The world’s navigable airspace
is divided into three-dimensional
segments, each of which is assigned to a specific
. Most nations adhere to the classification
specified by the International
Civil Aviation Organization (ICAO) and described below.
nations also designate Special Use Airspace
, which places
further rules on air navigation for reasons of national security or
On March 12 1990
adopted the current airspace classification scheme. The classes are
fundamentally defined in terms of flight
and interactions between aircraft
and Air Traffic Control
Some key concepts are:
Maintaining a specific minimum distance between an aircraft and
another aircraft or terrain to avoid collisions, normally by
requiring aircraft to fly at set levels or level bands, on set
routes or in certain directions, or by controlling an aircraft's
- Clearance: Permission given by ATC for an aircraft to proceed
under certain conditions contained within the clearance.
- Traffic Information: Information given by ATC on the position
and, if known, intentions of other aircraft likely to pose a hazard
The classifications adopted by ICAO are:
- Class A: All operations must be conducted under Instrument Flight Rules (IFR) or
Special visual flight
rules (SVFR) and are subject to ATC clearance. All flights are
separated from each other by ATC.
- Class B: Operations may be conducted under IFR, SVFR, or
Visual flight rules (VFR). All
aircraft are subject to ATC clearance. All flights are separated
from each other by ATC.
- Class C: Operations may be conducted under IFR, SVFR, or VFR.
All flights are subject to ATC clearance. Aircraft operating under
IFR and SVFR are separated from each other and from flights
operating under VFR. Flights operating under VFR are given traffic
information in respect of other VFR flights.
- Class D: Operations may be conducted under IFR, SVFR, or VFR.
All flights are subject to ATC clearance. Aircraft operating under
IFR and SVFR are separated from each other, and are given traffic
information in respect of VFR flights. Flights operating under VFR
are given traffic information in respect of all other flights.
- Class E: Operations may be conducted under IFR, SVFR, or VFR.
Aircraft operating under IFR and SVFR are separated from each
other, and are subject to ATC clearance. Flights under VFR are not
subject to ATC clearance. As far as is practical, traffic
information is given to all flights in respect of VFR flights.
- Class F: Operations may be conducted under IFR or VFR. ATC
separation will be provided, so far as practical, to aircraft
operating under IFR. Traffic Information may be given as far as is
practical in respect of other flights.
- Class G: Operations may be conducted under IFR or VFR. ATC
separation is not provided. Traffic Information may be given as far
as is practical in respect of other flights.
Classes A-E are referred to as controlled airspace
. Classes F and G are
As of 2004, ICAO is considering a proposal to reduce the number of
airspace classifications to three (N, K and U), which roughly
correspond to the current classes C, E and G.
These proposed classes are described at:
Use of airspace classes
Each national aviation authority determines how it uses the ICAO
classifications in its airspace design. In some countries, the
rules are modified slightly to fit the airspace rules and air
traffic services that existed before the ICAO
U.S. adopted a
slightly modified version of the ICAO system on September 16 1993, when
regions of airspace designated according to older classifications
were converted wholesale.
The exceptions are some Terminal
Radar Service Areas (TRSA), which have special rules and still
exist in a few places.
With some exceptions, Class A airspace is applied to all airspace
between and Flight Level
(approximately 60,000 ft). Above FL600, the airspace reverts to
Class E (Reference Order 7400.9P, Subpart E). The transition
altitude (see Flight level
also consistently . All operations in US Class A airspace must be
conducted under IFR. SVFR flight in Class A airspace is
airspace is used around major airports, in
an inverted wedding cake shape that is designed to contain arriving
and departing commercial air traffic operating under IFR, up to
above MSL (12,000 feet above Denver,
Class C airspace is used around airports with a moderate traffic
level. Class D is used for smaller airports that have a control
tower. The U.S. uses a modified version of the ICAO class C and D
airspace, where only radio contact with ATC rather than an ATC
clearance is required for VFR operations.
Other controlled airspace is designated as Class E - this includes
a large part of the lower airspace. Class E airspace exists in many
forms. It can serve as a surface-based extension to Class D
airspace to accommodate IFR approach/departure procedure areas.
Class E airspace can be designated to have a floor of 700' AGL
(above ground level) or 1,200' AGL. Class E airspace exists above
Class G surface areas from 14,500' MSL (mean sea level) to 18,000
MSL. Federal airways
AGL to 18,000 MSL within of the centerline of the airway is
designated Class E airspace. Airspace at any altitude over 60,000'
(the ceiling of Class A airspace) is designated Class E
The U.S. does not use ICAO Class F.
Class G airspace (Uncontrolled) is mostly used for a small layer of
airspace near the ground, but there are larger areas of Class G
airspace in remote regions.
Canada generally follows the United States in application of
airspace with some differences. For example, Canadian class "C"
procedurally equivalent to United States class "B" airspace.
Additionally, the term "Class F" is used for Special Use Airspace,
this includes Advisory airspace and Restricted airspace.
Germany, Classes A and B are not used at all.
C is used for Airspace above Flight
(FL) 100 (or FL 130 near the Alps
to FL 660. Airspace is divided into lower airspace
FL 245 and upper airspace
above FL 245.
