanes over the radio and by sighting them visually. Pilots must also talk to ATC before entering their airspace.
  • Class E: Airspace that does not require aircraft to talk to or be under the control of ATC unless the weather is worse than certain specified criteria. Aircraft avoid each other by looking out the window, and while this is true in the other airspace above, it is particularly important here (and in Class G) since many aircraft don't talk to ATC. There is an exception for airports in Class E that have a control tower; pilots landing at these airports must contact the tower before entering the airport's airspace.
  • Class F: Not used in the United States
  • Class G: Uncontrolled airspace that aircraft do not have to talk to ATC at all, except that are some airports in Class G that have a control tower; pilots landing at these airports must contact the tower before entering their airspace.
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  • $\begingroup$ So could I define Class G airspace as any airspace that is below 18,000 feet and not near any airports with towers, or near the smallest airports with towers while the weather is good? $\endgroup$ – hairboat Mar 31 '14 at 20:58
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    $\begingroup$ @AbbyT.Miller Nope, the official definition is "Class G airspace (uncontrolled) is that portion of airspace that has not been designated as Class A, Class B, Class C, Class D, or Class E airspace.". Class E is automatic above 14,500 ft., and in the US most class G is below 1,200 or 800 ft. above the ground, depending on the location. $\endgroup$ – Lnafziger Mar 31 '14 at 21:38
  • $\begingroup$ Your Class B and Class C definitions don't really mention that its an upside-down wedding-cake shape, and that uncontrolled GA traffic can be well inside the 30-mile radius, but below the class B space. $\endgroup$ – abelenky Jul 7 '14 at 17:34
  • $\begingroup$ @abelenky This is for non-aviation people and I said "approximately". No need to get bogged down with so many details for the purposes of this question. :) $\endgroup$ – Lnafziger Jul 7 '14 at 21:29
  • $\begingroup$ Abby, The airspace (away from airports) below 1,200' is usually Class G. There are also large areas of Class G airspace in the mountains of the Western US. The reason it is uncontrolled airspace is because radar coverage is not available. $\endgroup$ – JScarry Jul 11 '14 at 16:07
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The answers given are correct for the USA. If you're in one of the other 199 (or so) countries, it's more likely that something closer to the original ICAO (International Civil Aviation Organisation) definition of airspace applies.

They specify (a) what sort of traffic is allowed, and (b) who is responsible for avoiding other traffic, namely ATC (when separation is provided) or the pilot. Below, IFR refers to Instrument Flight Rules, while VFR refers to Visual Flight Rules.

  • Class A: IFR only. All aircraft are subject to ATC clearance. All flights are separated from each other by ATC.
  • Class B: IFR or VFR. All aircraft are subject to ATC clearance. All flights are separated from each other by ATC.
  • Class C: IFR or VFR. All aircraft are subject to ATC clearance. IFR flights are separated from each other and from VFR, but VFR flights are not separated from each other.
  • Class D: IFR or VFR. All flights are subject to ATC clearance. IFR flights are separated from each other, and are given traffic information in respect of VFR flights.
  • Class E: IFR or VFR. IFR flights are subject to ATC clearance. IFR flights are separated from each other, and are given traffic information in respect of VFR flights.
  • Class F: IFR or VFR (no SVFR). ATC separation will be provided, so far as practical, to aircraft operating under IFR.
  • Class G: IFR or VFR (no SVFR). ATC separation is not provided. Traffic Information may be given as far as is practical in respect of other flights.

Individual countries can (unfortunately...) deviate quite substantially from these airspace classes, and/or impose further structure on top of them.

Frequently, certain air space classes will have specific speed limits, equipment requirements, or minimum visibility requirements for VFR traffic.

