Thursday, January 12, 2012

Air Traffic Control

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.
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.
Flight Rules
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.
Navigation Systems
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.
Air Traffic Controllers
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.
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.
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.
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.
Stations and Personnel
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.
Labor and the FAA
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).
Improving Air Traffic Control
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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