Air traffic control
Air traffic control
3.1 Ground control 3.2 Air control or local control 3.3 Flight data and clearance delivery 3.4 Approach and terminal control
4 En route, center, or area control
4.1 General characteristics
5.1 Traffic 5.2 Weather
6 Callsigns 7 Technology 8 Air navigation service providers (ansps) and air traffic service providers (atsps) 9 Proposed changes
10 ATC regulations in the United States 11 See also 12 References 13 External links
Aeronautical phraseology and Aviation English
Pursuant to requirements of the International Civil Aviation
Organization (ICAO), ATC operations are conducted either in the
English language or the language used by the station on the ground.
In practice, the native language for a region is normally used;
however, the English language must be used upon request.
In 1920, Croydon Airport, London was the first airport in the world to
introduce air traffic control.
In the United States, air traffic control developed three divisions.
The first of air mail radio stations (AMRS) was created in 1922 after
World War I when the U.S. Post Office began using techniques developed
by the Army to direct and track the movements of reconnaissance
aircraft. Over time, the AMRS morphed into flight service stations.
Today's flight service stations do not issue control instructions, but
provide pilots with many other flight related informational services.
They do relay control instructions from ATC in areas where flight
service is the only facility with radio or phone coverage. The first
airport traffic control tower, regulating arrivals, departures and
surface movement of aircraft at a specific airport, opened in
Cleveland in 1930. Approach/departure control facilities were created
after adoption of radar in the 1950s to monitor and control the busy
airspace around larger airports. The first air route traffic control
center, which directs the movement of aircraft between departure and
destination was opened in Newark, NJ in 1935, followed in 1936 by
Chicago and Cleveland.
Air traffic control
São Paulo–Guarulhos International Airport's control tower
The primary method of controlling the immediate airport environment is visual observation from the airport control tower. The tower is a tall, windowed structure located on the airport grounds. Air traffic controllers are responsible for the separation and efficient movement of aircraft and vehicles operating on the taxiways and runways of the airport itself, and aircraft in the air near the airport, generally 5 to 10 nautical miles (9 to 18 km) depending on the airport procedures. Surveillance displays are also available to controllers at larger airports to assist with controlling air traffic. Controllers may use a radar system called secondary surveillance radar for airborne traffic approaching and departing. These displays include a map of the area, the position of various aircraft, and data tags that include aircraft identification, speed, altitude, and other information described in local procedures. In adverse weather conditions the tower controllers may also use surface movement radar (SMR), surface movement guidance and control systems (SMGCS) or advanced SMGCS to control traffic on the manoeuvring area (taxiways and runway). The areas of responsibility for tower controllers fall into three general operational disciplines; local control or air control, ground control, and flight data / clearance delivery—other categories, such as Apron control or ground movement planner, may exist at extremely busy airports. While each tower may have unique airport-specific procedures, such as multiple teams of controllers ('crews') at major or complex airports with multiple runways, the following provides a general concept of the delegation of responsibilities within the tower environment. Remote and virtual tower (RVT) is a system based on air traffic controllers being located somewhere other than at the local airport tower and still able to provide air traffic control services. Displays for the air traffic controllers may be live video, synthetic images based on surveillance sensor data, or both. Ground control Ground control (sometimes known as ground movement control) is responsible for the airport "movement" areas, as well as areas not released to the airlines or other users. This generally includes all taxiways, inactive runways, holding areas, and some transitional aprons or intersections where aircraft arrive, having vacated the runway or departure gate. Exact areas and control responsibilities are clearly defined in local documents and agreements at each airport. Any aircraft, vehicle, or person walking or working in these areas is required to have clearance from ground control. This is normally done via VHF/UHF radio, but there may be special cases where other procedures are used. Aircraft or vehicles without radios must respond to ATC instructions via aviation light signals or else be led by vehicles with radios. People working on the airport surface normally have a communications link through which they can communicate with ground control, commonly either by handheld radio or even cell phone. Ground control is vital to the smooth operation of the airport, because this position impacts the sequencing of departure aircraft, affecting the safety and efficiency of the airport's operation. Some busier airports have surface movement radar (SMR), such as, ASDE-3, AMASS or ASDE-X, designed to display aircraft and vehicles on the ground. These are used by ground control as an additional tool to control ground traffic, particularly at night or in poor visibility. There are a wide range of capabilities on these systems as they are being modernized. Older systems will display a map of the airport and the target. Newer systems include the capability to display