Brief History of the National Airspace System (NAS)

About two decades after the introduction of powered flight, aviation industry leaders believed that the airplane would not reach its full commercial potential without federal action to improve and maintain safety standards. In response to their concerns, the U.S. Congress passed the Air Commerce Act of May 20, 1926, marking the onset of the government’s hand in regulating civil aviation. The act charged the Secretary of Commerce with fostering air commerce, issuing and enforcing air traffic rules, licensing pilots, certifying aircraft, establishing airways, and operating and maintaining aids to air navigation. As commercial flying increased, the Bureau of Air Commerce— a division of the Department of Commerce—encouraged a group of airlines to establish the first three centers for providing ATC along the airways. In 1936, the bureau took over the centers and began to expand the ATC system. [Figure 1-2] The pioneer air traffic controllers used maps, blackboards, and mental calculations to ensure the safe separation of aircraft traveling along designated routes between cities.

On the eve of America’s entry into World War II, the Civil Aeronautics Administration (CAA)—charged with the responsibility for ATC, airman and aircraft certification, safety enforcement, and airway development— expanded its role to cover takeoff and landing operations at airports. Later, the addition of radar helped controllers to keep abreast of the postwar boom in commercial air transportation.

Following World War II, air travel increased, but with the industry's growth came new problems. In 1956 a midair collision over the Grand Canyon killed 128 people. The skies were getting too crowded for the existing systems of aircraft separation, and with the introduction of jet airliners in 1958 Congress responded by passing the Federal Aviation Act of 1958, which transferred CAA functions to the FAA (then the Federal Aviation Agency). The act entrusted safety rulemaking to the FAA, which also held the sole responsibility for developing and maintaining a common civil-military system of air navigation and air traffic control. In 1967, the new Department of Transportation (DOT) combined major federal transportation responsibilities, including the FAA (now the Federal Aviation Administration) and a new National Transportation Safety Board (NTSB).

By the mid-1970s, the FAA had achieved a semi-automated ATC system based on a marriage of radar and computer technology. By automating certain routine tasks, the system allowed controllers to concentrate more efficiently on the task of providing aircraft separation. Data appearing directly on the controllers’ scopes provided the identity, altitude, and groundspeed of aircraft carrying radar beacons. Despite its effectiveness, this system required continuous enhancement to keep pace with the increased air traffic of the late 1970s, due in part to the competitive environment created by airline deregulation.

To meet the challenge of traffic growth, the FAA unveiled the NAS Plan in January 1982. The new plan called for more advanced systems for en route and terminal ATC, modernized flight service stations, and improvements in ground-to-air surveillance and communication. Continued ATC modernization under the NAS Plan included such steps as the implementation of Host Computer Systems (completed in 1988) that were able to accommodate new programs needed for the future.[Figure 1-3]

In February 1991, the FAA replaced the NAS Plan with the more comprehensive Capital Investment Plan (CIP), which outlined a program for further enhancement of the ATC system, including higher levels of automation as well as new radar, communications, and weather forecasting systems. One of the CIP’s programs currently underway is the installation and upgrading of airport surface radars to reduce runway incursions and prevent accidents on airport runways and taxiways. The FAA is also placing a high priority on speeding the application of the GPS satellite technology to civil aeronautics. Another notable ongoing program is encouraging progress toward the implementation of Free Flight, a concept aimed at increasing the efficiency of high-altitude operations.

Flow Control

ATC provides IFR aircraft separation services for NAS users. Since the capabilities of IFR operators vary from airlines operating hundreds of complex jet aircraft to private pilots in single engine, piston-powered airplanes, the ATC system must accommodate the least sophisticated user. The lowest common denominator is the individual controller speaking to a single pilot on a VHF voice radio channel. While this commonality is desirable, it has led to a mindset where other opportunities to interact with NAS users have gone undeveloped. The greatest numbers of operations at the 20 busiest air carrier airports are commercial operators (airlines and commuters) operating IFR with some form of ground-based operational control. Since not all IFR operations have ground-based operational control, very little effort has been expended in developing ATC and Airline Operations Control Center (AOC) collaboration techniques, even though ground-based computer-to-computer links can provide great data transfer capacity. Until the relatively recent concept of Air Traffic Control-Traffic Flow Management (ATC-TFM), the primary purpose of ATC was aircraft separation, and the direct pilot-controller interaction was adequate to the task. Effective and efficient traffic flow management now requires a new level of control that includes the interaction of and information transfer among ATC, TFM, AOCs, and the cockpit. [Figure 1-17]