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    $\begingroup$ Hi Fab, welcome to the site! If you look under the question, you will see that this question is tagged faa-regulations, so is specifically asking about the USA and not ICAO or the other 199 countries. Because there are so many different possible answers to a question like this, we use the tags to limit the scope of an individual question. Your answer would be great for a similar question tagged icao-recommendations, and I would suggest adding an appropriate question along with your answer since you already went through the trouble to write it up! See How to Ask and How to Answer for details. $\endgroup$ – Lnafziger Apr 2 '14 at 21:59
  • $\begingroup$ Hi Lnafziger, thanks for your welcome and note. Good point! The tags are easy to overlook though - maybe one would want to delimit the scope in the question, as well. $\endgroup$ – Fab Apr 3 '14 at 5:43
  • $\begingroup$ Sometimes people do in their questions, but it just takes one comment to a new user and they know how it works. I tend not to on my own questions because it is redundant, but that's just me. :) $\endgroup$ – Lnafziger Apr 3 '14 at 11:38
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This question is quite broad, as airspace classes are used in varying ways by different jurisdictions. The only near-constant is what the airspace classes mean in terms of the services Air Traffic Control must provide and the conditions under which planes may use the space.

First, you must understand four basic modes of aircraft operation:

  • Visual Flight Rules (VFR) - Pilots fly the plane primarily by looking outside the cockpit, using the horizon to know how their aircraft is oriented, landmarks to navigate, and visual contact with other planes and airborne objects to safely "separate" his aircraft and avoid collisions (a mentality known as "see and avoid" which is required of all pilots when it is possible). This is the simplest mode of operation and the first that pilots learn, but requires good weather, especially good visibility distance (3 miles or better) and a high cloud ceiling (1,000 feet or higher). The minimal conditions required for VFR are known as "visual meteorological conditions" or VMC.
  • Instrument Flight Rules - Pilots fly the plane using primarily instruments inside the cockpit for orientation and navigation of the aircraft, while Air Traffic Control (ATC) issues directions to aircraft regarding course and to maintain separation between one plane and another and direct planes onto the correct course, especially for landings. This requires pilots to receive special training on the instruments they'll use and in radio procedures, and allows flight in any weather that doesn't meet the minimum criteria for VMC but still allows the flight of aircraft (fog, low clouds, light rain, dust storms), which is known as "instrument meteorological conditions" or IMC.
  • Special Visual Flight Rules (SVFR) - An exceptional case in which pilots are allowed to fly under VFR within controlled airspace in weather conditions marginally worse than the minimums in which VFR is normally allowed. ATC is required to track the aircraft and provide separation even if the airspace class wouldn't normally require ATC to do this for VFR traffic. This is typically used to get planes into or out of an airport that is experiencing localized worsening weather, without those pilots having to declare an emergency to override the controller's lack of clearance (normally if the airport is experiencing IMC it is illegal for a controller to clear a VFR flight for landing or even entry, as it's illegal for the aircraft to operate under VFR in such conditions).
  • VFR-On-Top - This is a special case of IFR which requires the same instrument training and certification, but allows a more VFR-like flight experience for the pilot. The pilot will be granted a "slice" of airspace by ATC, either altitude-only along his flight path or in three dimensions, within which the pilot has the freedom to choose his own altitude (typically to avoid clouds). The pilot gains primary responsibility to maintain minimum distances from other aircraft and from clouds within this airspace as he would under VFR.

As defined by ICAO, the classes are:

