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.

Ground Delay Program

Bad weather often forces the reconfiguration of runways at an airport or mandates the use of IFR arrival and departure procedures, reducing the number of flights per hour that are able to takeoff or land at the affected airport. To accommodate the degraded arrival capacity at the affected airport, the ATCSCC imposes a ground delay program (GDP), which allocates a reduced number of arrival slots to airlines at airports during time periods when demand exceeds capacity. The GDP suite of tools is used to keep congestion at an arrival airport at acceptable levels by issuing ground delays to aircraft before departure, as ground delays are less expensive and safer than in-flight holding delays. The FAA started GDP prototype operations in January 1998 at two airports and expanded the program to all commercial airports in the U.S. within nine months.

Ground Delay Program Enhancements (GDPE) significantly reduced delays due to compression—a process that is run periodically throughout the duration of a GDP. It reduces overall delays by identifying open arrival slots due to flight cancellations or delays and fills in the vacant slots by moving up operating flights that can use those slots. During the first two years of this program, almost 90,000 hours of scheduled delays were avoided due to compression, resulting in cost savings to the airline industry of more than $150 million. GDPE also has improved the flow of air traffic into airports; improved compliance to controlled times of departure; improved data quality and predictability; resulted in equity in delays across carriers; and often avoided the necessity to implement FAA ground delay programs, which can be disruptive to air carrier operations.

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.

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.

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.

Navigation Systems

Navigation systems are the basis for pilots to get from one place to another and know where they are and what course to follow. Since the 1930s, aircraft have navigated by means of a set of ground-based NAVAIDs. Today, pilots have access to over 2,000 such NAVAIDs within the continental U.S., but the system has its limitations:

• Constrained to fly from one NAVAID to the next, aircraft route planners need to identify a beacon-based path that closely resembles the path the aircraft needs to take to get from origin to destination. Such a path will always be greater in distance than a great circle route between the two points.
• Because the NAVAIDs are ground-based, navigation across the ocean is problematic, as is navigation in some mountainous regions.
• NAVAIDs are also expensive to maintain.

Since the 1980s, aircraft systems have evolved towards the use of SATNAV. Based on the GPS satellite constellation, SATNAV may provide better position information than a ground-based navigation system. GPS is universal so there are no areas without satellite signals. Moreover, a space-based system allows “off airway” navigation so that the efficiencies in aircraft route determination can be exacted. SATNAV is revolutionizing navigation for airlines and other aircraft owners and operators. A drawback of the satellite system, though, is the integrity and availability of the signal, especially during electromagnetic and other events that distort the Earth’s atmosphere. In addition, the signal from space needs to be augmented, especially in traffic-dense terminal areas, to guarantee the necessary levels of accuracy and availability.

The CAASD is helping the navigation system of the U.S. to evolve toward a satellite-based system. The CAASD analysts are providing the modeling necessary to understand the effects of atmospheric phenomena on the GPS signal from space, while the CAASD is providing the architecture of the future navigation system and writing the requirements (and computer algorithms) to ensure the navigation system’s integrity. Moving toward a satellite-based navigation system allows aircraft to divorce themselves from the constraints of ground-based NAVAIDs and formulate and fly those routes that aircraft route planners deem most in line with their own cost objectives.

With the advent of SATNAV, there are a number of applications that can be piggybacked to increase capacity in the NAS. Enhanced navigation systems will be capable of “random navigation,” that is, capable of treating any latitude-longitude point as a radio navigation fix, and being able to fly toward it with the accuracy we see today, or better. New routes into and out of the terminal areas are being implemented that are navigable by on-board systems. Properly equipped aircraft are being segregated from other aircraft streams with the potential to increase volume at the nation’s busy airports by keeping the arrival and departure queues full and fully operating.

The CAASD is working with the FAA to define the nation’s future navigation system architecture. By itself, the GPS satellite constellation is inadequate to serve all the system’s needs. Augmentation of the GPS signal via WAAS and LAAS is a necessary part of that new architecture. The CAASD is developing the requirements based on the results of sophisticated models to ensure the system’s integrity, security, and availability.

Electronic Flight Information System

The electronic flight information system (EFIS) found in advanced aircraft cockpits offer pilots a tremendous amount of information on a colorful, easy-to-read display. Glass cockpits are a vast improvement over the earlier generation of instrumentation. [Figure 1-16]

Primary flight, navigation, and engine information are presented on large display screens in front of the flight crew. Flight management CDUs are located on the center console. They provide data display and entry capabilities for flight management functions. The display units generate less heat, save space, weigh less, and require less power than traditional navigation systems. From a pilot’s point of view, the information display system is not only more reliable than previous systems, but also uses advanced liquid-crystal technology that allows displayed informa tion to remain clearly visible in all conditions, including direct sunlight.

