Air Traffic Control (ATC)
is a service provided by ground-based controllers who direct
aircraft on the ground and in the air. A controller's primary
task is to separate certain aircraft — to
prevent them from coming too close to each other horizontally and
vertically. Secondary tasks include ensuring orderly and
expeditious traffic flow and
providing advisories, such as weather information
and navigation directions (vectors).
In many countries, ATC services are provided throughout the
majority of airspace, and its services are available to all users
(private, military, and commercial). When controllers are
responsible for separating some or all aircraft, such airspace is
called "controlled airspace" in contrast to "uncontrolled
airspace." Depending on the type of flight and the class of
airspace, ATC may issue instructions that
pilots are required to follow, or
merely advisories to assist pilots operating
in the airspace. In all cases, however, the pilot has final
responsibility for the safety of the flight, and may deviate from
ATC instructions in an emergency.
Air traffic control services can be divided into two major
subspecialties, terminal
control and en-route control.
Terminal control includes the control of traffic (aircraft and
vehicles) on the airport surface and airborne aircraft within the
immediate airport environment. Generally, this is approximately a
30 to 50 nautical mile (56 to 93 km) radius of the airport, from
the surface to about 10,000 ft (about 3,050 m). Terminal
controllers work in facilities called control towers and terminal
area control (called Terminal Radar Approach
Control, or TRACON, in the U.S.). At some locations,
controllers are shared between tower control and the terminal
area control, while at others the tower and the terminal area
control are completely separate entities. For example,
Philadelphia International Airport is served by a combined
("up/down") facility, while Chicago's O'Hare Airport is served by
a control tower at the airport, and a remote TRACON located at
Elgin, Illinois.
En-route controllers control the traffic between the terminals.
They can also control traffic in and out of airports where the
traffic volume does not warrant the establishment of a terminal
ATC operation. En-route controllers work at facilities called
Area Control Centers or Air Route Traffic Control Centers.
The primary method of controlling the immediate airport
environment is visual observation from the control tower. The
tower is a tall, windowed structure located on the airport
grounds. Tower 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 2 to 5 nautical miles (4 to 9 km)
depending on the airport procedures.
Radar displays are also available to controllers at some
airports. Controllers may use a radar system called Secondary
Surveillance Radar also known as Airport 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, heading,
and other information described in local procedures.
The areas of responsibility for tower controllers fall into three
general operational disciplines; Ground
Control (Ground Movement Control,
or GMC in the
U.K.), Local
Control (Tower in North
America), and Clearance
Delivery (Planner in the
U.K.) -- other categories, such as Apron
Control, may exist at extremely busy airports. While
each tower's procedures will vary and while there may be multiple
teams in larger towers that control multiple runways, the
following provides a general concept of the delegation of
responsibilities within the tower environment.
Ground Control (sometimes known as Ground Movement
Control) is responsible for the airport "movement" areas, or
areas not released to the airlines or other users. This generally
includes all taxiways, holding areas, and some transitional
aprons or intersections where aircraft arrive having vacated the
runway and departure gates. 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 the ground controller. This is normally done via VHF radio,
but there may be special cases where other processes are used.
Most aircraft and airside vehicles have radios. Aircraft or
vehicles without radios will communicate with the tower via
aviation light signals or will be led by vehicles with radios.
People working on the airport surface normally have a
communications link through which they can reach or be reached by
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 might constrain the order in which
the aircraft will be sequenced to depart, which can affect the
safety and efficiency of the airport's operation.
Some busier airports have systems, such as, ASDE-3, AMASS or
ASDE-X, designed to display aircraft and vehicles on the ground.
These are used by the ground controller 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.
Local Control (most often referred to as the generic "Tower"
control, although Tower control can also refer to a combination
of the local, ground and clearance delivery positions) is
responsible for the active runway surfaces. Local control clears
aircraft for take off or landing and ensures the runway is clear
for these aircraft. To accomplish this, local control controllers
are normally given 2 to 5 nautical miles (4 to 9 km) of airspace
around the airport, allowing them to give the clearances
necessary for airport safety. If the local controller detects any
unsafe condition, a landing aircraft will be told to "go around"
and will be re-sequenced into the landing pattern by the terminal
area controller.
