During peak air travel times in the United States, there
are about 5,000 airplanes
in the sky every hour. This translates to approximately 50,000
aircraft operating in our skies each day. How do these
aircraft keep from colliding with each other? How does air
traffic move into and out of an airport or
across the country?
The task of ensuring safe operations of commercial and
private aircraft falls on air traffic controllers. They must
coordinate the movements of thousands of aircraft, keep them
at safe distances from each other, direct them during takeoff
and landing from airports, direct them around bad weather and
ensure that traffic flows smoothly with minimal delays.
When you think about air traffic control, the image of men
and women in the tower of an airport probably comes to mind.
However, the air traffic control system is much more complex
than that. In this edition of HowStuffWorks,
we will examine air traffic control in the United States.
We'll follow a flight from departure to arrival, looking at
the various controllers involved, what each one does, the
equipment they use and how they are trained.
Airspace and Air Traffic Control The United
States airspace is divided into 21 zones (centers), and
each zone is divided into sectors. Also within each zone are
portions of airspace, about 50 miles (80.5 km) in diameter,
called TRACON (Terminal Radar
Approach CONtrol) airspaces. Within each TRACON
airspace are a number of airports, each of which has its own
airspace with a 5-mile (8-km) radius.
Divisions of U.S.
airspace
Visual & Instrument
Flying
Some pilots of
small aircraft fly by vision only (visual flight
rules, or VFR). These pilots are not required by the
FAA to file flight plans and, except for FSS and local
towers, are not serviced by the mainstream air traffic
control system. Pilots of large commercial flights use
instruments to fly (instrument flight rules, or
IFR), so they can fly in all sorts of weather. They must
file flight plans and are serviced by the mainstream air
traffic control
system.
The air
traffic control system, which is run by the Federal Aviation
Administration (FAA), has been designed around these
airspace divisions. The air traffic control system divisions
are:
Air Traffic Control System Command Center
(ATCSCC) - The ATCSCC oversees all air traffic
control. It also manages air traffic control within centers
where there are problems (bad weather, traffic overloads,
inoperative runways).
Air route traffic control centers (ARTCC) - There
is one ARTCC for each center. Each ARTCC manages
traffic within all sectors of its center except for TRACON
airspace and local-airport airspace.
Terminal radar approach control - TRACON
handles departing and approaching aircraft within its space.
Air traffic control tower (ATCT) - An ATCT
is located at every airport that has regularly scheduled
flights. Towers handle all takeoff, landing, and ground
traffic.
Flight service station (FSS) - The FSS
provides information (weather, route, terrain, flight plan)
for private pilots flying into and out of small airports and
rural areas. It assists pilots in emergencies and
coordinates search-and-rescue operations for missing or
overdue aircraft.
The movement of aircraft through the various airspace
divisions is much like players moving through a "zone" defense
that a basketball or football team might use. As an aircraft
travels through a given airspace division, it is monitored by
the one or more air traffic controllers responsible for that
division. The controllers monitor this plane and give
instructions to the pilot. As the plane leaves that airspace
division and enters another, the air traffic controller passes
it off to the controllers responsible for the new airspace
division.
Let's follow a flight to demonstrate this system.
Flight Profile and Preflight Suppose you are
flying across the United States, perhaps from New York to San
Francisco. Your flight, like every other commercial
airline flight, follows a typical profile:
Preflight -This portion of the flight starts on
the ground and includes flight checks, push-back from the gate
and taxi to the runway.
Takeoff - The pilot powers up the aircraft and
speeds down the runway.
Departure - The plane lifts off the ground and
climbs to a cruising altitude.
En route - The aircraft travels through one or
more center airspaces and nears the destination airport.
Descent - The pilot descends and maneuvers the
aircraft to the destination airport.
Approach - The pilot aligns the aircraft with the
designated landing runway.
Landing - The aircraft lands on the designated
runway, taxis to the destination gate and parks at the
terminal.
Profile of a typical commercial
flight
Preflight While you
prepare for your flight by checking your bags and walking to
the gate, your pilot inspects your plane and files a flight
plan with the tower -- all IFR pilots must file a flight plan
at least 30 minutes prior to pushing back from the gate. Your
pilot reviews the weather along the intended route, maps the
route and files the plan. The flight plan includes:
Airline name and flight number
Type of aircraft and equipment
Intended airspeed and cruising altitude
Route of flight (departure airport, centers that will be
crossed and destination airport)
Your pilot
transmits this data to the tower.
