Air Traffic
Control
Paris Airport
The control tower at an airport in
Paris, France, stands over airplanes waiting at the terminals. Air traffic
controllers use radar, computers, and radio to track air traffic and issue
instructions for takeoffs and landings. Airport operations include a variety of
jobs necessary to assure smooth and safe transportation.
Air Traffic Control, various aircraft navigation
and communication systems that use computers, radar, radios, and other
instruments and devices to provide guidance to flying aircraft. Trained
personnel working as air traffic controllers at stations on the ground
constantly monitor these systems and track the locations and speeds of
individual aircraft. Controllers can warn aircraft should they come too close
to each other. Air traffic control is also used for the safe coordination of
landings and takeoffs at airports.
The goal of air traffic
control is to minimize the risk of aircraft collisions while maximizing the
number of aircraft that can fly safely at the same time. Aircraft pilots and
their onboard flight crews work closely with controllers to manage air traffic.
Air traffic control systems also provide updated weather information to
airports around the country, so aircraft can take off and land safely. This
information is important not only to airline passengers but also to industries
that rely on aviation for the timely transport of goods, materials, and
personnel.
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ELEMENTS OF AIR TRAFFIC CONTROL
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Air traffic control is a
combination of three general elements. The first element is the basic set of
flying rules that pilots follow in the air. These are much like the traffic
rules that motorists must obey. The second element is the multitude of
electronic navigation systems and instruments that pilots use to remain on
course. The third element is made up of air traffic controllers and the
computer systems they use to track aircraft during takeoff, flight, and
landing. These three elements work together to keep aircraft safely separated
in the air and to avoid collisions.
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Flight Rules
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The basic system of air
traffic control relies on the ability of pilots to provide their own navigation
in order to see and visually avoid other aircraft. This system is known as
Visual Flight Rules (VFR). Under VFR pilots navigate using charts that display
terrain features, airports, and landmarks. VFR pilots also may use radio
beacons or other ground-based navigational aids to monitor their flight path.
To avoid other aircraft, pilots fly at specified altitudes reserved for their
general direction of flight. Pilots also simply keep a constant lookout for
other aircraft. VFR works well where visibility is good, aircraft speeds are
fairly low, and air traffic is sparse. VFR pilots must remain clear of clouds
and have a range of visibility of at least 5 km (3 mi).
When any of the VFR conditions
cannot be met, or if a pilot is operating in a busy area, aircraft must be
operated under Instrument Flight Rules (IFR). IFR is a more complex set of
rules, and pilots flying under IFR must have an instrument pilot certificate.
IFR requires that pilots notify the airport control tower of their intended
route before takeoff, a procedure known as filing a flight plan. Once the tower
gives clearance, the pilot may take off. The pilot must also maintain radio
contact with air traffic controllers during the flight. IFR is required
whenever flight visibility is less than 5 km (3 mi), when pilots must fly
through clouds, or when pilots are flying in congested areas. Airlines and
larger aircraft normally operate under IFR at all times. In the United States,
the Federal Aviation Administration (FAA) is the federal agency that regulates
air travel. The FAA requires that all aircraft use IFR when flying near major
metropolitan areas or at the high altitudes normally used by commercial
airliners.
The flight crew of an
aircraft, made up of the pilot and any other personnel that fly or navigate the
aircraft, use various instruments when flying under IFR. These instruments are
designed to work in any weather condition, day or night, and tell the pilot the
direction and speed of the aircraft. The altimeter indicates altitude, and the
airspeed indicator shows how fast the aircraft is moving. The attitude
indicator shows how the aircraft is tilted in flight. Other instruments
indicate direction.
The flight crew also uses
radio to stay in contact with air traffic controllers. Flight crews file flight
plans with the control tower by radio, and ask for clearance before taking off
or landing at an airport. Another communications instrument used by aircraft is
an automatic device called a transponder. A transponder sends an electronic
identification signal to air traffic control centers on the ground. Controllers
use transponder signals to identify individual aircraft and track their
positions by computer.
