The increase in numbers of airplanes flying over the united states prompted the creation of demarcations on airspace, to provide for better traffic management and ensure the safety of all aircraft involved.  The following classifications are determined and regulated by the FAA and enforced via the several Air Traffic Control Centers throughout the US.

Class A Airspace

Extends from 18000 feet to 60000 feet.  All aircraft in this zone must adhere to Instrument Flying Rules (IFR) and must file a plan accordingly to the appropriate ATC and receive clearance for it.  They must also reset their instrument so that all the aircraft will have the same reading on their altimeters to ensure proper separation.  This airspace affect all 48 contiguous states and 12 nautical miles

Class B Airspace

Extends from the surface to 18000 feet.  This is most often modified to meet the needs of the airports it surrounds.  In order to enter Class B airspace, all aircraft must receive clearance from ATC and strictly stick with the heading, altitude and speed assigned to them for safety, since it's usually the busiest and most crowded.

Class C Airspace

Only found in airports that have an operating tower and can be serviced by radar approach control and support IFR operations.  It is individually tailored to meet the needs of airports.  Pilots must establish and maintain two way radio communications at all times with ATC controllers until they are handed off to the tower for landing.

Class D Airspace

Goes from the surface to 2500 feet above certain airports.  It is necessary to establish two way communications with ATC, although  no separation services are provided to aircraft flying under Visual Flying Rules (VFR).  It is also present in uncontrolled airports (without a tower).  In this case, a broadcast frequency will be used for pilots to inform other aircraft of their intentions (such as, landing, departing, etc.)  Responsability for aircraft separation falls in the hands of those flying the planes.

Class E Airspace

Controlled airspace that does not falls into the previous categories is called Class E.  It is usually used when transitioning between other controlled arieas under IFR.  VFR planes can use this airspace so long as they can mantain VFR conditions for separations, and olny up to 17500 feet.

Class G Airspace

This is uncontrolled airspace.  IFR doesn't operate here.  VFR planes can operate in this airspace if weather conditions allow for it.

Sources: FAA web site.
Airspace Classifications.

It's high time the UK was represented in this node; while the UK does have the same classifications of airspace as the US, their similarities end with their names.

Why do we have airspace classifications? Because pilots are the enemy.

They gather around smoke-shrouded tables at night and conspire to make the lives of air traffic controllers a misery, by doing things like flying through localisers, busting their levels, giving wrong readbacks, penetrating (oooh) controlled airspace without clearance, having TCAS fitted, and worst of all, taking off.

Yes yes, we're all professionals, abiding respect, yada yada yada. We're all for the reinstitution of surface to air missiles at airfields. We're all for the remote mining of taxiways, so anything with a piston engine never makes it from the stand to the holding point. We're all for taxi into position and hold, as long as a weapons-free AC-130 Spectre Gunship is on a 1-mile final. Come to think of it, why stop there? The Soviet Union had the right idea, churning out anti-aircraft missiles like sausages and scattering them around the country. Strategic defences would engage the transatlantic traffic early using nuclear warheads, long-range interceptors would break up the field as they got closer, handing them off to the medium/short-range SAMs to clean up any stragglers that actually got near to the airfields. No tyre-marks would ever tarnish our runways, no sir.

This would be bad news for the passengers, of course. But good news for the controllers - they'd know the score, they wouldn't have to work much traffic ("Lufthansa 458, traffic information: Stinger missile at 3 o'clock, 5 miles, impact in ten seconds. Have a nice day"), there's no major problems there. For a controller, an empty radar screen is a home run. You wouldn't find a better work-to-pay ratio this side of Paris Hilton.

Of course, the airlines wouldn't like it - passenger booking figures would probably fall off, that sort of thing. So sensibility prevailed and a system was devised, a derivation of which is still used today, in fact.

It provides a reasonably fair division of airspace so that fast, commercial traffic has some skyways allocated, but hobbyists who want to potter about in their Cessnas without any restrictions are also catered for and all get to travel in relative safety. The poor defenceless public transport flights are kept safe from those loose cannons in their Skyhawks and Rallyes, not to mention the Military. The classifications of airspace are freely available to the public (start at, so pilots know what to expect, and what is expected of them, in the airspace they want to fly in. Also, whether they are actually allowed to fly in the airspace they want to fly in.