In the tower, a controller called a flight data
person reviews the weather and flight-plan information and
enters the flight plan into the FAA host computer. The
computer generates a flight progress strip that will be
passed from controller to controller throughout your flight.
The flight progress strip contains all of the necessary data
for tracking your plane during its flight and is constantly
updated.
Example of a flight progress
strip
Once the flight plan has been approved, the flight data
person gives clearance to your pilot (clearance
delivery) and passes the strip to the ground controller in
the tower.
The ground controller is responsible for all
ground traffic, which includes aircraft taxiing from the gates
to takeoff runways and from landing runways to the gates. When
the ground controller determines that it is safe, he or she
directs your pilot to push the plane back from the gate
(airline personnel operate the tugs that actually push the
aircraft back and direct the plane out of the gate area). As
your plane taxis to the runway, the ground controller watches
all of the airport's taxiways and uses ground radar to track
all of the aircraft (especially useful in bad weather),
ensuring that your plane does not cross an active runway or
interfere with ground vehicles. The ground controller talks
with your pilot by radio and
gives him instructions, such as which way to taxi and which
runway to go to for takeoff. Once your plane reaches the
designated takeoff runway, the ground controller passes the
strip to the local controller.
Photo courtesy British
Airways An airplane taxis
to the runway under instructions from the ground
controller.
The local controller in the tower watches the skies
above the airfield and uses surface radar to track aircraft.
He or she is responsible for maintaining safe distances
between planes as they take off. The local controller gives
your pilot final clearance for takeoff when it is deemed safe,
and provides the new radio frequency for the departure
controller. Once clearance is given, your pilot must decide if
it is safe to takeoff. If it is safe, he accelerates the plane
down the runway. As you leave the ground, the local controller
hands your plane off electronically to the departure
controller at the TRACON facility that services your departure
airport, but still monitors the plane until it is 5 miles from
the airport. Your pilot now talks with the departure
controller.
Departure Once your plane takes off, your
pilot activates a transponder device inside the aircraft. The
transponder detects incoming radar
signals and broadcasts an amplified, encoded radio signal
in the direction of the detected radar wave. The transponder
signal provides the controller with your aircraft's flight
number, altitude, airspeed and destination. A blip
representing the airplane appears on the controller's radar
screen with this information beside it. The controller can now
follow your plane.
An airplane's transponder
transmits flight data to incoming radar
signals.
The departure controller is located in the TRACON
facility, which may have several airports within its airspace
(50-mile/80-km radius). He or she uses radar to monitor the
aircraft and must maintain safe distances between ascending
aircraft. The departure controller gives instructions to your
pilot (heading, speed, rate of ascent) to follow regular
ascent corridors through the TRACON airspace.
Photo courtesy Federal
Aviation Administration (FAA) Departure and approach corridors for westward
air traffic to and from airports in the San Francisco
Bay Area TRACON airspace. (Click on the image for a
larger
version.)
The departure controller monitors your flight during ascent
to the en route portion. When your plane leaves TRACON
airspace, the departure controller passes your plane off to
the center controller (ARTCC controller). Every time
your plane gets passed between controllers, an updated flight
progress slip gets printed and distributed to the new
controller.
En Route and Descent
Safe
Separation?
Safe
vertical separation between aircraft is considered to be
1,000 ft (305 m) at altitudes below 29,000 ft (8845 m)
and 2,000 ft (610 m) at altitudes above that. When
aircraft are at the same altitude, safe horizontal
separation is considered to be 5 miles (8 km). The Air
Transport Association has recommended that these
separations be reduced to use airspace more efficiently
and reduce airport
delays.
Once your
plane has left TRACON airspace, it enters a sector of the
ARTCC airspace, where it is monitored by at least two air
traffic controllers. The radar associate controller
receives the flight-plan information anywhere from five to 30
minutes prior to your plane entering that sector. The
associate controller works with the radar controller in charge
of that sector. The radar controller is in charge of
all air-to-ground communication, maintains safe separation of
aircraft within the sector and coordinates activities with
other sectors and/or centers. The controllers must monitor the
airspace at high altitude (above 24,000 ft/7320 m) and low
altitude (below 24,000 ft). The center controllers provide
your pilot with updated weather and air-traffic information.