higher quality mapping, radar target, data blocks, and safety alerts, and to interface with other systems such as digital flight strips. Air control or local control Air control (known to pilots as "tower" or "tower control") is responsible for the active runway surfaces. Air control clears aircraft for takeoff or landing, ensuring that prescribed runway separation will exist at all times. If the air controller detects any unsafe conditions, a landing aircraft may be instructed to "go-around" and be re-sequenced into the landing pattern. This re-sequencing will depend on the type of flight and may be handled by the air controller, approach or terminal area controller. Within the tower, a highly disciplined communications process between air control and ground control is an absolute necessity. Air control must ensure that ground control is aware of any operations that will impact the taxiways, and work with the approach radar controllers to create "gaps" in the arrival traffic to allow taxiing traffic to cross runways and to allow departing aircraft to take off. Ground control need to keep the air controllers aware of the traffic flow towards their runways in order to maximise runway utilisation through effective approach spacing. Crew resource management (CRM) procedures are often used to ensure this communication process is efficient and clear. Within ATC, it is usually known as TRM (Team Resource Management) and the level of focus on TRM varies within different ATC organisations. Flight data and clearance delivery Clearance delivery is the position that issues route clearances to aircraft, typically before they commence taxiing. These clearances contain details of the route that the aircraft is expected to fly after departure. Clearance delivery or, at busy airports, Ground Movement Planner (GMP) or Traffic Management Coordinator (TMC) will, if necessary, coordinate with the relevant radar centre or flow control unit to obtain releases for aircraft. At busy airports, these releases are often automatic and are controlled by local agreements allowing "free-flow" departures. When weather or extremely high demand for a certain airport or airspace becomes a factor, there may be ground "stops" (or "slot delays") or re-routes may be necessary to ensure the system does not get overloaded. The primary responsibility of clearance delivery is to ensure that the aircraft have the correct aerodrome information, such as weather and airport conditions, the correct route after departure and time restrictions relating to that flight. This information is also coordinated with the relevant radar centre or flow control unit and ground control in order to ensure that the aircraft reaches the runway in time to meet the time restriction provided by the relevant unit. At some airports, clearance delivery also plans aircraft push-backs and engine starts, in which case it is known as the Ground Movement Planner (GMP): this position is particularly important at heavily congested airports to prevent taxiway and apron gridlock. Flight data (which is routinely combined with clearance delivery) is the position that is responsible for ensuring that both controllers and pilots have the most current information: pertinent weather changes, outages, airport ground delays/ground stops, runway closures, etc. Flight data may inform the pilots using a recorded continuous loop on a specific frequency known as the automatic terminal information service (ATIS). Approach and terminal control
Potomac Consolidated TRACON
Many airports have a radar control facility that is associated with
the airport. In most countries, this is referred to as terminal
control; in the U.S., it is referred to as a
The training department at the Washington Air Route Traffic Control Center, Leesburg, Virginia, United States.
Main article: Area control center
ATC provides services to aircraft in flight between airports as well.
Pilots fly under one of two sets of rules for separation: visual
flight rules (VFR) or instrument flight rules (IFR). Air traffic
controllers have different responsibilities to aircraft operating
under the different sets of rules. While IFR flights are under
positive control, in the US VFR pilots can request flight following,
which provides traffic advisory services on a time permitting basis
and may also provide assistance in avoiding areas of weather and
flight restrictions. Across Europe, pilots may request for a "Flight
Information Service", which is similar to flight following. In the UK
it is known as a "traffic service".
En-route air traffic controllers issue clearances and instructions for
airborne aircraft, and pilots are required to comply with these
instructions. En-route controllers also provide air traffic control
services to many smaller airports around the country, including
clearance off of the ground and clearance for approach to an airport.
Controllers adhere to a set of separation standards that define the
minimum distance allowed between aircraft. These distances vary
depending on the equipment and procedures used in providing ATC
En-route air traffic controllers work in facilities called air traffic
control centers, each of which is commonly referred to as a "center".
The United States uses the equivalent term air route traffic control
center (ARTCC). Each center is responsible for many thousands of
square miles of airspace (known as a flight information region) and
for the airports within that airspace. Centers control IFR aircraft
from the time they depart from an airport or terminal area's airspace
to the time they arrive at another airport or terminal area's
airspace. Centers may also "pick up" VFR aircraft that are already
airborne and integrate them into the IFR system. These aircraft must,
however, remain VFR until the center provides a clearance.