Flight Management System

A flight management system (FMS) is a flight computer system that uses a large database to allow routes to be preprogrammed and fed into the system by means of a data loader. The system is constantly updated with respect to position accuracy by reference to conventional navigation aids, inertial reference system technology, or the satellite global positioning system. The sophisticated program and its associated database ensures that the most appropriate navigation aids or inputs are automatically selected during the information update cycle. A typical FMS provides information for continuous automatic navigation, guidance, and aircraft performance management, and includes a control display unit (CDU). [Figure 1-15]

FMS Control Display Unit. This depicts an aircraft
established on the Atlantic City, NJ, RNAV (GPS) Rwy 13
instrument approach procedure at the Atlantic City
International Airport, KACY. The aircraft is positioned at the
intermediate fix UNAYY inbound on the 128 degree magnetic
course, 5.5 nautical miles from PVIGY, the final approach fix.

Avionics and Instrumentation

The proliferation of advanced avionics and instrumentation has substantially increased the capabilities of aircraft in the IFR environment.

Terrain Awareness and Warning System

The Terrain Awareness and Warning System (TAWS) is an enhanced ground proximity warning capability being installed in many aircraft. TAWS uses position data from a navigation system, like GPS, and a digital terrain database to display surrounding terrain. TAWS equipment is mandatory for all U.S registered turbine powered airplanes with six or more passenger seats. FAA and NTSB studies have shown that a large majority of CFIT accidents could likely have been avoided had the aircraft been equipped with enhanced ground proximity warning systems.

Traffic Information Service

Traffic Information Service (TIS) provides many of the functions available in TCAS; but unlike TCAS, TIS is a ground-based service available to all aircraft equipped with Mode S transponders. TIS takes advantage of the Mode S data link to communicate collision avoidance information to aircraft. Information is pre sented to a pilot in a cockpit display that shows traffic within 5 nautical miles and a 1,200-foot altitude of other Mode S-equipped aircraft. The TIS system uses track reports provided by ground-based Mode S surveillance systems to retrieve traffic information. Because it is available to all Mode S transponders, TIS offers an inexpensive alternative to TCAS. The increasing availability of TIS makes collision avoidance technology more accessible to the general aviation community. Beginning in 2005, the use of Mode S TIS is being discontinued at some sites as the ground radar systems are upgraded. In all, 23 sites are expected to lose TIS capability by 2012.

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.

Mode S

The first comprehensive proposal and design for the Mode S system was delivered to the FAA in 1975. However, due to design and manufacturing setbacks, few Mode S ground sensors and no commercial Mode S transponders were made available before 1980. Then, a tragic mid-air collision over California in 1986 prompted a dramatic change. The accident that claimed the lives of 67 passengers aboard the two planes and fifteen people on the ground was blamed on inadequate automatic conflict alert systems and surveillance equipment. A law enacted by Congress in 1987 required all air carrier airplanes operating within U.S. airspace with more than 30 passenger seats to be equipped with Traffic Alert and Collision Avoidance System (TCAS II) by December 1993. Airplanes with 10 to 30 seats were required to employ TCAS I by December 1995.

Due to the congressional mandate, TCAS II became a pervasive system for air traffic control centers around the world. Because TCAS II uses Mode S as the standard air-ground communication datalink, the widespread international use of TCAS II has helped Mode S become an integral part of air traffic control systems all over the world. The datalink capacity of Mode S has spawned the development of a number of different services that take advantage of the two-way link between air and ground. By relying on the Mode S datalink, these services can be inexpensively deployed to serve both the commercial transport aircraft and general aviation communities. Using Mode S makes not only TCAS II, but also other services available to the general aviation community that were previously accessible only to commercial aircraft. These Mode S-based technologies are described below.

Data Link

The CAASD is working with the FAA and the airlines to define and test a controller-pilot data link communication (CPDLC), which provides the capability to exchange information between air traffic controllers and flight crews through digital text instead of voice messages. With CPDLC, communications between the ground and the air would take less time, and would convey more information (and more complex information) than by voice alone. Communications would become more accurate as up-linked information would be collected, its accuracy established, and then displayed for the pilot in a consistent fashion.

By using digital data messages to replace conventional voice communications (except during landing and departure phases and in emergencies) CPDLC is forecast to increase airspace capacity and reduce delays. Today the average pilot/controller voice exchange takes around 20 seconds, compared to one or two seconds with CPDLC. In FAA simulations, air traffic controllers indicated that CPDLC could increase their productivity by 40 percent without increasing workload. Airline cost/benefit studies indicate average annual savings that are significant in the terminal and en route phases, due to CPDLC-related delay reductions.

CPDLC for routine ATC messages, initially offered in Miami Center, will be implemented via satellite at all oceanic sectors. Communications between aircraft and FAA oceanic facilities will be available through satellite data link, high frequency data link (HFDL), or other subnetworks, with voice via HF and satellite communications remaining as backup. Eventually, the service will be expanded to include clearances for altitude, speed, heading, and route, with pilot initiated downlink capability added later.

Communications

Most air traffic control communications between pilots and controllers today are conducted via voice. Each air traffic controller uses a radio frequency different from the ones used by surrounding controllers to communicate with the aircraft under his or her jurisdiction. With the increased traffic, more and more controllers have been added to maintain safe separation between aircraft. While this has not diminished safety, there is a limit to the number of control sectors created in any given region to handle the traffic. The availability of radio frequencies for controller-pilot communications is one limiting factor. Some busy portions of the U.S., such as the BostonChicago- Washington triangle are reaching toward the limit. Frequencies are congested and new frequencies are not available, which limits traffic growth to those aircraft that can be safely handled.