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Navigation Systems
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VOR Station
VOR (very-high-frequency omnidirectional
range) stations are radio antennas on the ground that broadcast navigation
signals in all directions to aircraft. By using a special receiver, a pilot can
determine his or her aircraft's direction of travel relative to a VOR station.
Pilots navigate from station to station along corridors known as airways.
Navigation systems assist pilots
in flying from one airport to another. These systems help both pilots and air
traffic controllers determine an aircraft’s position relative to the ground and
to other aircraft. At high altitudes, or during bad weather, navigation systems
are essential for safe aircraft flight. Navigation systems have developed from
fairly inaccurate ground-based radio transmitters to sophisticated space-based
systems.
The earliest navigational
aids were simple radio beacons, in use since 1924. Radio beacons provided the
pilot with only the ability to head toward the beacon. Although fairly
inaccurate, beacons were inexpensive to install and were at one time fairly
numerous. Advances in navigation technology led the FAA to decommission many of
these navigation aids.
The basic electronic navigation
system in use is the VHF omnidirectional range (VOR) system. VOR consists of a
series of radio stations that beam direction information to aircraft. Most VOR
stations also have distance-measuring equipment (DME). A display indicator in the
aircraft reads the signals and tells the pilots if they are on course and how
far they are from the station. VOR-DME systems are limited in range to 260 km
(160 mi) and can only provide direct courses to or from a given station. This
limitation compelled the FAA to install thousands of ground stations across the
United States and to provide over 8,000 airway segments connecting each VOR-DME
station to another.
Researchers have been working
since the 1950s to increase the flexibility of the VOR system. Area navigation
systems have been developed that permit a pilot to fly directly from one
airport to another, bypassing the VOR airways. Loran (long range
navigation) is a radio system that automatically calculates an aircraft’s
position and provides direct navigation guidance to any location. However, the
charged particles in the layer of the atmosphere known as the ionosphere limit
the radio range of Loran signals and can sometimes cause interference.
Flight Navigation System (VOR)
An omnirange station broadcasts radio
beams that pilots within a radius of 160 km (100 mi) may use for navigation.
The VOR (Very High Frequency Omnidirectional Range) station uses a central
antenna to broadcast a continuous reference signal and four variable-signal
antennas that produce a signal rotated at 1,800 rpm. A pilot sets a desired
course manually, then relies on electronic equipment to interpret and process
the signals received from the VOR station. The airplane receiver compares the
phases of the signals to determine the bearing of the plane, then indicates
whether the plane is to the left or right of the desired course.
Satellites provide a better
system of area navigation than ground-based radio stations. In the 1980s the
U.S. Department of Defense developed a highly accurate satellite-based
navigation system known as the Global Positioning System, or GPS. GPS and other
satellite navigation systems provide highly accurate positioning information to
anyone using an appropriate receiver.
GPS-type systems are so
accurate that the FAA and its international counterpart, the International
Civil Aviation Organization (ICAO), have agreed that satellite navigation will
become the standard for international aviation navigation. Satellite navigation
provides adequate accuracy for in-flight navigation, but will need to be
improved if it is to guide aircraft during the more complex landing procedure.
Two systems have been developed and are planned for installation by the FAA.
One system, called the Wide Area Augmentation System (WAAS), uses a satellite
transmitter to send accuracy corrections to all aircraft operating over the
continental United States. The other, the Local Area Augmentation System
(LAAS), will be installed at airports to provide guidance information that will
allow automated aircraft landings in any type of weather.
One type of instrument
navigation that does not rely on radio or satellite transmissions is inertial
guidance. Inertial guidance uses mechanical or laser gyroscopes to determine
precisely an aircraft’s direction of flight. When an inertial guidance system
has been programmed correctly, it can provide direction to any point in the
world. Although inertial guidance is fairly costly, its biggest advantage is
that it is a self-contained system, independent of either ground or space-based
transmitters.