Flow Control Restriction

As the first step in modernizing the traffic flow management infrastructure, the FAA began reengineering traffic flow management software using commercial off-the-shelf products. In FY 1996, the FAA and NASA collaborated on new traffic flow management research and development efforts for the development of collaborative decision making tools that will enable FAA traffic flow managers to work cooperatively with airline personnel in responding to congested conditions. Additionally, the FAA provided a flight scheduling software system to nine airlines.

Surveillance Systems

Surveillance systems are set up to enable the ATC system to know the location of an aircraft and where it is heading. Position information from the surveillance system supports many different ATC functions. Aircraft positions are displayed for controllers as they watch over the traffic to ensure that aircraft do not violate separation criteria. In the current NAS, surveillance is achieved through the use of long-range and terminal radars. Scanning the skies, these radars return azimuth and slant range for each aircraft that, when combined with the altitude of the aircraft broadcast to the ground via a transceiver, is transformed mathematically into a position. The system maintains a list of these positions for each aircraft over time, and this time history is used to establish short-term intent and short-term conflict detection. Radars are expensive to maintain, and position information interpolated from radars is not as good as what the aircraft can obtain with SATNAV. ADS-B technology may provide the way to reduce the costs of surveillance for air traffic management purposes and to get the better position information to the ground.

New aircraft systems dependent on ADS-B could be used to enhance the capacity and throughput of the nation’s airports. Electronic flight following is one example: An aircraft equipped with ADS-B could be instructed to follow another aircraft in the landing pattern, and the pilot could use the on-board displays or computer applications to do exactly that. This means that visual rules for landing at airports might be used in periods where today the airport must shift to instrument rules due to diminishing visibility. Visual capacities at airports are usually higher than instrument ones, and if the airport can operate longer under visual rules (and separation distances), then the capacity of the airport is maintained at a higher level longer.

ATC Facilities

FAA figures show that the NAS includes more than 18,300 airports, 21 ARTCCs, 197 TRACON facilities, over 460 air traffic control towers (ATCTs), 58 flight service stations and automated flight service stations (FSSs/AFSSs), and approximately 4,500 air navigation facilities. Several thousand pieces of maintainable equipment including radar, communications switches, ground-based navigation aids, computer displays, and radios are used in NAS operations, and NAS components represent billions of dollars in investments by the government. Additionally, the aviation industry has invested significantly in ground facilities and avionics systems designed to use the NAS. Approximately 47,000 FAA employees provide air traffic control, flight service, security, field maintenance, certification, system acquisition, and other essential services.

Differing levels of ATC facilities vary in their structure and purpose. Traffic management at the national level is led by the Command Center, which essentially “owns” all airspace. Regional Centers, in turn, sign Letters of Agreement (LOAs) with various approach control facilities, delegating those facilities chunks of airspace in which that approach control facility has jurisdiction. The approach control facilities, in turn, sign LOAs with various towers that are within that airspace, further delegating airspace and responsibility. This ambiguity has created difficulties in communication between the local facilities and the Command Center. However, a decentralized structure enables local flexibility and a tailoring of services to meet the needs of users at the local level. Improved communications between the Command Center and local facilities could support enhanced safety and efficiency while maintaining both centralized and decentralized aspects to the ATC system.