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.

Bright Radar Indicator Terminal Equipment

Bright Radar Indicator Terminal Equipment (BRITE) provides radar capabilities to towers, a system with tremendous benefits for both pilots and controllers. Unlike traditional radar systems, BRITE is similar to a television screen in that it can be seen in daylight. BRITE was so successful that the FAA has installed the new systems in towers, and even in some TRACONs. In fact, the invention of BRITE was so revolutionary that it launched a new type of air traffic facility . the TRACAB, which is a radar approach control facility located in the tower cab of the primary airport, as opposed to a separate room.

In the many facilities without BRITE, the controllers use strictly visual means to find and sequence traffic. Towers that do have BRITE may have one of several different types. Some have only a very crude display that gives a fuzzy picture of blips on a field of green, perhaps with the capability of displaying an extra slash on transponder-equipped targets and a larger slash when a pilot hits the ident button. Next in sophistication are BRITEs that have alphanumeric displays of various types, ranging from transponder codes and altitude to the newest version, the DBRITE (digital BRITE). A computer takes all the data from the primary radar, the secondary radar (transponder information), and generates the alphanumeric data. DBRITE digitizes the image, and then sends it all, in TV format, to a square display in the tower that provides an excellent presentation, regardless of how bright the ambient light.

One of the most limiting factors in the use of the BRITE is in the basic idea behind the use of radar in the tower. The radar service provided by a tower controller is not, nor was it ever intended to be, the same thing as radar service provided by an approach control or Center. The primary duty of tower controllers is to separate airplanes operating on runways, which means controllers spend most of their time looking out the window, not staring at a radar scope.

Precision Approach Radar

While ASR provides pilots with horizontal guidance for instrument approaches via a ground-based radar, Precision Approach Radar (PAR) provides both horizontal and vertical guidance for a ground controlled approach (GCA). In the U.S., PAR is mostly used by the military. Radar equipment in some ATC facilities operated by the FAA and/or the military services at joint-use locations and military installations are used to detect and display azimuth, elevation, and range of aircraft on the final approach course to a runway. This equipment may be used to monitor certain non- radar approaches, but it is primarily used to conduct a precision instrument approach.

Standard Terminal Automation Replacement System

The FAA has begun modernizing the computer equipment in the busiest terminal airspace areas. The newer equipment is called STARS, for Standard Terminal Automation Replacement System. The system's improvements will enhance safety while reducing delays by increasing system reliability and lowering life- cycle operating and maintenance costs. STARS also will accommodate the projected growth in air traffic and provide a platform for new functions to support FAA initiatives such as Free Flight. STARS offers many advantages, including an open architecture and expansion capability that allow new software and capacity to be added as needed to stay ahead of the growth in air traffic. Under the first phase of terminal modernization, STARS is being deployed to 47 air traffic control facilities. As of July 2005, 37 FAA and 22 Department of Defense sites were fully operational with STARS. The first phase is expected to be complete in fiscal year 2007. By then, STARS will be operational at 18 of the FAA's 35 most critical, high-volume airports, which together handle approximately 50 percent of air traffic. STARS consists of new digital, color displays and computer software and processors that can track 435 aircraft at one time, integrating six levels of weather information and 16 radar feeds.

For the terminal area and many of the towers, STARS is the key to the future, providing a solid foundation for new capabilities. STARS was designed to provide the software and hardware platform necessary to support future air traffic control enhancements.

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.

Graphical Weather System

The Graphical Weather Service (GWS) provides a graphical representation of weather information that is transmitted to aircraft and displayed on the cockpit display unit. The service is derived from ground-based Mode S sensors and offers information to all types of aircraft, regardless of the presence of on-board weather avoidance equipment. The general aviation community has been very pro-active in evaluating this technology, as they have already participated in field evaluations in Mode S stations across the U.S. The service is provided through one of two types of flight information services (FIS) systems. Broadcast only systems, called FIS-B, include a ground- or space-based transmitter, an aircraft receiver, and a portable or installed cockpit display device. They allow pilots to passively collect weather and other operational data and to display that data at the appropriate time. They can display graphical weather products such as radar composite/mosaic images, temporary flight restricted airspace and other NOTAMs. In addition to graphical weather products, they can also show textual information, such as Aviation Routine Weather Reports (METARs)/Aviation Selected Special Weather Reports (SPECIs) and Terminal Area Forecasts (TAFs).

Two-way FIS systems are request/reply systems, that is, they permit the pilot to make specific requests for weather and other operational information. An FIS service provider will then prepare a reply in response to that specific request and transmit the product to that specific aircraft for display in the cockpit.

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.

Equipment and Avionics

By virtue of distance and time savings, minimizing traffic congestion, and increasing airport and airway capacity, the implementation of RNAV routes, direct routing, RSVM, PRM, and other technological innovations would be advantageous for the current NAS. Some key components that are integral to the future development and improvement of the NAS are described below. However, equipment upgrades require capital outlays, which take time to penetrate the existing fleet of aircraft and ATC facilities. In the upcoming years while the equipment upgrade is taking place, ATC will have to continue to accommodate the wide range of avionics used by pilots in the nation’s fleet.