The navigation instruments that
pilots use to land aircraft during foul weather are more sensitive than those
used to navigate during flight. The systems mentioned above only guide aircraft
to within 2 km (1 mi) of the end of an airport runway. To guide aircraft to a
safe landing, many runways have been equipped with the Instrument Landing
System (ILS). The ILS uses two transmitters to guide aircraft to within 800 m
(0.5 mi) of the runway. One transmitter provides altitude information as the
aircraft approaches the runway, and the other transmitter alerts the pilot if
the aircraft drifts to the left or right of the runway path. More sophisticated
versions of the ILS guide aircraft to within 400 meters (0.25 mi) of the
runway, or to the runway itself for an automatic landing. The combination of
the satellite-based WAAS and LAAS is planned to replace ILS and should provide
approaches to the major runways in the United States.
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Air Traffic Controllers
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Air traffic controllers make up
the third segment of air traffic control, managing the location of aircraft to
ensure the safest and most efficient use of airspace. Controllers use radar and
transponder signals to monitor aircraft positions and altitudes within a given
area of airspace. Controllers also track hazardous weather and obstructions to
flight, and relay this information to flight crews. Air traffic controllers
work in one of three different types of stations. Air Route Traffic Control
Centers (ARTCC) are located nationwide and track all air traffic within their
airspace. Flight Service Stations provide weather information to pilots, and
are also located nationwide. Control towers are located at airports, and
coordinate aircraft landings and takeoffs.
The ARTCCs are responsible
for the separation of IFR aircraft as they fly between airports. ARTCC
controllers also guide IFR aircraft operating from small airports that do not
have control towers. There are 22 ARTCCs in the United States, each employing
hundreds of controllers. Each ARTCC is centered around a huge room that houses
radio and computer equipment and between 50 and 100 radar displays. Each
display is assigned to an individual sector, or area of airspace, within radio
range of the ARTCC. Each sector is monitored by as many as four controllers at
a time. The controllers include a radar controller, a radar associate (who acts
as an assistant), a flight data controller (who performs much of the routine
computer entries), and a coordinator (who communicates information to
surrounding sectors). ARTCCs also employ traffic management controllers, who
monitor overall traffic flow and make any traffic adjustments needed to reduce
aircraft delays.
The FAA also operates about
90 flight service stations. These stations provide weather briefings and pass
along weather and flight planning information to pilots. They also record
flight plans from pilots, provide in-flight assistance to VFR aircraft, and
coordinate search and rescue operations. Most flight service stations are
automated.
Airport control towers
coordinate landings and takeoffs, and are probably the air traffic control
facilities most visible to the public. The first towers were small glassed-in
rooms built on top of airport terminal buildings. Modern towers are hundreds of
feet high, with room for a dozen controllers to work at one time. The local
controller has responsibility for ensuring that the runways are clear before
permitting landings and takeoffs. The ground controller is responsible for
aircraft taxiing to and from runways. Clearance delivery controllers issue IFR
clearances to pilots, while flight data controllers operate the computer
equipment. The busiest towers also employ traffic management controllers to
help coordinate traffic flows. Major metropolitan airports also use radar to
guide aircraft safely in and out of the busy airspace around the airport. These
radar facilities, known as TRACONs, perform many of the functions of an ARTCC,
but within the airspace surrounding an airport.
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HOW AIR TRAFFIC CONTROL WORKS
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Radar Dish
Radar dishes, such as this one at
London's Heathrow airport, enable air-traffic controllers to safely and
efficiently direct airplanes in flight. The shape of the dish is designed to
focus radar waves into a beam that scatters off aircraft. The part of the beam
that gets reflected is detected by the radar dish and gives important
information about the airplane, such as its altitude, heading, and speed.
Before departure, IFR pilots
file a flight plan and contact the clearance delivery controller to receive
their clearance to fly. A clearance includes the route and flight altitude, the
frequencies for radio and transponder use, and departure instructions. At
airports with a control tower, both IFR and VFR pilots contact the ground
controller to receive taxi instructions, which tell the pilot which runway to
use, and when to proceed. When ready for departure, the pilot contacts the
local controller. When the local controller is confident that the runway and
all intersections are clear of traffic, the airplane is cleared for takeoff.