Center controllers are responsible for issuing instructions to pilots
to climb their aircraft to their assigned altitude while, at the same
time, ensuring that the aircraft is properly separated from all other
aircraft in the immediate area. Additionally, the aircraft must be
placed in a flow consistent with the aircraft's route of flight. This
effort is complicated by crossing traffic, severe weather, special
missions that require large airspace allocations, and traffic density.
When the aircraft approaches its destination, the center is
responsible for issuing instructions to pilots so that they will meet
altitude restrictions by specific points, as well as providing many
destination airports with a traffic flow, which prohibits all of the
arrivals being "bunched together". These "flow restrictions" often
begin in the middle of the route, as controllers will position
aircraft landing in the same destination so that when the aircraft are
close to their destination they are sequenced.
As an aircraft reaches the boundary of a center's control area it is
"handed off" or "handed over" to the next Area Control Center. In some
cases this "hand-off" process involves a transfer of identification
and details between controllers so that air traffic control services
can be provided in a seamless manner; in other cases local agreements
may allow "silent handovers" such that the receiving center does not
require any co-ordination if traffic is presented in an agreed manner.
After the hand-off, the aircraft is given a frequency change and
begins talking to the next controller. This process continues until
the aircraft is handed off to a terminal controller ("approach").
Unmanned radar on a remote mountain
Centers also exercise control over traffic travelling over the world's
ocean areas. These areas are also flight information regions (FIRs).
Because there are no radar systems available for oceanic control,
oceanic controllers provide ATC services using procedural control.
These procedures use aircraft position reports, time, altitude,
distance, and speed to ensure separation. Controllers record
information on flight progress strips and in specially developed
oceanic computer systems as aircraft report positions. This process
requires that aircraft be separated by greater distances, which
reduces the overall capacity for any given route. See for example the
North Atlantic Track
For more information see Air traffic flow management.
Intersecting contrails of aircraft over London, an area of high air traffic.
The day-to-day problems faced by the air traffic control system are
primarily related to the volume of air traffic demand placed on the
system and weather. Several factors dictate the amount of traffic that
can land at an airport in a given amount of time. Each landing
aircraft must touch down, slow, and exit the runway before the next
crosses the approach end of the runway. This process requires at least
one and up to four minutes for each aircraft. Allowing for departures
between arrivals, each runway can thus handle about 30 arrivals per
hour. A large airport with two arrival runways can handle about 60
arrivals per hour in good weather. Problems begin when airlines
schedule more arrivals into an airport than can be physically handled,
or when delays elsewhere cause groups of aircraft – that would
otherwise be separated in time – to arrive simultaneously. Aircraft
must then be delayed in the air by holding over specified locations
until they may be safely sequenced to the runway. Up until the 1990s,
holding, which has significant environmental and cost implications,
was a routine occurrence at many airports. Advances in computers now
allow the sequencing of planes hours in advance. Thus, planes may be
delayed before they even take off (by being given a "slot"), or may
reduce speed in flight and proceed more slowly thus significantly
reducing the amount of holding.
Air traffic control
Airplane taking off from
Dallas/Fort Worth International Airport
Beyond runway capacity issues, the weather is a major factor in
traffic capacity. Rain, ice, snow or hail on the runway cause landing
aircraft to take longer to slow and exit, thus reducing the safe
arrival rate and requiring more space between landing aircraft. Fog
also requires a decrease in the landing rate. These, in turn, increase
airborne delay for holding aircraft. If more aircraft are scheduled
than can be safely and efficiently held in the air, a ground delay
program may be established, delaying aircraft on the ground before
departure due to conditions at the arrival airport.
In Area Control Centers, a major weather problem is thunderstorms,
which present a variety of hazards to aircraft. Aircraft will deviate
around storms, reducing the capacity of the en-route system by
requiring more space per aircraft or causing congestion as many
aircraft try to move through a single hole in a line of thunderstorms.
Occasionally weather considerations cause delays to aircraft prior to
their departure as routes are closed by thunderstorms.
Much money has been spent on creating software to streamline this
process. However, at some ACCs, air traffic controllers still record
data for each flight on strips of paper and personally coordinate
their paths. In newer sites, these flight progress strips have been
replaced by electronic data presented on computer screens. As new
equipment is brought in, more and more sites are upgrading away from
paper flight strips.
A prerequisite to safe air traffic separation is the assignment and
use of distinctive call signs. These are permanently allocated by ICAO
on request usually to scheduled flights and some air forces and other
military services for military flights. They are written callsigns
with a 3-letter combination like KLM, BAW, VLG followed by the flight
number, like AAL872, VLG1011. As such they appear on flight plans and
ATC radar labels. There are also the audio or Radiotelephony callsigns
used on the radio contact between pilots and air traffic control.