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.

Air Route Surveillance Radar

The long-range radar equipment used in controlled airspace to manage traffic is the air route surveillance radar (ARSR) system. There are approximately 100 ARSR facilities to relay traffic information to radar controllers throughout the country. Some of these facilities can detect only transponder-equipped aircraft and are referred to as beacon-only sites. Each air route surveillance radar site can monitor aircraft flying within a 200-mile radius of the antenna, although some stations can monitor aircraft as far away as 600 miles through the use of remote sites.

The direction and coordination of IFR traffic in the U.S. is assigned to air route traffic control centers (ARTCCs). These centers are the authority for issuing IFR clearances and managing IFR traffic; however, they also provide services to VFR pilots. Workload permitting, controllers will provide traffic advisories and course guidance, or vectors, if requested.

Air Route Surveillance Radar

The long-range radar equipment used in controlled airspace to manage traffic is the air route surveillance radar (ARSR) system. There are approximately 100 ARSR facilities to relay traffic information to radar controllers throughout the country. Some of these facilities can detect only transponder-equipped aircraft and are referred to as beacon-only sites. Each air route surveillance radar site can monitor aircraft flying within a 200-mile radius of the antenna, although some stations can monitor aircraft as far away as 600 miles through the use of remote sites.

The direction and coordination of IFR traffic in the U.S. is assigned to air route traffic control centers (ARTCCs). These centers are the authority for issuing IFR clearances and managing IFR traffic; however, they also provide services to VFR pilots. Workload permitting, controllers will provide traffic advisories and course guidance, or vectors, if requested.

Airport Surveillance Radar

The direction and coordination of IFR traffic within specific terminal areas is delegated to airport surveillance radar (ASR) facilities. Approach and departure control manage traffic at airports with ASR. This radar system is designed to provide relatively short-range coverage in the airport vicinity and to serve as an expeditious means of handling terminal area traffic. The ASR also can be used as an instrument approach aid. Terminal radar approach control facilities (TRACONs) provide radar and nonradar services at major airports. The primary responsibility of each TRACON is to ensure safe separation of aircraft transitioning from departure to cruise flight or from cruise to a landing approach.

Most ASR facilities throughout the country use a form of automated radar terminal system (ARTS). This system has several different configurations that depend on the computer equipment and software programs used. Usually the busiest terminals in the country have the most sophisticated computers and programs. The type of system installed is designated by a suffix of numbers and letters. For example, an ARTS-IIIA installation can detect, track, and predict primary, as well as secondary, radar returns. [Figure 1-13]

ARTS-III Radar Display

On a controller’s radar screen, ARTS equipment automatically provides a continuous display of an aircraft’s position, altitude, groundspeed, and other pertinent information. This information is updated continuously as the aircraft progresses through the terminal area. To gain maximum benefit from the system, each aircraft in the area must be equipped with a Mode C altitude encoding transponder, although this is not an operational requirement. Direct altitude readouts eliminate the need for time consuming verbal communication between controllers and pilots to verify altitude. This helps to increase the number of aircraft that may be handled by one controller at a given time.

The FAA has begun replacing the ARTS systems with newer equipment in some areas. The new system is called STARS, for Standard Terminal Automation Replacement System. STARS is discussed in more detail later in this chapter.

FAA Radar Systems

The FAA operates two basic radar systems; airport surveillance radar (ASR) and air route surveillance radar (ARSR). Both of these surveillance systems use primary and secondary radar returns, as well as sophisticated computers and software programs designed to give the controller additional information, such as aircraft speed and altitude.

Reduced Vertical Separation Minimums

The U.S. domestic reduced vertical separation minimums (DRVSM) program has reduced the vertical separation from the traditional 2,000-foot minimum to a 1,000-foot minimum above FL 290, which allows aircraft to fly a more optimal profile, thereby saving fuel while increasing airspace capacity. The FAA has implemented DRVSM between FL 290 and FL 410 (inclusive) in the airspace of the contiguous 48 states, Alaska, and in Gulf of Mexico airspace where the FAA provides air traffic services. DRVSM is expected to result in fuel savings for the airlines of as much as $5 billion by 2016. Full DRVSM adds six additional usable altitudes above FL 290 to those available using the former vertical separation minimums. DRVSM users experience increased benefits nationwide, similar to those already achieved in oceanic areas where RVSM is operational. In domestic airspace, however, operational differences create unique challenges. Domestic U.S. airspace contains a wider variety of aircraft types, higher-density traffic, and an increased percentage of climbing and descending traffic. This, in conjunction with an intricate route structure with numerous major crossing points, creates a more demanding environment for the implementation of DRVSM than that experienced in applying RVSM on international oceanic routes. As more flights increase demands on our finite domestic airspace, DRVSM helps to reduce fuel burn and departure delays and increases flight level availability, airspace capacity, and controller flexibility.