Once airborne, IFR pilots contact the departure controller to receive heading
and altitude instructions, guiding the airplane to the appropriate airway. VFR
pilots usually navigate visually to their destination airport.
In most cases, airliners
and business aircraft file IFR flight plans and use the ATC system during their
entire flight, even if the weather is suitable for visual navigation. This is a
safety requirement of both the FAA and the airlines, since these flights occur
at high speed and in congested areas. Small, privately owned aircraft usually
operate under VFR once they have left the immediate vicinity of the airport.
Since VFR pilots operate at low altitudes where airliners do not typically fly,
and at much slower speeds, it is easier for them to see and avoid other
aircraft. If they operate exclusively from small airports, they may never need
to contact a controller at all. But once within the vicinity of a large
airport, they are required to make contact with a controller so that separation
of all aircraft can be provided.
Air traffic controllers watch
radar displays that show the locations of individual aircraft. These displays
also predict future positions and altitudes of aircraft. If the computer
detects that two aircraft might come too close to each other or that one
aircraft might descend to an inappropriately low altitude, it will sound an
alert and the controller will tell the pilots to change course. A similar
computer system installed in most airliners is called the traffic
alert/collision avoidance system, or TCAS. TCAS independently monitors the
positions of nearby aircraft and determines whether a potential for collision
exists. If TCAS predicts a potential problem, it alerts the pilots
automatically and issues course and altitude changes to avoid a collision.
Once an aircraft has flown
50 km (30 mi) from the airport, the departure controller transfers, or hands
off, the tracking signal to a succession of ARTCC controllers. ARTCC
controllers monitor the aircraft’s progress, separate it from other aircraft,
and issue route or speed changes when needed to avoid bad weather or to keep
the aircraft in the proper flow of traffic. As an aircraft flies out of range
and toward another ARTCC, the tracking controller hands off the signal to a
controller at the next ARTCC, who monitors the aircraft as it continues on its
journey.
Once the aircraft is close
to its destination, the controller issues arrival instructions to the pilot,
and then hands the aircraft off to the approach controller at the airport. VFR
pilots usually contact approach control 50 km (30 mi) from the destination
airport. Approach control is responsible for lining inbound aircraft up for the
runway. Once aircraft are properly spaced, local control takes over and issues
landing instructions. If there is a delay in landing, an aerial traffic jam can
develop. To avoid this, aircraft waiting to land are directed to a holding area
away from the runway. At the holding area the waiting aircraft circle a radio
beacon at different altitudes, forming a stack of aircraft. When a runway
becomes available, an airplane at the bottom of the stack is instructed to
land, and the waiting aircraft spiral down one layer. After the aircraft has
landed and taxied off the runway, ground control issues taxi instructions that
direct the aircraft to parking.
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ADMINISTRATION AND MANAGEMENT
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Air traffic control in the
United States is organized and regulated by the FAA. The FAA provides
substantial air traffic control coverage to the airspace in the United States.
Other countries provide their own air traffic control systems, which can differ
widely in technology and sophistication. The FAA and other air traffic control
agencies are planning to modernize air traffic control systems with satellite
tracking. New satellite systems will improve safety by enhancing the tracking
ability of air traffic controllers.
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Organization
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The FAA has overall responsibility
for air traffic control in the United States. Airspace in the United States is
divided into a number of flight information regions, each under the control of
one ARTCC. Some airspace is reserved for military use, while the remaining
airspace is broken into smaller, more manageable areas called sectors. Sectors
are designed around traffic flows and usually control either low- or
high-altitude aircraft. A team of controllers manages the traffic in each
sector. Airspace surrounding busier airports is delegated to either air traffic
control towers or terminal radar approach controls.