These are not always identical to their written counterparts. An
example of an audio callsign would be "Speedbird 832", instead of the
written "BAW832". This is used to reduce the chance of confusion
between ATC and the aircraft. By default, the callsign for any other
flight is the registration number (tail number) of the aircraft, such
as "N12345", "C-GABC" or "EC-IZD". The short Radiotelephony callsigns
for these tail numbers is the last 3 letters using the NATO phonetic
alphabet (i.e. ABC spoken alpha-bravo-charlie for C-GABC) or the last
3 numbers (i.e. three-four-five for N12345). In the United States, the
prefix may be an aircraft type, model or manufacturer in place of the
first registration character, for example, "N11842" could become
"Cessna 842". This abbreviation is only allowed after
communications have been established in each sector.
Before around 1980
International Air Transport Association
The air traffic control tower at Hartsfield–Jackson Atlanta International Airport.
Many technologies are used in air traffic control systems. Primary and
secondary radar are used to enhance a controller's situation awareness
within his assigned airspace – all types of aircraft send back
primary echoes of varying sizes to controllers' screens as radar
energy is bounced off their skins, and transponder-equipped aircraft
reply to secondary radar interrogations by giving an ID (Mode A), an
altitude (Mode C) and/or a unique callsign (Mode S). Certain types of
weather may also register on the radar screen.
These inputs, added to data from other radars, are correlated to build
the air situation. Some basic processing occurs on the radar tracks,
such as calculating ground speed and magnetic headings.
Usually, a flight data processing system manages all the flight plan
related data, incorporating – in a low or high degree – the
information of the track once the correlation between them (flight
plan and track) is established. All this information is distributed to
modern operational display systems, making it available to
Flight data processing systems: this is the system (usually one per center) that processes all the information related to the flight (the flight plan), typically in the time horizon from gate to gate (airport departure/arrival gates). It uses such processed information to invoke other flight plan related tools (such as e.g. MTCD), and distributes such processed information to all the stakeholders (air traffic controllers, collateral centers, airports, etc.). Short-term conflict alert (STCA) that checks possible conflicting trajectories in a time horizon of about 2 or 3 minutes (or even less in approach context – 35 seconds in the French Roissy & Orly approach centres) and alerts the controller prior to the loss of separation. The algorithms used may also provide in some systems a possible vectoring solution, that is, the manner in which to turn, descend, increase/decrease speed, or climb the aircraft in order to avoid infringing the minimum safety distance or altitude clearance. Minimum safe altitude warning (MSAW): a tool that alerts the controller if an aircraft appears to be flying too low to the ground or will impact terrain based on its current altitude and heading. System coordination (SYSCO) to enable controller to negotiate the release of flights from one sector to another. Area penetration warning (APW) to inform a controller that a flight will penetrate a restricted area. Arrival and departure manager to help sequence the takeoff and landing of aircraft.
The departure manager (DMAN): A system aid for the ATC at airports, that calculates a planned departure flow with the goal to maintain an optimal throughput at the runway, reduce queuing at holding point and distribute the information to various stakeholders at the airport (i.e. the airline, ground handling and air traffic control (ATC)). The arrival manager (AMAN): A system aid for the ATC at airports, that calculates a planned arrival flow with the goal to maintain an optimal throughput at the runway, reduce arrival queuing and distribute the information to various stakeholders. Passive final approach spacing tool (pFAST), a CTAS tool, provides runway assignment and sequence number advisories to terminal controllers to improve the arrival rate at congested airports. pFAST was deployed and operational at five US TRACONs before being cancelled. NASA research included an active FAST capability that also provided vector and speed advisories to implement the runway and sequence advisories.
Converging runway display aid (CRDA) enables approach controllers to
run two final approaches that intersect and make sure that go arounds
In the US, user request evaluation tool (URET) takes paper strips out of the equation for en route controllers at ARTCCs by providing a display that shows all aircraft that are either in or currently routed into the sector. In Europe, several MTCD tools are available: iFACTS (NATS), VAFORIT (DFS), new FDPS (MUAC). The SESAR programme should soon launch new MTCD concepts.
URET and MTCD provide conflict advisories up to 30 minutes in advance and have a suite of assistance tools that assist in evaluating resolution options and pilot requests.