  • Class A - All flights must operate using IFR; no VFR or SVFR flight is allowed and VFR-On-Top is typically not granted due to the high activity level. Planes entering Class A airspace or making any self-directed change in flight path require ATC clearance. ATC provides full separation services, providing instructions to aircraft regarding required heading and altitude to keep aircraft on-course and well-spaced.
  • Class B - Flights can use all four flight rulesets, however all aircraft must be in contact with ATC and receive explicit clearance for entry or flight path deviation. ATC provides full separation services to all aircraft as well as directional guidance to IFR and SVFR.
  • Class C - Flights can use any of the four flight rulesets. Aircraft must be in contact with ATC and (except in certain countries including the U.S.) require explicit ATC clearance. ATC provides IFR guidance and separation of IFR and SVFR craft from each other and from VFR craft, while VFR craft receive "traffic advisories" but must maintain their own separation from each other.
  • Class D - Flights can use all four rulesets. Aircraft must be in contact with ATC and (except in certain countries including the U.S.) must receive explicit clearance for entry. ATC provides separation of IFR and SVFR flights from each other and provides traffic advisories to these craft of nearby VFR planes; VFR traffic may receive traffic advisories but is responsible for their own separation from all other aircraft.
  • Class E - Flights can use IFR, VOT or VFR; SVFR is allowed only when the airspace is established around a towered airport. IFR and SVFR flights are separated by ATC and require ATC communication and clearance prior to entry if the airspace is established around an airport; VFR aircraft do not require clearance or ATC communication, and do not receive separation services (though they may receive traffic advisories, ATC workload permitting).
  • Class F - Flights can use IFR, VOT or VFR but not SVFR. No clearance is required and ATC communication is only required for IFR. ATC provides separation between IFR flights and traffic advisories based on ability and workload, but separation is not guaranteed and IFR flights gain responsibility to "see and avoid". VFR flights may receive traffic advisories from ATC on a workload-permitting basis.
  • Class G - Flights may use IFR, VOT or VFR but not SVFR. No clearance is required and communication is only required for IFR. ATC has no authority or responsibility within this airspace; it provides traffic advisories as it is able prioritizing IFR flights.

In the United States, the classes are used as follows:

  • A - Airliner cruising altitudes, beginning at about 18,000 feet up to 60,000 feet within U.S. territorial boundaries and additional monitored airspace in international waters.
  • B - Surrounds large national and international hub airports in a big "inverted wedding cake" shape beginning at the surface 10 miles out and proceeding upward in wider layers to 10,000-12,000 feet at 20-30 miles out (the exact shape and number of layers depends on the specific needs of the airport and surrounding airfields or terrain); examples include Chicago O'Hare (ORD), Hartsfield-Jackson Atlanta (ATL), the Dallas-Fort Worth area (DFW/DAL), Boston Logan (BOS), the Washington, DC area (IAD/DCA/BWI), Los Angeles (LAX), San Francisco (SFO), Denver (DEN), the Houston area (IAH/HOU) and a few others.
  • C - Surrounds any airport that isn't Class B or within the footprint of one, but has significant scheduled service from national airlines (basically if it sees regular service from a 737 or larger it's at least Class C). Airspace shape is a smaller "inverted birthday cake" with an inner cylinder at the surface to 2,000 feet 5 miles in radius, and an upper cylinder from 2,000' to 4,000' feet at 10 miles out, with tweaks as necessary to accommodate the airport's specific needs.
  • D - Surrounds any airport that isn't class B or C but has a tower and instrument approach services. Includes most U.S. airports with commercial service using smaller narrow-body jets and turboprops, as well as most military air bases (AFB/NAS/JRB). Airspace shape is typically a single cylinder 4 miles in radius extending to 2500 feet; small variations are allowed.
  • E - Any airspace underneath Class A that isn't B, C, or D but can still be controlled by ATC. Typically starts at 1,200 feet above ground and extends upward to class A; a lower 700' "floor" is established 5 to 10 miles outside and underneath the airspace near most towered airports, with airstrips that do not have their own tower but do have instrument approach services also having a Class E "surface footprint" of 3 miles radius. This airspace is typically where ATC will grant "VFR-On-Top" clearance.
  • F - Not used in the U.S.; by FAA law, all "controlled" airspace requires ATC to guarantee IFR-IFR separation; any space in which it cannot do so is "uncontrolled" Class G.
  • G - Anything that isn't Class A-E. Typically airspace from the surface to 1,200 feet AGL, except where the Class E floor is lowered below 1,200'AGL, or any region above 1,200'AGL where radar or radio communications is disrupted by surrounding terrain.
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