Airliners

Though commercial air carrier aircraft traditionally make up less than 5 percent of the civil aviation fleet, they account for about 30 percent of the instrument operations flown in civil aviation. Commercial air carriers are the most homogenous category of airspace users, although there are some differences between U.S. trunk carriers (major airlines) and regional airlines (commuters) in terms of demand for ATC services. Generally,

U.S. carriers operate large, high performance airplanes that cruise at altitudes above 18,000 feet. Conducted exclusively under IFR, airline flights follow established schedules and operate in and out of larger and better- equipped airports. In terminal areas, however, they share airspace and facilities with all types of traffic and must compete for airport access with other users. Airline pilots are highly proficient and thoroughly familiar with the rules and procedures under which they must operate.

Some airlines are looking toward the use of larger aircraft, with the potential to reduce airway and terminal congestion by transporting more people in fewer aircraft. This is especially valuable at major hub airports, where the number of operations exceeds capacity at certain times of day. On the other hand, the proliferation of larger aircraft also requires changes to terminals (e.g., double-decker jetways and better passenger throughput), rethinking of rescue and fire-fighting strategies, taxiway fillet changes, and perhaps stronger runways and taxiways.

Commuter airlines also follow established schedules and are flown by professional pilots. Commuters characteristically operate smaller and lower performance aircraft in airspace that must often be shared by general aviation (GA) aircraft, including visual flight rules (VFR) traffic. As commuter operations have grown in volume, they have created extra demands on the airport and ATC systems. At one end, they use hub airports along with other commercial carriers, which contributes to growing congestion at major air traffic hubs. IFR-equipped and operating under IFR like other air carriers, commuter aircraft cannot be used to full advantage unless the airport at the other end of the flight, typically a small community airport, also is capable of IFR operation. Thus, the growth of commuter air service has created pressure for additional instrument approach procedures and control facilities at smaller airports. A growing trend among the major airlines is the proliferation of regional jets (RJs). RJs are replacing turboprop aircraft and they are welcomed by some observers as saviors of high-quality jet aircraft service to small communities. RJs are likely to be a regular feature of the airline industry for a long time because passengers and airlines overwhelmingly prefer RJs to turboprop service. From the passengers’ perspective, they are far more comfortable; and from the airlines’ point of view, they are more profitable. Thus, within a few years, most regional air traffic in the continental U.S. will be by jet, with turboprops filling a smaller role.

FAA and industry studies have investigated the underlying operational and economic environments of RJs on the ATC system. They have revealed two distinct trends: (1) growing airspace and airport congestion is exacerbated by the rapid growth of RJ traffic, and (2) potential airport infrastructure limitations may constrain airline business. The FAA, the Center for Advanced Aviation System Development (CAASD), major airlines, and others are working to find mitigating strategies to address airline congestion. With nearly 2,000 RJs already in use—and double that expected over the next few years—the success of these efforts is critical if growth in the regional airline industry is to be sustained. [Figure 1-8]

Traffic Alert and Collision Avoidance System

The traffic alert and collision avoidance system (TCAS) is designed to provide a set of electronic eyes so the pilot can maintain awareness of the traffic situation in the vicinity of the aircraft. The TCAS system uses three separate systems to plot the positions of nearby aircraft. First, directional antennae that receive Mode S transponder signals are used to provide a bearing to neighboring aircraft . accurate to a few degrees of bearing. Next, Mode C altitude broadcasts are used to plot the altitude of nearby aircraft. Finally, the timing of the Mode S interrogation/response protocol is measured to ascertain the distance of an aircraft from the TCAS aircraft.

Traffic Alert and Collision Avoidance System

TCAS I allows the pilot to see the relative position and velocity of other transponder-equipped aircraft within a 10 to 20-mile range. [Figure 1-14] More importantly, TCAS I provides a warning when an aircraft in the vicinity gets too close. TCAS I does not provide instructions on how to maneuver in order to avoid the aircraft, but does supply important data with which the pilot uses to evade intruding aircraft.