They also give directions to your pilot regarding such aspects
as speed and altitude to maintain a safe separation between
aircraft within their sector. They monitor your plane until it
leaves their sector. Then they pass it off to another sector's
controller.
The various air traffic control facilities
encountered by a plane during its
flight
Another controller, called the radar hand-off
controller, assists the radar and associate radar
controllers during times of heavy traffic, watching the radar
screen and helping to maintain smooth air-traffic flow.
Photo courtesy NASA ARTCC, showing various
controllers
While you are enjoying your meal, snack, in-flight movie or
the view outside the window, your plane gets passed from
sector to sector and center to center. In each sector, center
controllers radio instructions to the pilots. The path of your
plane may have to be changed from the original flight plan to
move around bad weather or avoid a congested sector. Your
pilots may request a change in altitude to avoid or reduce
turbulence. This back and forth between pilots and center
controllers continues until you are about 150 miles (241 km)
from San Francisco (your destination). At this point, the
center controller directs all planes flying into San Francisco
to move from high altitudes to low altitudes and merges the
descending aircraft into a single file line toward the
airport. The controller gives instructions to your pilot, such
as changes in heading, speed and altitude, to place your plane
in line with these other aircraft. Depending on traffic
conditions, the controller may have to place your plane into a
holding pattern, which is a standard route around each
airport, where you wait until the airport can handle your
arrival. The controller continues to give directions to your
pilot until your plane is within TRACON airspace.
Traffic Management
Advisor
To assist the
center controllers in scheduling descents to airports
within their airspace, the FAA uses Traffic Management
Advisor (TMA) software developed by NASA and the Federal
Aviation Administration (FAA). TMA assists the
center coordinator in scheduling when each plane should
arrive and what order it should be placed in for
descent.
Photo courtesy NASA A center coordinator consults the TMA
displays.
TMA uses flight-plan information, aircraft
performance data and wind predictions to compute,
predict and schedule when a particular airplane should
reach its destination. It also looks at the number of
planes allowed to land within a given period of time at
an airport (the airport's capacity) and compares it to
the number of planes scheduled. If the scheduled number
exceeds the capacity (rush alert), it calculates
the number that can be landed safely and makes
recommendations to the coordinator for adjustments in
the air-traffic pattern. This information is passed on
to the controllers, who then give appropriate directions
to the pilots.
Approach When your descending plane is 50
miles from the San Francisco airport, it is within TRACON
airspace. An approach controller directs your pilot to
adjust the aircraft's heading, speed and altitude to line up
and prepare to land along standard approach corridors. Your
pilot then aligns your plane with the runway. When you are 10
miles (16 km) from the runway, the approach controller passes
your plane off to the local controller in the airport tower.
Photo courtesy Federal
Aviation Administration (FAA) Departure and approach corridors for eastward
air traffic to and from airports in the San Francisco
Bay Area TRACON airspace. (Click on the image for a
larger
view.)
FAST
Software
The approach
controller uses another software program developed by
NASA and the FAA called the final approach spacing
tool (FAST). The FAST program assists controllers in
choosing the landing order and runway for each
approaching aircraft. The program does the following:
Projects each aircraft's flight path based on
flight plan and radar tracking
Predicts arrival time
Suggests landing order and runway assignment based
on calculations that take into account aircraft size,
aircraft performance capability and wind directions
The FAST software helps to ensure that no
single runway or controller gets overloaded with planes,
helping to minimize unnecessary
delays.
Landing The local controller in the airport
tower checks the runways and the skies above the runways with
binoculars and surface radar (local and ground controllers are
the only controllers licensed to use visual information in
performing their duties). When the local controller determines
that it is safe, he or she gives your pilot clearance to land.
The local controller also updates weather conditions for your
pilot and monitors the spacing between your plane and other
landing aircraft.
Photo courtesy NASA Air traffic controllers in the tower monitor
takeoffs, landings and ground traffic with visual and
radar
tools.
Once you've landed, the local controller directs your plane
to an exit taxiway, tells your pilot the new radio frequency
for the ground controller and passes your plane off to the
ground controller.