GPS-Based Helicopter Operations

The synergy between industry and the FAA created during the development of the Gulf of Mexico GPS grid system and approaches is an excellent example of what can be accomplished to establish the future of helicopter IFR SATNAV. The Helicopter Safety Advisory Council (HSAC), National Air Traffic Controllers Association (NATCA), helicopter operators, and FAA Flight Standards Divisions all worked together to develop this infrastructure. The system provides both the operational and cost-saving features of flying direct to a destination when offshore weather conditions deteriorate below VFR and an instant and accurate aircraft location capability that is invaluable for rescue operations.

The expansion of helicopter IFR service for emergency medical services (EMS) is another success story. The FAA worked with EMS operators to develop helicopter GPS nonprecision instrument approach procedures and en route criteria. As a result of this collaborative effort, EMS operators have been provided with hundreds of EMS helicopter procedures to medical facilities. Before the GPS IFR network, EMS helicopter pilots had been compelled to miss 30 percent of their missions due to weather. With the new procedures, only about 11 percent of missions are missed due to weather.

The success of these operations can be attributed in large part to the collaborative efforts between the helicopter industry and the FAA. There are currently 289 special use helicopter procedures, with more being added. There are also 37 public use helicopter approaches. Of these, 18 are to runways and 19 are to heliports or points-inspace (PinS).

Global Positioning System

The FAA’s implementation activities of the Global Positioning System (GPS) are dedicated to the adaptation of the NAS infrastructure to accept satellite navigation (SATNAV) technology through the management and coordination of a variety of overlapping NAS implementation projects. These projects fall under the project areas listed below and represent different elements of the NAS infrastructure:

• Avionics Development - includes engineering support and guidance in the development of current and future GPS avionics minimum operational performance standards (MOPS), as well as FAA Technical Standard Orders (TSOs) and establishes certification standards for avionics installations.
• Flight Standards - includes activities related to instrument procedure criteria research, design, testing, and standards publication. The shift from ground-based to space-based navigation sources has markedly shifted the paradigms used in obstacle clearance determination and standards development. New GPS-based Terminal Procedures (TERPS) manuals are in use today as a result of this effort.
• Air Traffic - includes initiatives related to the development of GPS routes, phraseology, procedures, controller GPS training and GPS outage simulations studies. GPS-based routes, developed along the East Coast to help congestion in the Northeast Corridor, direct GPS-based Caribbean routes, and expansion of RNAV activities are all results of SATNAV sponsored implementation projects.
• Procedure Development - includes the provision of instrument procedure development and flight inspection of GPS-based routes and instrument procedures. Today over 3,500 GPS-based IAPs have been developed.
• Interference Identification and Mitigation - includes the development and fielding of airborne, ground, and portable interference detection systems. These efforts are ongoing and critical to ensuring the safe use of GPS in the NAS.

To use GPS, WAAS, and/or LAAS in the NAS, equipment suitable for aviation use (such as a GPS receiver, WAAS receiver, LAAS receiver, or multi-modal receiver) must be designed, developed, and certified for use. To ensure standardization and safety of this equipment, the FAA plays a key role in the development and works closely with industry in this process. The avionics development process results in safe, standardized SATNAV avionics, developed in concurrence with industry. Due to the growing popularity of SATNAV and potential new aviation applications, there are several types of GPS-based receivers on the market, but only those that pass through this certification process can be used as approved navigation equipment under IFR conditions. Detailed information on GPS approach procedures is provided in Chapter 5–Approach.

Required Navigation Performance

Required Navigation Performance

The continuing growth of aviation places increasing demands on airspace capacity and emphasizes the need for the best use of the available airspace. These factors, along with the accuracy of modern aviation navigation systems and the requirement for increased operational efficiency in terms of direct routings and track-keeping accuracy, have resulted in the concept of required navigation performance—a statement of the navigation performance accuracy necessary for operation within a defined airspace. Required Navigation Performance (RNP) is a statement of the navigation performance necessary for operation within a defined airspace. RNP includes both performance and functional requirements, and is indicated by the RNP value. The RNP value designates the lateral performance requirement associated with a procedure. [Figure 1-11]

NRP Approach Chart

RNP includes a navigation specification including requirements for on-board performance monitoring and alerting. These functional and performance standards allow the flight paths of participating aircraft to be both predictable and repeatable to the declared levels of accuracy. More information on RNP is contained in subsequent chapters.
The term RNP is also applied as a descriptor for air-the capability of both the available infrastructure (navigaspace, routes, and procedures — including departures, tion aids) and the aircraft. Washington National Airport arrivals, and instrument approach procedures (IAPs). (KDCA) introduced the first RNP approach procedure in The descriptor can apply to a unique approach procedure September 2005. An example of an RNP approach chart or to a large region of airspace. RNP applies to navigation is shown in Figure 1-12.