TCAS II provides pilots with airspace surveillance, intruder tracking, threat detection, and avoidance maneuver generations. TCAS II is able to determine whether each aircraft is climbing, descending, or flying straight and level, and commands an evasive maneuver to either climb or descend to avoid conflicting traffic. If both planes in conflict are equipped with TCAS II, then the evasive maneuvers are well coordinated via air-to-air transmissions over the Mode S datalink, and the commanded maneuvers do not cancel each other out.

TCAS and similar traffic avoidance systems provide safety independent of ATC and supplement and enhance ATC’s ability to prevent air-to-air collisions. Pilots currently use TCAS displays for collision avoidance and oceanic station keeping (maintaining miles-in-trail separation). Recent TCAS technology improvements enable aircraft to accommodate reduced vertical separation above FL 290 and the ability to track multiple targets at longer ranges. The Airborne Collision Avoidance System (ACAS) is an international ICAO standard that is the same as the latest TCAS II, which is sometimes called “Change 7” or “Version 7” in the United States. ACAS has been mandated, based on varying criteria, throughout much of the world.

System Capacity

On the user side, there are more than 740,000 active pilots operating over 319,000 commercial, regional, general aviation, and military aircraft. This results in more than 49,500 flights per day. Figure 1-6 depicts over 5,000 aircraft operating at the same time in the U.S. shown on this Air Traffic Control System Command Center (ATCSCC) screen. 

Approximately 5,000 Aircraft in ATC System at One Time.

Automated Radar Terminal System

Most medium-to-large radar facilities in the U.S. use some form of automated radar terminal system (ARTS), which is the generic term for the functional capability afforded by several automated systems that differ in functional capabilities and equipment. “ARTS” followed by a suffix Roman numeral denotes a specific system, with a subsequent letter that indicates a major modification to that particular system. In general, the terminal controller depends on ARTS to display aircraft identification, flight plan data, and other information in conjunction with the radar presentation. In addition to enhancing visualization of the air traffic situation, ARTS facilitates intra- and inter-facility transfers and the coordination of flight information. Each ARTS level has the capabilities of communicating with other ARTS types as well as with ARTCCs.

As the primary system used for terminal ATC in the U.S., ARTS had its origin in the mid-1960’s as ARTS I, or Atlanta ARTS and evolved to the ARTS II and ARTS III configurations in the early to mid-1970’s. Later in the decade, the ARTS II and ARTS III configurations were expanded and enhanced and renamed ARTS IIA and ARTS IIIA respectively. The vast majority of the terminal automation sites today remain either IIA or IIIA configurations, except for about nine of the largest IIIA sites, which are ARTS IIIE candidate systems. Selected ARTS IIIA/IIIE and ARTS IIA sites are scheduled to receive commercial off the shelf (COTS) hardware upgrades, which replace portions of the proprietary data processing system with standard off-the-shelf hardware.

IFR Operations in the National Airspace System

Today’s National Airspace System (NAS) consists of a complex collection of facilities, systems, equipment, procedures, and airports operated by thousands of people to provide a safe and efficient flying environment. The NAS includes:

• More than 690 air traffic control (ATC) facilities with associated systems and equipment to provide radar and communication service.
• Volumes of procedural and safety information necessary for users to operate in the system and for Federal Aviation Administration (FAA) employees to effectively provide essential services.
• More than 19,800 airports capable of accommodating an array of aircraft operations, many of which support instrument flight rules (IFR) departures and arrivals.
• Approximately 11,120 air navigation facilities.
• Approximately 45,800 FAA employees who provide air traffic control, flight service, security, field maintenance, certification, systems acquisitions, and a variety of other services.
• Approximately 13,000 instrument flight procedures as of September 2005, including 1,159 instrument landing system (ILS), 121 ILS Category (CAT) II, 87 ILS CAT III, 7 ILS with precision runway monitoring (PRM), 3 microwave landing system (MLS), 1,261 nondirectional beacon (NDB), 2,638 VHF omnidirectional range (VOR), and 3,530 area navigation (RNAV), 30 localizer type directional aid (LDA), 1,337 localizer (LOC), 17 simplified directional facility (SDF), 607 standard instrument departure (SID), and 356 standard terminal arrival (STAR).
• Approximately 48,200,000 instrument operations logged by FAA towers annually, of which 30 percent are air carrier, 27 percent air taxi, 37 percent general aviation, and 6 percent military.