The ground controller watches the runways and taxiways and
uses ground radar information to ensure that your taxiing
aircraft does not cross active runways or interfere with
ground vehicles. He or she directs your plane to the
appropriate terminal gate. Ground personnel from the airline
use hand signals to assist your pilot in parking the airplane
at the gate.
Gate to Gate
CD-ROM
NASA Ames Research
Center and the FAA have released an excellent,
educational, multi-media CD-ROM entitled "Gate to
Gate" that explains modern air traffic control.
There is also an accompanying Student
Activity and Career Guidance Package that
teachers can use. You can request the free CD-ROM by
contacting Karen
Stewart at the FAA.
Career Training
Air Traffic
Controllers
Work at local towers, TRACONs, ARTCC centers, FSS
and the ATCSCC
Work 40 hours per week (also some additional
over-time hours)
Earn between $36,000 and $87,000 depending upon
experience (median income = $64,880 as of 1998)
Receive 13 to 28 days paid vacation and 13 days paid
sick leave
Can retire as early as age 50 with 20 years active
service or at any age after 25 years active service
What does it take
to be an air traffic controller? To be a ground controller,
you have to memorize the position of aircraft on the runways
and taxiways with a single, short glance. Local, TRACON and
ARTCC controllers must be able to think and visualize in three
dimensions. All controllers must be able to gather information
from what they hear, make decisions quickly and know the
geography of their own airspace, as well as that of others.
They must be able to read and interpret symbols as well as
predict the whereabouts of aircraft from course headings and
speeds, and they must be able to concentrate intensely. To
test your skills as an air traffic controller, see pages 17
through 67 of the "Gate
to Gate" CD ROM: Student Activity and Career Guidance
Package.
Air traffic controllers at all levels are employed by the
FAA. To
become an air traffic controller, you must apply through the
federal civil-service system and pass a written test that
assesses your abilities to perform a controller's duties.
Abstract reasoning and 3-D spatial visualization are tested on
the exam. Applicants must have three years of work experience,
a four-year college degree or some combination of the two.
If you are accepted into the training program, you will
attend the FAA Academy in Oklahoma City, Oklahoma, for seven
months of training. While there, you will learn the air
traffic control system, equipment, regulations, procedures and
about aircraft performance. You will need to pass a final
examination before you graduate.
Photo courtesy NASA Air traffic controller training on TRACON
systems
After graduation, you will accumulate work experience at
various sites across the country, from airport towers to
ARTCCs. You must be certified for various positions, such as
ground controller, associate radar controller and radar
hand-off controller. You will be required to pass annual
physical examinations, semi-annual performance examinations
and periodic drug screenings. Air traffic control positions
are highly competitive jobs, and the controller workforce is
relatively young (most were hired after the air traffic
controller strike in the 1980s, when President Ronald Reagan
ordered that all striking controllers be fired).
Air Traffic Control Problems
Air
Crash
In the event of
an air crash or runway incursion (two planes on a
collision course), the National Transportation Safety
Board (NTSB) investigates. The NTSB team reconstructs
the air-traffic services given to the plane, examines
radar tracking data and studies transcripts of the
controller-pilot conversations. See NTSB:
The Investigative Process for details.
Air travel has increased
dramatically since the U.S. federal government deregulated the
airline
industry in the 1970s. However, the construction of new airports
and runways has not kept pace with the increase in air
traffic. This has put excessive pressure on the air traffic
control system to handle the nearly 50,000 flights per day, a
number projected to increase in the near future. To handle
these flights and avoid delays and collisions, the FAA and NASA
have developed modern software, upgraded existing host
computers and voice communications systems and instituted
full-scale GPS (global
positioning system) capabilities to help air traffic
controllers track and communicate with aircraft. The FAA is
currently redesigning U.S. airspace to make more room for
increased traffic. For example, the U.S. military has freed
previously restricted airspace off the coast of North Carolina
for use by commercial aircraft. These efforts should help ease
traffic and minimize delays in the short term; however,
increasing airport capacity by building new runways and
airports is ultimately the way to handle the problem. For more
information, see Airlines
Announce "Top Ten" Air Traffic Control Priorities.
For more information about air traffic control, check out
the links on the next page.
