Air Traffic Control - Airspace Classification
The airspace around the world is divided into numerous Flight Information Regions, or FIRs, and consist of both controlled and uncontrolled airspaces. Futhermore, the airspace is divided into so-called airspace classes which denotes the level of ATC service provided and the rules and regulations applicable to the specific airspace you are flying in. Worldwide there are seven airspace classes defined as either Class A, Class B, Class C, Class D, Class E, Class F or Class G airspace. Class A represents the most restrictive airspace where no VFR traffic is allowed while Class G is the least restrictive and is related to uncontrolled airspace.

Class A Airspace

Aircraft flying within Class A airspace typically operate at high speeds in the higher levels of the atmosphere. As it would be impractical for pilots to reset their altimeters every 100 nautical miles, every aircraft operator is required to use a standard altimeter setting of 29.92 in. Hg. (or 1013 hPa outside the United States) in Class A airspace.This means that all aircraft maintain their assigned altitudes as all use the same altimeter reference. In addition, when the standard altimeter setting is used once the transition altitude is crossed, altitudes are prefaced by the letter FL, meaning flight level, with the last two zeros omitted. For example, 39000 feet is calld FL390.

Aircraft seen while flying in Class A airspace

In order to fly in Class A airspace, there are certain guidelines that must be adhered:

  1. The pilot in command must be rated and current for instrument flight.
  2. The flight should be operated under an IFR plan and in accordance with an ATC clearance at specified flight levels.
  3. The aircraft must be equipped with instruments required for IFR operations, including a transponder and encoding altimeter. Besides this, the aircraft also needs to have a radio that provides direct pilot/controller communication on the frequency specified by ATC for the area the flight is operating in.
  4. In case VOR equipment is required for navigation, the aircraft operated should also have Distance Measuring Equipment (DME) if the flight is conducted at or above 24.000 feet MSL. If a DME failure occurs, ATC should be informed instantly and the flight may continue at or above 24.000 feet MSL and proceed to the nearest airport or airport of intended landing.
Class B Airspace

ATC Class B airspace

Class B airspace is frequently established at the country's busiest airports and is used to separate all arriving and departing traffic. The airspace consists of a surface area and two or more layers which are specifcally designed to facilitate traffic separation at a particular terminal. As in most airspaces, pilot participation is required and an ATC clearance must be optained prior to entering Class B airspace. In some countries Class B airspace areas have VFR corridors to allow pilots of VFR aircraft to pass through them without contacting ATC.

In order to fly in Class B airspace, the aircraft must be equipped with a two-way radio and a 4096-code transponder with C mode capability. However, a Mode S transponder can be used anywhere a Mode C transponder is required for operation. For IFR operations, the aircraft must also have a VOR receiver. In addition, the pilot in command should at least hold a private pilot certificate.

Class C Airspace

ATC Class C airspace

At certain airports Class C airspace areas are designated, which basiscally means that ATC is equipped to provide radar service for all aircraft flying in this particular area. Class C airspace normally consists of two circular areas which extend outward from the primary airport and are referred to as the "5 nautical mile radius core area" and the "ten nautical mile radius shelf area". In addition, an outer area extends ten nautical miles beyond the outer circle. Pilot participation within this area is not required although strongly encouraged. Before operating within the core and shelf areas, the pilot must estabilish two-way communication with ATC and maintain radio contact at all times.
In case a take-off is commenced at a satellite airport located within Class C airspace, the pilot must establish two-way communication with ATC as soon as practible.

In order to fly in Class C airspace, all aircraft must be equipped with an operable transponder with Mode C capability. Aircraft operating underneath Class C airspace are not obliged to be equipped with a transponder having Mode C capability.

Class D Airspace

ATC Class D airspace

Class D airspace areas are designated at airports that have operating control towers which are not associated with Class B or Class C airspace. Before entering Class D airspace the pilot must establish and maintain two-way radio communication with ATC (in this case the control tower). Departing from an airport situated within Class D airspace requires two-way radio communication with the tower as well untill the moment the aircraft leaves this particular area and switches to another frequency.

Some satellite airports are situated within the same Class D airspace designated for the primary airport. In case this satellite airport is a non-tower airfield, both arriving and departing aircraft must establish radio contact with the primary airport's control tower. If the satellite airport has a control tower, similar two-way radio communication requirements with that tower prevail for both arrivals and departures.

Class D airspace usually extends to 2.500 feet above the surface of the airport converted to mean sea level or MSL. The ceiling of Class D airspace is usually shown in hundreds of feet MSL on sectional charts. Furthermore, Class D airspace consists of a circular area with a four nautical mile radius but may vary depending on instrument procedures established for that particular airfield.

Class E Airspace
Much of the remaining airspace is designated as Class E airspace. Flying within Class E airspace does not mean that there are additional operating requirements beyond those mentioned previously. To give an example, the aircraft must be equipped with a Mode C capable transponder and the pilot must apply the appropriate cloud clearance and visibility requirements when flying under VFR. As Class E airspace normally extends up to the lower limit of Class A airspace, it is important to remember that operating under VFR is prohibited when crossing this limit. Another segment of Class E airspace are the so-called Victor airways, or Federal Airways, and are based on VOR to VORTAC navigation connecting one navaid to another. These airways are identified by a V and the airway number.

Class E airspace transitional areas have also been established between airports and the airway route system to allow IFR traffic to remain in controlled airspace while transitioning between the enroute and airport environments.

Class F Airspace
Class F airspace is designated to confine activities that are potentially hazardous to aircraft that are not involved in the activity. Class F airspace is usually divided into two categories, Advisory, and Restricted.

Airspace within an Advisory area may be controlled or uncontrolled, or a combination of these two. Class F Advisory airspace is commonly designated to those parts of the airspace where activities such as gliding, high traffic training areas, military operations take place and it would be of benefit to aircraft operators to be aware that such activities are taking place within this particular area. Overall, VFR traffic should try to avoid these areas if practible while IFR traffic is not permitted to enter Class F airspace unless the pilot has stated he has obtained permission to do so.

Restricted areas are more dangerous than those designated as Advisory. They are established when safety or security is an issue, for example when live firing or high speed training takes place or around high importance locations. Although IFR traffic occasionally may cross Advisory areas, aircraft are kept clear of Restricted areas by the appropriate vertical minimum.

Class G Airspace
Obviously, Class G airspace is that area which has not been designated as either Class A, Class B, Class C, Class D, Class E or Class F airspace and is essentially uncontrolled by ATC. Airspace below Class E airspace or below a Victor airway is normally uncontrolled. As already pointed out, within Class G airspace ATC does not have responsibility for or authority over aircraft operating in this particular area. However, most of the regulations that affect pilots and aircraft still apply. Although it is not necessary to file a flight plan for IFR flight in uncontrolled airspace, both pilot and aircraft must still be fully qualified to operate under IFR. In many cases, the day weather minimums for VFR flight are reduced from those in controlled airspace as can be seen in table 1.

Table 1: Weather minimums for VFR operations in Class G airspace.
Altitude Flight Visibility Distance From Clouds
1.200 feet or less above surface

Day: 1 Statute Mile

Night: 3 Statute Miles

Clear of Clouds

500 Feet Below
1.000 Feet Above
2.000 Feet Horizontal

More than 1.200 feet above surface, but less than 10.000 feet MSL

Day: 1 Statute Mile

Night: 3 Statute Miles

500 Feet Below
1.000 Feet Above
2.000 Feet Horizontal
More than 1.200 feet above the surface and at or above 10.000 feet MSL
Day & Night: 5 Statute Miles

1.000 Feet Below
1.000 Feet Above
1 Statute Mile Horizontal