America’s aviation industry is projecting continued increases in business, recreation, and personal travel. The FAA expects airlines in the United States (U.S.) to carry about 45 percent more passengers by the year 2015 than they do today. [Figure 1-1]

ATC Radar Equipment

All ARTCC radars in the conterminous U.S., as well as most airport surveillance radars, have the capability to interrogate Mode C and display altitude information to the controller. However, there are a small number of airport surveillance radars that are still two-dimensional (range and azimuth only); consequently, altitude information must be obtained from the pilot.

At some locations within the ATC environment, secondary only (no primary radar) gap filler radar systems are used to give lower altitude radar coverage between two larger radar systems, each of which provides both primary and secondary radar coverage. In the geographical areas serviced by secondary radar only, aircraft without transponders cannot be provided with radar service. Additionally, transponder- equipped aircraft cannot be provided with radar advisories concerning primary targets and weather.

An integral part of the air traffic control radar beacon system (ATCRBS) ground equipment is the decoder, which enables the controller to assign discrete transponder codes to each aircraft under his/her control. Assignments are made by the ARTCC computer on the basis of the National Beacon Code Allocation Plan (NBCAP). There are 4,096 aircraft transponder codes that can be assigned. An aircraft must be equipped with Civilian Mode A (or Military Mode 3) capabilities to be assigned a transponder code. Another function of the decoder is that it is also designed to receive Mode C altitude information from an aircraft so equipped. This system converts aircraft altitude in 100-foot increments to coded digital information that is transmitted together with Mode C framing pulses to the interrogating ground radar facility. The ident feature of the transponder causes the transponder return to “blossom” for a few seconds on the controller’s radarscope.

Radar Coverage

A full approach is a staple of instrument flying, yet some pilots rarely, if ever, have to fly one other than during initial or recurrency or proficiency training, because a full approach usually is required only when radar service is not available, and radar is available at most larger and busier instrument airports. Pilots come to expect radar vectors to final approach courses and that ATC will keep an electronic eye on them all the way to a successful conclusion of every approach. In addition, most en route flights are tracked by radar along their entire route in the 48 contiguous states, with essentially total radar coverage of all instrument flight routes except in the mountainous West. Lack of radar coverage may be due to terrain, cost, or physical limitations.

New developing technologies, like ADS-B, may offer ATC a method of accurately tracking aircraft in non- radar environments. ADS-B is a satellite-based air traffic tracking system enabling pilots and air traffic controllers to share and display the same information. ADS-B relies on the Global Positioning System (GPS) to determine an aircraft’s position. The aircraft’s precise location, along with other data such as airspeed, altitude, and aircraft identification, then is instantly relayed via digital datalink to ground stations and other equipped aircraft. Depending on the location of the ground based transmitters (GBT), ADS-B has the potential to work well at low altitudes, in remote locations, and mountainous terrain where little or no radar coverage exists.

Operational Tools

Airports are one of the main bottlenecks in the NAS, responsible for one third of the flight delays. It is widely accepted that the unconstrained increase in the number of airports or runways may not wholly alleviate the congestion problem and, in fact, may create more problems than it solves. The aim of the FAA is to integrate appropriate technologies, in support of the OEP vision, with the aim of increasing airport throughput.

The airport is a complex system of systems and any approach to increasing capacity must take this into account. Numerous recent developments contribute to the overall solution, but their integration into a system that focuses on maintaining or increasing safety while increasing capacity remains a major challenge. The supporting technologies include new capabilities for the aircraft and ATC, as well as new strategies for improving communication between pilots and ATC.