Within the tower, a highly disciplined communications process
between local and ground control is an absolute necessity. Ground
control must request and gain approval from local control to
cross any runway with any aircraft or vehicle. Likewise, local
control must ensure ground control is aware of any operations
that impact the taxiways and must work with the arrival radar
controllers to ensure "holes" in the arrival traffic are created
(where necessary) to allow taxiing traffic to cross runways and
to allow departures aircraft to take off. Crew resource
management procedures are often used to ensure this communication
process is efficient and clear.
Clearance delivery (sometimes known as Planner) is
the position that coordinates with the national command center
and the en-route center to obtain releases for aircraft. Under
normal conditions, this is more or less automatic. When weather
or extremely high demand for a certain airport become a factor,
there may be ground "stops" (or delays), or re-routes to ensure
the system does not get overloaded. The primary responsibility of
the clearance delivery position is to ensure that the aircraft
have the proper route and release time. This information is also
coordinated with the en-route center and the ground controller in
order to ensure the aircraft reaches the runway in time to meet
the release time provided by the command center.
Larger airports have a radar control facility that is associated
with the control tower. In most countries, this is referred to
asTerminal Area Control; in the U.S., it is often still
referred to as a TRACON
or Terminal Radar Approach CONtrol
facility (sometimes referred to as Approach or Departure
control). While every airport varies, terminal controllers
usually handle traffic in a 30 to 50 nautical mile (56 to 93 km)
radius from the airport and from the surface up to 10,000 feet.
The actual airspace boundaries and altitudes assigned to a TRACON
are based on factors such as traffic flows and terrain, and vary
widely from airport to airport.
Terminal area controllers are responsible for providing all ATC
services within their airspace. Traffic flow is broadly divided
into departures, arrivals, overflights, and VFR aircraft. As
aircraft move in and out of the terminal airspace, they are
handed off to the next appropriate control facility (a control
tower, an en-route control facility, or a bordering terminal area
control). Terminal is responsible for ensuring that aircraft are
at an appropriate altitude when they are handed off, and that
aircraft arrive at a slow enough rate to permit safe landing
times.
Not all airports have terminal area control available. In this
case, the en-route center will coordinate directly with the tower
and provide this type of service where radar coverage permits.
Under these circumstances, the separation minimums are usually
increased.
ATC provides services to aircraft in flight between airports as
well. The level of service is dependent on the type of flight the
aircraft falls under (IFR or VFR), the type of airspace the
aircraft is in and the services requested by the pilots.
En-route Air Traffic Controllers issue clearances and
instructions for airborne aircraft, and pilots are required to
comply with these instructions. 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 services.
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.
Pilots flying under VFR assume responsibility for their
separation from all other aircraft and are not assigned routes or
altitudes by ATC (outside of positive control airspace). They fly
on their own using a "see and be seen" separation criteria. In
busier controlled airspace, VFR aircraft are required to have a
transponder. This amplifies the radar signal (as well
broadcasting altitude level and a transponder code), and is used
to allow controllers to warn IFR aircraft of any potential
conflict. Governing agencies establish strict VFR "weather
minima" for visibility, distance from clouds, and altitude to
ensure that VFR pilots can be seen from a far enough distance.
VFR pilots can request, and ATC can elect to provide "VFR
Advisory Services," if the controllers' workload permits. This is
also referred to as "Flight following." Under this environment,
the controllers will radar identify the VFR aircraft and provide
traffic information and weather advisory services for the VFR
pilot. Controllers do not provide any instructions concerning
direction of flight, altitude, or speed to the VFR pilot
receiving advisory services, and they do not provide separation
services. This is an optional service and may be discontinued by
ATC or the pilot at any time.
Pilots flying under IFR in controlled airspace typically file a
flight plan with ATC and accept any revisions ATC requests to
their route or altitude. In return, controllers will ensure that
pilots flying IFR are separated from all other IFR aircraft and
terrain by the appropriate minimum separation, either through
radar services or by tracking flights through mandatory radio
reports from pilots. The IFR pilot, however, must maintain a
close watch for VFR aircraft since ATC may have no control over
these aircraft, depending on the altitude and the class of
airspace. In many areas, VFR aircraft are restricted to lower
altitudes (typically below 18,000 feet MSL in the U.S.) and must
have an operating transponder in congested airspace. In some
countries, all aircraft, VFR or IFR, must operate under positive
ATC control.
En-route air traffic controllers work in facilities called Area
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 the aircraft departs
an airport or leaves the terminal area's airspace or until the
aircraft approaches the airspace controlled by a terminal area or
if the airport does not have terminal area control, until the
aircraft lands. Centers may also "pick up" aircraft that are
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 climbing the aircraft to
their requested 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 cross traffic, severe weather, special missions
that require large airspace allocations, and traffic density.