- Class C is used for controlled zones above and around
airports and airspace above FL 100 (or FL 130 near the Alps) up to
- Class D is used for controlled zones or above and
around airspace class C designated zones where CVFR is not necessary.
- Class E is used for airspace between usually . AGL
(around airports . or 1700 ft. AGL) and FL 100.
- Class F is used for IFR-Flight in
- Class G is used below AGL (around airports below AGL,
then rises via a step at to AGL)
Lithuania, Classes A and B are generally not used at
Classes C and D are used in the following areas of
controlled airspace of the Republic of Lithuania:
- in control zones (CTR);
- in terminal control areas (TMA);
- in control area (CTA);
- in upper control area (UTA).
: Airfield Guide Lithuania, 29 SEP 2005, ENR 1.1-1
airways up to FL 195 with the exception of airways lying within the
Belfast CTR/TMA and the Scottish TMA.
Terminal Control Areas (TMAs) around London and Manchester.
London Control Zone around Heathrow and the Channel
Islands Control Zone; these areas are thus off-limits to VFR
flights (however Special VFR is used as
a get-around for this).
- The CTAs of Daventry, Cotswold and Worthing.
All UK airspace between FL 195 and FL 660.N.b: The Upper Flight
Information Region (UIR) boundary begins at FL 245
the Belfast and Scottish TMAs and a small part of the Durham Tees
"Advisory Routes" (ADRs): regularly used routes similar to airways
but where traffic levels are not high enough to warrant
establishment of an airway.
All remaining airspace, comprising by far the largest part of the
airspace below FL 195. The UK is unusual in that IFR flight in
Class G airspace is relatively common and ATC units may provide an
"as far as is practical" form of separation between some such
A clearance is not required for VFR flights within Class E
airspace, however pilots are strongly advised to contact the
In addition the UK has a couple of special classes of airspace that
do not fall within the ICAO classes:
Aerodrome Traffic Zones
(ATZ) are zones around an
airport with a radius of 2 nm or 2.5 nm, extending from
the surface to AAL. Aircraft within an ATZ must obey the
instructions of the tower controller (if present), or must make
radio contact with the Aerodrome Flight Information Service unit or
Air/Ground Communication Service unit for the aerodrome before
entering the zone (in the case of an uncontrolled airfield), or
must obey ground signals if non-radio.
Military Air Traffic Zones
(MATZ) are zones from
the surface to AAL (above aerodrome level) set up around military
air bases in class G airspace. Military aircraft treat these as if
they are controlled airspace; civilian traffic are advised but not
obliged to do the same.
has adopted a civil airspace
system based on the United States National Airspace System
- Class A is used above FL 180 along the populated
coastal areas, and above FL 245 elsewhere.
- Class B is not used.
- Class C is used in a 360° funnel shape in the Terminal
Control Zones of the major international airports, extending up to
the base of the Class A, generally at FL 180 over these airports.
It also overlays Class D airspace at smaller airports.
- Class D is used for the Terminal Control Zones of
medium sized airports, extending from the surface up to . Above
this, Class C airspace is used, although generally only in a
sector, and not 360° around the airport.
- Class E is used along the populated coastal areas,
from to the base of the overlying Class A or Class C airspace.
- Class F is not used.
- Class G is used wherever other classes are not -
almost always from the surface to the base of the overlying Class
A, C, D or E airspace.
In addition, Australia has a non-standard class of airspace for use
at the capital
city general aviation
called a General Aviation Airport Procedures Zone (GAAP Zone). A
control tower provides procedural clearances for all aircraft
inside the zone. Additionally, any aircraft operating within
5 nm of the zone must obtain a clearance. VFR aircraft arrive
and depart using standard arrival and departure routes, while
instrument arrival and departure procedures are published for IFR
operations. During visual meteorological
(VMC), IFR aircraft are not provided with full IFR
services. During instrument meteorological
(IMC), or marginal VMC, VFR operations are
restricted in order to facilitate full IFR service for IFR
Airspace classes and VFR
Authorities use the ICAO definitions to derive additional rules for
VFR cloud clearance, visibility, and equipment requirements.
For example, consider Class E airspace. An aircraft operating under
VFR may not be in communication with ATC, so it is imperative that
its pilot be able to see and avoid other aircraft (and vice versa).
That includes IFR flights emerging from a cloud, so the VFR flight
must keep a designated distance from the edges of clouds above,
below, and laterally, and must maintain at least a designated
visibility, to give the two aircraft time to observe and avoid each
other. The low-level speed limit of 250 knot
does not apply above , so the visibility
requirements are higher.
On the other hand, in Class B airspace, separation is provided by
ATC to all flights. Now the VFR flight only needs to see where it
is going, so visibility requirements are reduced and there is no
designated minimum distance from clouds.
Similar considerations determine whether a VFR flight must use a
and/or a transponder
Special use airspace
Each national authority designates areas of special use airspace
(SUA), primarily for reasons of national security. This is not a
separate classification from the ATC-based classes; each piece of
SUA is contained in one or more zones of letter-classed
SUAs range in restrictiveness, from areas where flight is always
prohibited except to authorized aircraft, to areas that are not
charted but are used by military for potentially hazardous
operations (in this case, the onus is on the military personnel to
avoid conflict). Refer to the external links for more specific