IFR Slots

During peak traffic, ATC uses IFR slots to promote a smooth flow of traffic. This practice began during the late 1960s, when five of the major airports (LaGuardia Airport, Ronald Reagan National Airport, John F. Kennedy International Airport, Newark International Airport, and Chicago O’Hare International Airport) were on the verge of saturation due to substantial flight delays and airport congestion. To combat this, the FAA in 1968 proposed special air traffic rules to these five high-density airports (the “high density rule”) that restricted the number of IFR takeoffs and landings at each airport during certain hours of the day and provided for the allocation of “slots” to carriers for each IFR landing or takeoff during a specific 30 or 60-minute period. A more recent FAA proposal offers an overhaul of the slot-reservation process for JFK, LaGuardia, and Reagan National Airport that includes a move to a 72hour reservation window and an online slot-reservation system.
The high density rule has been the focus of much examination over the last decade since under the restrictions, new entrants attempting to gain access to high density airports face difficulties entering the market. Because slots are necessary at high density airports, the modification or elimination of the high density rule could subsequently have an effect on the value of slots. Scarce slots hold a greater economic value than slots that are easier to come by.
The current slot restrictions imposed by the high density rule has kept flight operations well below capacity, especially with the improvements in air traffic control technology. However, easing the restrictions imposed by the high density rule is likely to affect airport oper ations. Travel delay time might be affected not only at the airport that has had the high density restrictions lifted, but also at surrounding airports that share the same airspace. On the other hand, easing the restrictions on slots at high density airports should help facilitate international air travel and help increase the number of passengers that travel internationally.
Slot controls have become a way of limiting noise, since it caps the number of takeoffs and landings at an airport. Easing the restrictions on slots could be politically difficult since local delegations at the affected airports might not support such a move. Ways other than imposing restrictions on slots exist that could diminish the environmental impacts at airports and their surrounding areas. Safeguards, such as requiring the quietest technology available of aircraft using slots and frequent consultations with local residents, have been provided to ensure that the environmental concerns are addressed and solved.

Precision Runway Monitoring

Precision runway monitor (PRM) is a high-update-rate radar surveillance system that is being introduced at selected capacity-constrained U.S. airports. Certified to provide simultaneous independent approaches to closely spaced parallel runways, PRM has been operational at Minneapolis since 1997. ILS/PRM approaches are conducted at Philadelphia International Airport. Simultaneous Offset Instrument Approach (SOIA)/PRM operations are conducted at San Francisco International and Cleveland Hopkins International Airports. Since the number of PRM sites is increasing, the likelihood is increasing that you may soon be operating at an airport conducting closely spaced parallel approaches using PRM. Furthermore, St. Louis Lambert International Airport began SOIA/PRM operations in 2005, and Atlanta Hartsfield International Airport will begin PRM operations in 2006. PRM enables ATC to improve the airport arrival rate on IFR days to one that more closely approximates VFR days, which means fewer flight cancellations, less holding, and decreased diversions.

PRM not only maintains the current level of safety, but also increases it by offering air traffic controllers a much more accurate picture of the aircraft’s location on final approach. Whereas current airport surveillance radar used in a busy terminal area provides an update to the controller every 4.8 seconds, PRM updates every second, giving the controller significantly more time to react to potential aircraft separation problems. The controller also sees target trails that provide very accurate trend information. With PRM, it is immediately apparent when an aircraft starts to drift off the runway centerline and toward the non-transgression zone. PRM also predicts the aircraft track and provides aural and visual alarms when an aircraft is within 10 seconds of penetrating the non-transgression zone. The additional controller staffing that comes along with PRM is another major safety improvement. During PRM sessions, there is a separate controller monitoring each final approach course and a coordinator managing the overall situation.

PRM is an especially attractive technical solution for the airlines and business aircraft because it does not require any additional aircraft equipment, only special training and qualifications. However, all aircraft in the approach streams must be qualified to participate in PRM or the benefits are quickly lost and controller workload increases significantly. The delay-reduction benefits of PRM can only be fully realized if everyone participates. Operators that choose not to participate in PRM operations when arriving at an airport where PRM operations are underway can expect to be held until they can be accommodated without disrupting the PRM arrival streams.