Air Traffic Control - Airspace
Within every country, airspace is either classified as "controlled" or "uncontrolled". Besides these, there are several special use airspaces which also includes either controlled or uncontrolled segments. As already pointed out in the previous article, pilots must know which flight restrictions or aircraft equipment requirements are applicable in these different types of airspace. The operational requirements within each airspace depends on the class of airspace and the altitude at which the aircraft flies at.

Controlled Airspace

Controlled airspace means specific layers of air of defined dimensions within which air traffic control service is being provided to traffic flying under both IFR and VFR rules in accordance with the airspace classification. When we are talking about controlled airspace we mean those airspaces that are classified as either Class A, Class B, Class C, Class D or Class E airspace. Operating within one of these airspaces means that the pilot should make sure the flight is operated conform ATC clearances and the transponder must be on, including Mode C if installed. During the flight, ATC provides separation between all aircraft, and if workload permits ATC also provides traffic advisories for VFR operation. An important issue for pilots to remember is that ATC is not required to separate IFR traffic from VFR traffic as they cannot provide such service for aircraft not appearing on the radar display. It is therefore the responsibility of the pilot to see and avoid aircraft operating under VFR rules when flying under IFR rules in VFR weather conditions. Table 1 provides an overview of the basic weather minimums which apply in Class A,B,C,D and E controlled airspace for VFR operations.

Table 1: Basic weather minimums applicable for VFR flight in controlled airspace (FAR 91.155).
Class Flight Visibility Distance From Clouds
Class A
Not Applicable
Not Applicable
Class B
3 Statute Miles
Clear of Clouds
Class C & Class D
3 Statute Miles
2.000 Feet Horizontal
1.000 Feet Above
500 Feet Below
Class E (Less than 10.000 ft MSL
3 Statute Miles
2.000 Feet Horizontal
1.000 Feet Above
500 Feet Below
Class E (At or above 10.000 ft MSL
5 Statute Miles
1 Statute Mile Horizontal
1.000 Feet Above
1.000 Feet Below

This table clearly shows that VFR flight is not permitted in Class A airspace. Furthermore, in order to operate within controlled airspace aircraft should meet certain requirements. Aircraft should have a transponder with Mode C capability in Class A airspace, Class B airspace, within 30 nautical miles of Class B primary airports, and in or above Class C airspace. When flying under IFR, Mode C should be operated at all times unless instructed otherwise by ATC.

Uncontrolled Airspace
Within densely populated areas not so many uncontrolled airspaces can be found, and if any these are relatively small. Uncontrolled airspace is used primarily by leisure traffic or military practicing purposes. Flying within an uncontrolled airspace does not necessarily mean that there is not radio communication between the pilot and air traffic control. However, in these circumstances, ATC merely provides the pilot or crew with flight information such as AFIS or weather reports. Within the boundaries of the Amsterdam FIR for example, uncontrolled airspace extends from ground level up to the lower level limits of the TMA- and CTA-areas and partly above the North Sea extending from ground level up to an altitude of 3000 feet. According to the airspace classes set by ICAO both Class F and Class G respresent uncontrolled airspace.