All sections of airspace in the UK are designated with a letter from A to G. Classes A-E are collectively classed as ‘Controlled’ airspace; the remainder cleverly titled ‘Uncontrolled’. There isn’t currently any Class B airspace in the UK but I will describe it anyway. There are also some airspace divisions within those classifications, some of which are covered elsewhere. Each class of airspace has different restrictions on what aircraft may enter, what is required of aircraft that do enter, and what level of air traffic service will be provided to them.

One major difference between the UK and US classifications of airspace is the vertical restrictions. Such limits do exist under the US system but generally they are not applied specifically to airspace classifications in the UK. A specific area of airspace may have a vertical limit, but that will usually be independent of its classification.

Class A Airspace

This is like the friendly dictatorship of the air, and the preferred domain of most public transport flights. Only IFR (instrument flight rules) flights are allowed into Class A airspace, and not without a clearance from air traffic control or a flight plan. IFR flights navigate using instruments and electronic navigation aids on the ground, as opposed to VFR (visual flight rules) flights which navigate using visual references – landmarks, terrain features and such. ATC will separate all aircraft in Class A airspace, and pilots are required to maintain a ‘listening watch’ on the radio and comply with ATC instructions.

Class A airspace is the one exception to the ‘vertical limit’ caveat mentioned earlier, and it is not so much a property as it is a general rule – Class A airspace does not exist above Flight Level 195 (about 19,500ft).

It is worth noting that the terms ‘airway’ and ‘Class A airspace’ are generally interchangeable; virtually all airways are Class A and similarly, virtually all Class A airspace is airways. The only exception to this is the Control Zone that surrounds London Heathrow Airport (LHR/EGLL), which is Class A (generally, Control Zones that surround aerodromes are Class D). Heathrow’s traffic volumes have afforded it certain statuses, for example that small single or twin-engined private aircraft are not allowed to use it. See, these guys are with me. There are plenty of other restrictions for flights that want to enter this airspace but they are beyond the scope of this writeup.

Class B Airspace

Again, no airspace in the UK has been designated Class B since 2007. This was ostensibly done to bring the UK in line with vertical airspace divisions in the rest of Europe, but I like to think it’s because Class B airspace was such a pain in the ass for controllers.

It used to be that all airspace in the UK above FL245 was Class B, all the way up to FL660 (about 66,000ft, above which it was — and is — Class G). Class B has some interesting restrictions, and by ‘restrictions’ I mean ‘unbridled freedoms.’

Of course I exaggerate – pilots that want to enter Class B airspace still have to have filed a flight plan, still have to get a clearance from ATC before they enter, still have to listen out on the radio and still have to follow ATC instructions once they're in. However, clearance permitting, any flight may enter Class B airspace, IFR or VFR. And the worst part: ATC are required to separate them all.

In other classes of airspace, there are caveats such as ATC being required to separate IFR flights from VFR flights, but only having to give the VFR flights traffic information on other flights nearby. In Class B airspace, it is the responsibility of ATC to separate:

  • IFR flights from other IFR flights
  • IFR flights from VFR flights, and
  • VFR flights from other VFR flights!

This is a royal pain, because as I’ve explained elsewhere, ATC have very little control over VFR flights. VFR flights can’t be told to climb, descend or turn (although arguably, in grave/dire/exceptional circumstances, ATC may break this rule) because to follow that instruction may take the aircraft into cloud. It would then no longer be able to operate under VFR and if the pilot were not instrument-rated, would be operating illegally.

Generally, the only direct instruction that may be given to a VFR flight (beyond, for example, a takeoff or landing clearance) is an altitude limitation: in giving a takeoff clearance to a VFR flight, the controller in the Tower may say “Maintain at or below 2000ft before departing <my airspace>,” or in the UK, “Not above 2000ft.” The pilot can fly whatever altitude he wishes within that band.

Imagine trying to separate two aircraft when you have virtually no control over what one of them is doing. Or both of them! Not an enviable situation, and it used to be possible over the entire UK. The practical reality of this still baffles me, since its use pre-dates my entry into the aviation industry. It could be circumvented fairly easily, though, by refusing entry clearance to an aircraft that might cause the controller problems under this remit.