The RNP value designates the lateral performance requirement associated with a procedure. The required performance is obtained through a combination of aircraft capability and the level of service provided by the corresponding navigation infrastructure. From a broad perspective:

Aircraft Capability + Level of Service = Access 

In this context, aircraft capability refers to the airworthiness certification and operational approval elements (including avionics, maintenance, database, human factors, pilot procedures, training, and other issues). The level of service element refers to the NAS infrastructure, including published routes, signal-in-space performance and availability, and air traffic management. When considered collectively, these elements result in providing access. Access provides the desired benefit (airspace, procedures, routes of flight, etc.).
A key feature of RNP is the concept of on-board monitoring and alerting. This means the navigation equipment is accurate enough to keep the aircraft in a specific volume of airspace, which moves along with the aircraft. The aircraft is expected to remain within this block of airspace for at least 95 percent of the flight time. Additional airspace outside the 95 percent area is provided for continuity and integrity, so that the combined areas ensure aircraft containment 99.9 percent of the time. RNP levels are actual distances from the centerline of the flight path, which must be maintained for aircraft and obstacle separation. Although additional FAA-recognized RNP levels may be used for specific operations, the United States currently supports three standard RNP levels:

• RNP 0.3 – Approach
• RNP 1.0 – Terminal
• RNP 2.0 – Terminal and En Route

RNP 0.3 represents a distance of 0.3 nautical miles (NM) either side of a specified flight path centerline. The specific performance required on the final approach segment of an instrument approach is an example of this RNP level.

For international operations, the FAA and ICAO member states have led initiatives to apply RNP concepts to oceanic routes. Here are the ICAO RNP levels supported for international operations:

• RNP-1 – European Precision RNAV (P-RNAV)
• RNP-4 – Projected for oceanic/remote areas where 30 NM horizontal separation is applied
• RNP-5 – European Basic RNAV (B-RNAV)
• RNP-10 – Oceanic/remote areas where 50 NM lateral separation is applied

NOTE: Specific operational and equipment performance requirements apply for P-RNAV and B-RNAV.

RNAV IFR Terminal Transition Routes

The FAA is moving forward with an initiative to chart RNAV terminal transition routes through busy airspace. In 2001, some specific RNAV routes were implemented through Charlotte’s Class B airspace, allowing RNAV- capable aircraft to cross through the airspace instead of using costly and time-consuming routing around the Class B area. The original RNAV terminal transition routes have evolved into RNAV IFR terminal transition routes, or simply RITTRs.

Beginning in March 2005, with the publication of the notice of proposed rulemaking (NPRM) for the Charlotte, North Carolina, RITTRs, the FAA advanced the process of establishing and charting the first RITTRs on IFR en route low altitude charts. The five new RITTRs through Charlotte's Class B airspace took effect on September 1, 2005, making them available for pilots to file on their IFR flight plans. Additional RITTRs are planned for Cincinnati, Ohio, and Jacksonville, Florida.

The RITTRs allow IFR overflights through the Class B airspace for RNAV-capable aircraft. Without the RITTRS, these aircraft would be routinely routed around the Class B by as much as 50 miles.

Application of Area Navigation

RNAV airways provide more direct routings than the current VOR-based airway system, giving pilots easier access through terminal areas, while avoiding the circuitous routings now common in many busy Class B areas. RNAV airways are a critical component to the transition from ground-based navigation systems to GPS navigation. RNAV routes help maintain the aircraft flow through busy terminals by segregating arrival or departure traffic away from possibly interfering traffic flows.
Further, RNAV provides the potential for increasing airspace capacity both en route and in the terminal area in several important ways.
Strategic use of RNAV airways nationwide is reducing the cost of flying and providing aircraft owners more benefits from their IFR-certified GPS receivers. Several scenarios have been identified where RNAV routes provide a substantial benefit to users.

• Controllers are assigning routes that do not require overflying ground-based NAVAIDs such as VORs.
• The lateral separation between aircraft tracks is being reduced.
• RNAV routes lower altitude minimums on existing Victor airways where ground-based NAVAID performance (minimum reception altitude) required higher minimums.
• RNAV routes may allow continued use of existing airways where the ground-based NAVAID has been decommissioned or where the signal is no longer suitable for en route navigation.
• The route structure can be modified quickly and easily to meet the changing requirements of the user community.
• Shorter, simpler routes can be designed to minimize environmental impact. Dozens of new RNAV routes have been designated, and new ones are being added continuously. In order to designate RNAV airways, the FAA developed criteria, en route procedures, procedures for airway flight checks, and created new charting specifications. Some of the considerations include:
  
• Navigation infrastructure (i.e., the ground-based and space-based navigation positioning systems) provides adequate coverage for the proposed route/procedure.
• Navigation coordinate data meets International Civil Aviation Organization (ICAO) accuracy and integrity requirements. This means that all of the coordinates published in the Aeronautical Information Publication (AIP) and used in the aircraft navigation databases must be referenced to WGS 84, and the user must have the necessary assurance that this data has not been corrupted or inadvertently modified.
• Airborne systems meet airworthiness performance for use on the RNAV routes and procedures.
• Flight crews have the necessary approval to operate on the RNAV routes and procedures.

In the future, as aircraft achieve higher levels of navigation accuracy and integrity, closely spaced parallel routes may be introduced, effectively multiplying the number of available routes between terminal areas. RNAV can be used in all phases of flight and, when implemented correctly, results in:

• Improved situational awareness for the pilot.
• Reduced workloads for both controller and pilot.
• Reduced environmental impact from improved route and procedure designs.
• Reduced fuel consumption from shorter, more direct routes.