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Stations and Personnel
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The FAA operates over
32,500 different air navigation and air traffic control systems. These
facilities include 90 flight service stations, over 350 control towers, 190
radar approach controls, and 22 air route traffic control facilities. The FAA
also operates and maintains research and development facilities, a major
training academy, and numerous regulatory offices. The air traffic control
system is responsible for the separation of over 200,000 takeoffs and landings
every day. This totals over 73 million per year. The busiest airports in the
United States in 1998 were Dallas-Fort Worth (with almost 782,000 flight
operations), Chicago O'Hare, Atlanta Hartsfield-Jackson, and Los Angeles
International Airport.
Almost 20,000 air traffic
controllers are employed in the United States. Most controllers work for the
FAA, which is an agency of the federal Department of Transportation. Additional
controllers are employed by private organizations and usually work at smaller
airports. Controller salaries are based on experience and the complexity of the
facility.
Controllers must complete a set
of screening examinations and training courses to become certified. Selected
individuals are employed by the FAA and sent to its training facility in Oklahoma
City, Oklahoma, for a 15-week training program. New controllers complete
between one and three years of on-the-job training before working by
themselves.
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Labor and the FAA
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The FAA has a long history
of labor relations problems. Until the late 1960s, most FAA employees were
former military controllers. When aviation began to grow, the FAA began to hire
employees with little or no aviation background, and conflict between the
former military employees and the new civilian staff eventually arose.
In the 1960s air traffic
controllers voted to create a union called the Professional Air Traffic
Controllers Organization (PATCO). PATCO immediately made funding and staffing
demands on the FAA. When these demands were not met, controllers protested in
1970 by not showing up for work. Although the protest lasted only a couple of
days, it proved to be a warm-up for what was to come. Still dissatisfied with
the FAA, PATCO sponsored an illegal controllers strike in 1981. The leaders of
PATCO felt that public sympathy would force the government to meet many of
their demands. The administration of President Ronald Reagan felt that this
challenge to its authority must be met and that most of the controllers would
abandon their union. The FAA gave the controllers two days to return to work or
be fired. Both groups miscalculated. Few controllers returned to work, and over
11,000 of the 15,000 controllers were subsequently fired. The FAA then began a
massive training program that was completed around the year 1990. Most air
traffic controllers fired during the strike left the profession, although a few
gained employment at private air traffic control facilities. The FAA rehired
some former employees in the late 1990s, after the administration of President
Bill Clinton lifted Reagan’s ban on reemployment. Although most of the new
controllers were thought to be unsympathetic to unionization, in the 1990s they
raised many of the same concerns as PATCO, and formed a new union, the National
Air Traffic Controllers Association (NATCA).
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Improving Air Traffic Control
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There is a fairly standardized
system of air traffic control worldwide. Through membership in the
International Civil Aviation Organization (IACO), almost every nation has
agreed to provide air traffic control services to aircraft operating within its
borders. ICAO standards include the use of English as the common language and
the use of VOR and satellite systems as the primary navigation tools. Every
nation that is a member of the ICAO is required to provide service to any civil
aircraft overflying its borders. Some countries offer this service for free,
while others charge for their services. Air traffic control procedures in other
countries can vary from very sophisticated to almost nonexistent. Countries
whose standard of living is similar to that in the United States usually
operate modern air traffic control systems. Countries lacking in financial
resources often operate less sophisticated systems.
Many different approaches to
improving the efficiency of air traffic control have been considered. In some
countries, the government contracts with private companies to operate segments
of the air traffic control system. In other countries, the entire system is
operated as a private or public corporation. In the United States, the FAA
currently contracts out the operation of many smaller air traffic control
towers. Air traffic control systems in other countries, such as Canada and New
Zealand, currently operate as private corporations.
The FAA is embarking on
a major project to modernize the air traffic control system. The FAA plans for
communications, navigation, and air traffic surveillance to be handled by
satellite. Sophisticated computers will help the controllers manage the flow of
air traffic. Pilots will be able to select their own routes and altitudes and
will be able to modify them at will. The new system will monitor each aircraft
and will alert the pilot and controller to any possible conflicts. The pilot
and controller will then work together to determine a solution. This method of
air traffic control is known as free flight, and is planned to become the
standard in the United States by the year 2010.
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