As an aircraft reaches the boundary of a Center's control area it
is "handed off" to the next Area Control Center. This "hand-off"
process is simply a transfer of identification between
controllers so that air traffic control services can be provided
in a seamless manner. Once the hand-off is complete, 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 area controller ("approach").
Since centers control a large airspace area, they will typically
use long range radar that has the capability to see aircraft
within 200 nautical miles (370 km) of the radar antenna. They may
also use TRACON radar data to control when it provides a better
"picture" of the traffic or when it can fill in a portion of the
area not covered by the long range radar.
In the U.S. system, at higher altitudes, over 90% of the U.S.
airspace is covered by radar and often by multiple radar systems;
however, coverage may be inconsistent at lower altitudes used by
unpressurized aircraft due to high terrain or distance from radar
facilities. A center may require numerous radar systems to cover
the airspace assigned to them, and may also rely on pilot
position reports from aircraft flying below the floor of radar
coverage. This results in a large amount of data being available
to the controller. To address this, automation systems have been
designed that consolidate the radar data for the controller. This
consolidation includes eliminating duplicate radar returns,
ensuring the best radar for each geographical area is providing
the data, and displaying the data in an effective format.
Centers also exercise control over traffic travelling over the
world's ocean areas. These areas are also FIRs. Due to the fact
that there are no radar systems available for oceanic control,
oceanic controllers provide ATC services using "non-radar"
procedures. 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.
Some Air Navigation Service Providers (e.g Airservices Australia,
Alaska Center, etc.) are implementing Automatic dependent
Surveillance - Broadcast (ADS-B) as part of their surveillance
capability. This new technology reverses the radar concept.
Instead of radar "finding" a target by interrogating the
transponder, ADS transmits the aircraft's position several times
a second. ADS also has other modes such as the "contract" mode
where the aircraft reports a position based on a pre-determined
time interval. This is significant because it can be used where
it is not possible to locate the infrastructure for a radar
system (e.g. over water). Computerised radar displays are now
being designed to accept ADS inputs as part of the display. As
this technology develops, oceanic ATC procedures will be
modernised to take advantage of the benefits this technology
provides.
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 end of the runway. This process
requires between 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 typical large airport with
two arrival runways can thus 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 was a common occurrence at airports. Advances in
computers now allow controllers to predict transit times and
sequence planes hours in advance. Thus, planes may be delayed
before they even take off, or may reduce power in flight and
proceed more slowly in order to fit perfectly into a landing
sequence without holding.
Beyond runway capacity issues, weather is a major factor in
traffic capacity. Rain or ice and snow 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. This, in turn, increases 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 ACCs, 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 Area Control Centers, 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. By default, the callsign is
the registration number (tail number) of the aircraft, such as
"N12345" or "CG-ABC".
For scheduled flights, military flights, and some other flights,
the operators obtain permission to use an airline call sign
followed by a flight number, instead of a registration number. In
this arrangement, an identical call sign might well be used for
the same scheduled journey each day it is operated, even if the
departure time varies a little across different days of the week.
The call sign of the return flight often differs only by the
final digit, from the outbound flight. Generally, airline flight
numbers are even if eastbound, and odd if westbound. In air
traffic control terminology, a block of airspace of predetermined
size assigned to a radar air traffic controller is called a
"sector". Depending on various factors (traffic density, etc.), a
controller may be responsible for one or more sectors at any
given time.
Many technologies are used in air traffic control systems.
Primary and secondary radar are used to enhance a controller's
"situational 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.
Other correlations with electronic flight plans are also
available to controllers on modern operational display systems.
Some tools are available in different domains to help the
controller further:
- Conflict Alert (CA): a tool that checks possible
conflicting trajectories and alerts the controller.
- 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.
- Converging Runway Display Aid (CRDA) enables Approach
controllers to run two final approaches that intersect and make
sure that go arounds are minimized
- Final Approach Spacing Tool (FAST) gives aircraft a runway
assignment that the Approach Controller will give to the
aircraft. FAST can also suggest vectors for downwind and base
with the correct timing.
- User Request Evaluation Tool (URET) takes paper strips out
of the equation for En Route controllers at ARTCC's. By
providing a display that shows all aircraft in or entering the
sector. Provides conflict resolution up to 30 minutes in
advance.