RNAV Departure Routes

For example, take the situation at Philadelphia International Airport, located in the middle of some highly popular north-south traffic lanes carrying New York and Boston traffic to or from Washington, Atlanta, and Miami. Philadelphia’s position is right underneath these flows. Chokepoints resulted from traffic departing Philadelphia, needing to wait for a “hole” in the traffic above into which they could merge. The CAASD helped US Airways and Philadelphia airport officials establish a set of RNAV departure routes that do not interfere with the prevailing established traffic. Traffic heading north or south can join the established flows at a point further ahead when higher altitudes and speeds have been attained. Aircraft properly equipped to execute RNAV procedural routes can exit the terminal area faster — a powerful inducement for aircraft operators to upgrade their navigation equipment.
Another example of an RNAV departure is the PRYME TWO DEPARTURE from Washington Dulles International. Notice in Figure 1-10 the RNAV way- points not associated with VORs help free up the flow of IFR traffic out of the airport by not funneling them to one point through a common NAVAID.

Managing Safety and Capacity System Design

MANAGING SAFETY AND CAPACITY SYSTEM DESIGN

The CAASD is aiding in the evolution towards free flight with its work in developing new procedures necessary for changing traffic patterns and aircraft with enhanced capabilities, and also in identifying traffic flow constraints that can be eliminated. This work supports the FAA’s Operational Evolution Plan in the near-term. Rapid changes in technology in the area of navigation performance, including the change from ground-based area navigation systems, provide the foundation for aviation’s global evolution. This progress will be marked by combining all elements of communication, navigation, and surveillance (CNS) with air traffic management (ATM) into tomorrow’s CNS/ATM based systems. The future CNS/ATM operating environment will be based on navigation defined by geographic waypoints expressed in latitude and longitude since instrument procedures and flight routes will not require aircraft to overfly ground-based navigation aids defining specific points.

Expect Departure Clearance Time

Another tool that the FAA is implementing to increase efficiency is the reduction of the standard expect departure clearance time3 (EDCT) requirement. The FAA has drafted changes to augment and modify procedures contained in Ground Delay Programs (GDPs). Airlines may now update their departure times by arranging their flights’ priorities to meet the controlled time of arrival. In order to evaluate the effectiveness of the new software and the airline-supplied data, the actual departure time parameter in relation to the EDCT has been reduced. This change impacts all flights (commercial and GA) operating to the nation’s busiest airports. Instead of the previous 25-minute EDCT window (5 minutes prior and 20 minutes after the EDCT), the new requirement for GDP implementation is a 10-minute window, and aircraft are required to depart within 5 minutes before or after their assigned EDCT. Using reduced EDCT and other measures included in GDPs, ATC aims at reducing the number of arrival slots issued to accommodate degraded arrival capacity at an airport affected by weather. The creation of departure or ground delays is less costly and safer than airborne holding delays in the airspace at the arrival airport.

Release Time

ATC uses an IFR release time2 in conjunction with traffic management procedures to separate departing aircraft from other traffic. For example, when controlling departures from an airport without a tower, the controller limits the departure release to one aircraft at any given time. Once that aircraft is airborne and radar identified, then the following aircraft may be released for departure, provided they meet the approved radar separation (3 miles laterally or 1,000 feet vertically) when the second aircraft comes airborne. Controllers must take aircraft performances into account when releasing successive departures, so that a B-747 HEAVY aircraft is not released immediately after a departing Cessna 172. Besides releasing fast aircraft before slow ones, another technique commonly used for successive departures is to have the first aircraft turn 30 to 40 degrees from runway heading after departure, and then have the second aircraft depart on a SID or runway heading. Use of these techniques is common practice when maximizing airport traffic capacity.

Flight Plans

Prior to flying in controlled airspace under IFR conditions or in Class A airspace, pilots are required to file a flight plan. IFR (as well as VFR) flight plans provide air traffic center computers with accurate and precise routes required for flight data processing (FDP1). The computer knows every route (published and unpub lished) and NAVAID, most intersections, and all airports, and can only process a flight plan if the proposed routes and fixes connect properly. Center computers also recognize preferred routes and know that forecast or real-time weather may change arrival routes. Centers and TRACONs now have a computer graphic that can show every aircraft on a flight plan in the U.S. as to its flight plan information and present position. Despite their sophistication, center computers do not overlap in coverage or information with other Centers, so that flight requests not honored in one must be repeated in the next.

Flight Service Stations

Flight Service Stations (FSSs) and Automated Flight Service Stations (AFSSs) are air traffic facilities which provide pilot briefings, en route communications and VFR search and rescue services, assist lost aircraft and aircraft in emergency situations, relay ATC clearances, originate Notices to Airmen, broadcast aviation weather and NAS information, receive and process IFR flight plans, and monitor navigational aids (NAVAIDs). In addition, at selected locations, FSSs/AFSSs provide En route Flight Advisory Service (Flight Watch), take weather observations, issue airport advisories, and advise Customs and Immigration of transborder flights.