Mode S: provides a data downlink of flight parameters via secondary surveillance radars allowing radar processing systems and therefore controllers to see various data on a flight, including airframe unique id (24-bits encoded), indicated airspeed and flight director selected level, amongst others. CPDLC: controller-pilot data link communications – allows digital messages to be sent between controllers and pilots, avoiding the need to use radiotelephony. It is especially useful in areas where difficult-to-use HF radiotelephony was previously used for communication with aircraft, e.g. oceans. This is currently in use in various parts of the world including the Atlantic and Pacific oceans. ADS-B: automatic dependent surveillance broadcast – provides a data downlink of various flight parameters to air traffic control systems via the transponder (1090 MHz) and reception of those data by other aircraft in the vicinity. The most important is the aircraft's latitude, longitude and level: such data can be utilized to create a radar-like display of aircraft for controllers and thus allows a form of pseudo-radar control to be done in areas where the installation of radar is either prohibitive on the grounds of low traffic levels, or technically not feasible (e.g. oceans). This is currently in use in Australia, Canada and parts of the Pacific Ocean and Alaska. The electronic flight strip system (e-strip):
Electronic flight progress strip system at São Paulo Intl. control tower – ground control
A system of electronic flight strips replacing the old paper strips is
being used by several service providers, such as Nav Canada, MASUAC,
DFS, DECEA. E-strips allows controllers to manage electronic flight
data online without paper strips, reducing the need for manual
functions, creating new tools and reducing the ATCO's workload. The
firsts electronic flight strips systems were independently and
simultaneously invented and implemented by
Screen content recording: Hardware or software based recording function which is part of most modern automation system and that captures the screen content shown to the ATCO. Such recordings are used for a later replay together with audio recording for investigations and post event analysis. Communication navigation surveillance / air traffic management (CNS/ATM) systems are communications, navigation, and surveillance systems, employing digital technologies, including satellite systems together with various levels of automation, applied in support of a seamless global air traffic management system.
Air navigation service providers (ansps) and air traffic service
Main article: Air
Azerbaijan – AzərAeroNaviqasiya
Albania – Agjencia Nacionale e Trafikut Ajror
Algeria – Etablissement National de la
Guatemala – Dirección General de Aeronáutica Civil (DGAC) El Salvador Honduras Nicaragua – Empresa Administradora Aeropuertos Internacionales (EAAI) Costa Rica – Dirección General de Aviación Civil Belize
Chile – Dirección General de Aeronáutica Civil (DGAC)
Colombia – Aeronáutica Civil Colombiana (UAEAC)
Croatia – Hrvatska kontrola zračne plovidbe (Croatia Control Ltd.)
Cuba – Instituto de Aeronáutica Civil de Cuba (IACC)
Czech Republic – Řízení letového provozu ČR
Proposed changes In the United States, some alterations to traffic control procedures are being examined.
The Next Generation Air Transportation System examines how to overhaul the United States national airspace system. Free flight is a developing air traffic control method that uses no centralized control (e.g. air traffic controllers). Instead, parts of airspace are reserved dynamically and automatically in a distributed way using computer communication to ensure the required separation between aircraft.
In Europe, the SESAR (Single European Sky ATM Research) programme
plans to develop new methods, technologies, procedures, and systems to
accommodate future (2020 and beyond) air traffic needs.
Change in regulation in admittance for possible A.T.C.'s regarding
their eye-refraction and correction thereof by technology has been
Many countries have also privatized or corporatized their air
navigation service providers. There are several models that can be
used for ATC service providers. The first is to have the ATC services
be part of a government agency as is currently the case in the United
States. The problem with this model is that funding can be
inconsistent and can disrupt the development and operation of
services. Sometimes funding can disappear when lawmakers cannot
approve budgets in time. Both proponents and opponents of
privatization recognize that stable funding is one of the major
factors for successful upgrades of ATC infrastructure. Some of the
funding issues include sequestration and politicization of
projects. Proponents argue that moving ATC services to a private
corporation could stabilize funding over the long term which will
result in more predictable planning and rollout of new technology as
well as training of personnel.
Another model is to have ATC services provided by a government
corporation. This model is used in Germany, where funding is obtained
through user fees. Yet another model is to have a for-profit
corporation operate ATC services. This is the model used in the United
Kingdom, but there have been several issues with the system there
including a large-scale failure in December 2014 which caused delays
and cancellations and has been attributed to cost-cutting measures put
in place by this corporation. In fact, earlier that year, the
corporation owned by the German government won the bid to provide ATC
services for Gatwick
Air traffic service Flight information service officer Flight planning
Forward air control Global air-traffic management Tower en route control (TEC)
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