Air traffic control History
The air age arrived on Dec. 17, 1903, when the Wright brothers succeeded in a 120-foot flight in a heavier-than-air craft at Kitty Hawk, N.C., U.S. It is difficult to imagine the rapid technological advances that now allow interplanetary travel by unmanned, but directly controlled, satellites and probes. The earliest common uses of aviation were by the military and the civilian postal service. With infrequent flights and virtually no carriage of passengers, the primary concern was for the integrity of the aircraft and the management of safe takeoffs and landings. One of the principal distinguishing characteristics of aviation, compared to other transportation modes, is the high speed and "vertical" nature of operations. Because of these unique features, aviation has always posed the highest risk of severe injuries and fatalities, given an accident, of almost any transportation mode. When passengers began to be carried in significant volumes in the 1920s, it became clear that a systematic set of air traffic control principles were needed to handle the increasing volumes at several critical airports.
Airplanes travel along established routes called airways, which are analogous to guideways, even though they are not physical constructions. They are defined by a particular width (e.g., 32 miles) and also have defined altitudes, which separate air traffic moving in opposite directions along the same airway. Because of the ability to vertically separate aircraft, it is possible for through traffic to fly over airports while operations continue underneath. The economics of air travel require relatively long-distance travel from origin to destination in order to retain economic viability. For the vehicle operator (i.e., the pilot), this means short periods of high concentration and stress (takeoffs and landings) with relatively long periods of low activity and arousal. During this long-haul portion of a flight, a pilot is much more concerned with monitoring aircraft status than looking around for nearby planes. This is markedly different from highways, in which a collision threat is nearly always apparent. While midair collisions have occurred away from airports, the scenario most feared by safety analysts is a midair collision near or at an airport because of a traffic control misunderstanding. These concerns led to the evolution of the present air traffic control system.
The first attempt to develop air traffic control rules occurred in 1922 under the auspices of the International Commission on Air Navigation (ICAN) under the direction of the League of Nations. The first air traffic controller, Archie League of St. Louis, Mo., U.S., began working in 1929. The long distances traveled by aircraft show why aviation quickly became an international concern. The capabilities of aircraft to fly hundreds or thousands of miles at several hundred miles per hour created a market for long-distance, high-speed transportation. Two immediate concerns were in the areas of language and equipment compatibility. Pilots from many countries and with many native languages needed to communicate with each other and with controllers on the ground. Electronic equipment including radios and, more recently, computers needed to exchange information. English was established as the international language of air traffic control, but even within this context, there was a need for precise use of phrases and strings of words. These common practices have their conceptual roots in the same issues of uniformity that are directly applied to highways. The operator needs to be given clear and simple information that meets a direct need. In road transportation, this is conveyed through verbal or symbolic visual images; in aviation, it is achieved through the spoken word, supplemented by aircraft instrumentation. The initial international activity in navigation also distinguishes air transport: finding a way to a destination was an area of principal concern in the early years of aviation. Because aircraft could not operate without fixed land references (particularly on long-distance trips), it became necessary to develop an elaborate system of navigation aids (first visual, using beacons, now electronic, using radar) to help indicate the current aircraft position. Availability of inertial navigation units for commercial aircraft has reduced the need for this communication in the passenger sector; en route information is still provided through a variety of communication media on long-distance trips to warn of impending delays or other conditions.
Traffic elements
The elements that make up the air traffic control system must provide the capability to assist aircraft in traveling between airports as well as in landing and taking off. Air route traffic control centres are responsible for controlling and monitoring movement between origin and destination airports. Each centre is responsible for a defined geographic area; as an aircraft continues on a flight, crossing these areas, the responsibility for monitoring the plane is transferred ("handed off") to the next air route centre. The flight continues to be transferred until it reaches the control area at its destination. At this point, typically within five miles of the destination airport, the air traffic control function is turned over to an airport controller, and the plane is guided through a sequence of locations in order to land.