Class C Airspace

Class C airspace has kinda-sorta replaced Class B airspace in the UK. The vertical divider between it and the airspace below has dropped from FL245 to FL195, but similar to before, all UK airspace above that level is Class C. This is shared by other EU countries (most of whom don’t use Class B airspace at all).

The entry requirements for this airspace class are the same as those for Classes A and B, but the controller’s responsibilities are reduced; in Class C airspace they must:

  • separate IFR flights from other IFR flights
  • separate IFR flights and VFR flights
  • give VFR flights traffic information on other VFR flights so they can avoid it themselves.

The VFR provisions there generally do not crop up often, certainly above FL195, as it has been decreed that VFR flight at or above that level will only be permitted under exceptional circumstances.

Class D Airspace

Class D airspace generally surrounds aerodromes. Many of them, certainly the larger ones, are surrounded by a block of airspace called a ‘Control Zone’ (CTR – yes, I know the abbreviation is illogical) for the protection of traffic landing, departing, and flying in the immediate vicinity (aircraft flying training circuits of the airfield, for example). Aside from London Heathrow’s CTR, most are Class D.

They are not uniform in shape, but seen from above frequently resemble a circle with a slice taken off opposite sides. Their longest dimension generally follows the centreline of the aerodrome’s main runway. Their horizontal and vertical extents are all specified in legal documents.

Continuing the more-relaxed trend, ATC only has to separate IFR flights from other IFR flights in Class D airspace. They give IFR flights traffic information on VFR flights that might be in the way, and give VFR flights traffic information on any other flights that might be in their way.

Class E Airspace

Class E airspace is rather a white elephant and is the least-common active classification of airspace in the UK. The reason for this is most likely a nonsensical rule included in its specification, which is that while IFR flights have to get a clearance from ATC before entering Class E airspace, VFR flights do not!

Although higher classes of airspace impose increasing requirements on the controller for keeping aircraft apart, the controller is still free to refuse entry to an aircraft that may come into conflict with others. This provision/get-out is partially absent in Class E airspace. VFR flights can come and go as they please. Thankfully, controllers only have to give IFR flights traffic information about these unruly VFR interlopers. They are only required to actually separate IFR flights from each other.

There are only three small areas of Class E airspace in the UK. The areas of other classifications number in the dozens or hundreds.

Class F Airspace

This airspace class is also called ‘Advisory Airspace.’ The operating conditions here are virtually identical to Class D airspace, except that the phrase ‘participating aircraft’ is inserted in front of all requirements. ATC separate participating IFR flights from each other, but do not give any sort of separation or traffic information service to VFR flights.

As this is advisory airspace, unlike higher airspace classes no control service (also called ‘positive control,’) can technically be provided by ATC. However, in practice, Radar Advisory Services are provided in a fashion that is virtually identical to positive control services, with the pilot just stating if they do not intend to follow the ‘advice’ they are being given.

Class G Airspace

This is all the rest. Any airspace that is not designated class A-F is class G. This is uncontrolled airspace, and anyone can fly around doing what they like in this airspace with few restrictions. Bandit country.

In certain areas it is still possible for flights to be provided with air traffic services in Class G airspace, though this is always on the pilot’s request. Some airfields have their own radar unit (virtually all large airports in the UK do not have their approach control handled on-site, but at a remote Area centre) which can provide air traffic services to aircraft flying near the airfield.

The types of services that can be given are different to those provided inside controlled airspace but they are beyond the scope of this writeup.

Interestingly, since all UK airspace above FL660 is unclassified and therefore uncontrolled, in theory it is possible to fly VFR up there. The only obstacle is all of the controlled airspace you would have to climb through to get there, which if you’re flying VFR would be problematic.

archiewood wonders whether the SR-71 flew VFR...

This covers all of the classifications of UK airspace but not all of the divisions – the country is replete with Terminal Control Areas, Military Training Areas, Areas of Intense Aerial Activity, Aerial Tactics Areas, Air to Air Refueling Areas, High Intensity Radio Transmission Areas, Prohibited Areas, Restricted Areas and Danger Areas that inhabit these classifications, as well as…others. I could describe them all but then there’d be no reason to read any of my other writeups. Ahem.

Despite my bluster, I actually like the ATC job.

Source: UK AIP

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