The airport traffic control tower has direct responsibility for managing handling, takeoffs, and all movement within the airport terminal control area. Flight service stations are located at airports and air route centres, providing updated weather and other information of relevance to incoming and departing pilots.
Air traffic controllers and aircraft pilots occupy a unique position in the air traffic control system. There is no other mode of transportation that relies so heavily on the communication and coordination of these two sets of individuals. As part of an overall objective to maintain safe and efficient air traffic flow, the pilot is required to comply with requests and instructions directed to him by the controller, subject to the pilot's ultimate responsibility for the safety of the aircraft. Particularly in the vicinity of airports, and particularly when arranging for landing or takeoff, clear communication is essential. Conflicts can arise between the control responsibilities of the air traffic controller and the authority of the pilot in the aircraft. Traditional approach control using stacks (see below) placed a heavy burden on the airport traffic controllers to monitor many planes in the air. After the 1981 air traffic controller strike in the United States and the subsequent dismissal of approximately 10,000 controllers, the Federal Aviation Administration instituted a policy of flow controls. These controls required an aircraft to remain at its origin airport unless a landing opportunity was estimated to be available at the destination airport at the estimated arrival time. This results in a significantly reduced workload for the terminal air traffic controllers at the destination airport. It is an understandable source of frustration for travelers because they are not informed of a flow control delay until after the plane is pushed away from the gate at its origin and the pilot requests a landing slot. While air traffic controller staffing levels have gradually increased, the flow control system is retained because it reduces air traffic controller stress and workload by delaying flights on the ground, not in the air.
Aids to navigation are a critical element in the air traffic control system. The navigation function needs to be satisfied by a variety of technologies to supplement destination finding when visual references are limited by weather or ambient light. The earliest navigation aids were lighted beacons along the ground; these suffered obvious problems during adverse weather and were replaced by radio direction-finding equipment. The radio technologies are able to transmit the heading and distance to an intended destination. These aircraft-mounted technologies are supplemented by air route surveillance radar, which monitors aircraft within each designated sector of the air route traffic control system. The radar-based systems form the backbone of the navigation aids for privately owned aircraft and small passenger-carrying planes. Major commercial jets are now supplied with inertial navigation units, which allow an aircraft to independently navigate to a destination. A computer and gyroscope are used to sense direction and, with speed sensors, track direction and distance to the destination. The navigation units can fly virtually automatically until in the vicinity of an airport, at which time the pilot and controller interact to safely control the landing.
An aircraft leaves the holding stack (a series of elliptical patterns flown at assigned altitudes while awaiting clearance to land), if there is one, and approaches a runway through an outer and an inner marker. Airport surveillance radar and approach lights are used to assist the pilot. The landing occurs on a runway that is designed to carry the impact load of the aircraft on landing. An important role is played by exit taxiways in expeditiously clearing aircraft from the runway in order to allow another operation (either landing or takeoff). The electronic landing aids, approach lights, and exit taxiways should work as a system to safely land and clear the runway for another operation.
The final element in the air traffic control system is the ability to control and direct aircraft on the ground. Arriving flights must be safely guided to a terminal, departing flights to the proper runway. For smaller airports, under satisfactory weather conditions, this can be done visually. At larger airports, ground movement radar is needed to track planes on the ground, just as in the air. Part of an air traffic controller's duties is to conduct this guidance of planes along taxiways and near terminals. Ground movement problems have been exacerbated in the United States by the hub-and-spoke network that has evolved for most carriers since deregulation in 1978. Carriers now operate in and out of hub airports, which are the focal points of large numbers of flights. Waves of aircraft arrive tightly spaced in a narrow time window and depart similarly bunched. Passengers frequently reach their destinations by changing planes at the hub. This allows airlines to minimize transfer times and schedule efficiently, but it can result in extreme ground delays when many aircraft exchange gate positions simultaneously. Airlines generally resist attempts to move flights significantly from on-the-hour or half-hour departures because of a perception of passenger inconvenience. Expansion of hub-and-spoke operations will continue the pressure on ground operations.