Pilot Briefers at flight service stations render preflight, in-flight, and emergency assistance to all pilots on request. They give information about actual weather conditions and forecasts for airports and flight paths, relay air traffic control instructions between controllers and pilots, assist pilots in emergency situations, and initiate searches for missing or overdue aircraft. FSSs/AFSSs provide information to all airspace users, including the military. In October 2005, operation of all FSSs/AFSSs, except those in Alaska, was turned over to the Lockheed Martin Corporation. In the months after the transition, 38 existing AFSSs are slated to close, leaving 17 “Legacy” stations and 3 “Hub” stations. Services to pilots are expected to be equal to or better than prior to the change, and the contract is expected to save the government about $2.2 billion over ten years.

Control Tower

Controllers in this type of facility manage aircraft operations on the ground and within specified airspace around an airport. The number of controllers in the tower varies with the size of the airport. Small general aviation airports typically have three or four controllers, while larger international airports can have up to fifteen controllers talking to aircraft, processing flight plans, and coordinating air traffic flow. Tower controllers manage the ground movement of aircraft around the airport and ensure appropriate spacing between aircraft taking off and landing. In addition, it is the responsibility of the control tower to determine the landing sequence between aircraft under its control. Tower controllers issue a variety of instructions to pilots, from how to enter a pattern for landing to how to depart the airport for their destination.

Terminal Radar Approach Control

Terminal Radar Approach Control

Terminal Radar Approach Control (TRACON) controllers work in dimly lit radar rooms located within the control tower complex or in a separate building located on or near the airport it serves. [Figure 1-9] Using radarscopes, these controllers typically work an area of airspace with a 50-mile radius and up to an altitude of 17,000 feet. This airspace is configured to provide service to a primary airport, but may include other airports that are within 50 miles of the radar service area. Aircraft within this area are provided vectors to airports, around terrain, and weather, as well as separation from other aircraft. Controllers in TRACONs determine the arrival sequence for the control tower’s designated airspace.

Air Route Traffic Control Center

A Center’s primary function is to control and separate air traffic within a designated airspace, which may cover more than 100,000 square miles, may span several states, and extends from the base of the underlying controlled airspace up to Flight Level (FL) 600. There are 21 Centers located throughout the U.S., each of which is divided into sectors. Controllers assigned to these sectors, which range from 50 to over 200 miles wide, guide aircraft toward their intended destination by way of vectors and/or airway assignment, routing aircraft around weather and other traffic. Centers employ 300 to 700 controllers, with more than 150 on duty during peak hours at the busier facilities. A typical flight by a commercial airliner is handled mostly by the Centers.

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.

Military

From an operational point of view, military flight activities comprise a subsystem that must be fully integrated within NAS. However, military aviation has unique requirements that often are different from civil aviation users. The military’s need for designated training areas and low-level routes located near their bases sometimes conflicts with civilian users who need to detour around these areas. In coordinating the development of ATC systems and services for the armed forces, the FAA is challenged to achieve a maximum degree of compatibility between civil and military aviation objectives.

General Aviation

The tendency of GA aircraft owners to upgrade the performance and avionics of their aircraft increases the demand for IFR services and for terminal airspace at airports. In response, the FAA has increased the extent of controlled airspace and improved ATC facilities at major airports. The safety of mixing IFR and VFR traffic is a major concern, but the imposition of measures to separate and control both types of traffic creates more restrictions on airspace use and raises the level of aircraft equipage and pilot qualification necessary for access.

Corporate and Fractional Ownerships

Though technically considered under the GA umbrella, the increasing use of sophisticated, IFR-equipped aircraft by businesses and corporations has created a niche of its own. By using larger high performance airplanes and equipping them with the latest avionics, the business portion of the GA fleet has created demands for ATC services that more closely resemble commercial operators than the predominately VFR general aviation fleet.

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]

Users

Despite a drop in air traffic after the September 11 terrorist attacks, air travel returned to 2000 levels within three years and exceeded them in 2004. Industry forecasts predict growth in airline passenger traffic of around 4.3 percent per year. Commercial aviation is expected to exceed one billion passengers by 2015. The system is nearing the point of saturation, with limited ability to grow unless major changes are brought about.

Adding to the growth challenge, users of the NAS cover a wide spectrum in pilot skill and experience, aircraft types, and air traffic service demands, creating a challenge to the NAS to provide a variety of services that accommodate all types of traffic. NAS users range from professional airline, commuter, and corporate pilots to single-engine piston pilots, as well as owner-operators of personal jets to military jet fighter trainees.

How The System Components Work Together

The NAS comprises the common network of U.S. airspace, air navigation facilities, equipment, services, airports and landing areas, aeronautical charts, information and services, rules and regulations, procedures, technical information, manpower, and material. Included are system components shared jointly with the military. The underlying demand for air commerce is people’s desire to travel for business and pleasure and to ship cargo by air. This demand grows with the economy independent of the capacity or performance of the NAS. As the economy grows, more and more people want to fly, whether the system can handle it or not. Realized demand refers to flight plans filed by the airlines and other airspace users to access the system. It is moderated by the airline’s understanding of the number of flights that can be accommodated without encountering unacceptable delay, and is limited by the capacity for the system.

Air Traffic Control System Command Center

The task of managing the flow of air traffic within the NAS is assigned to the Air Traffic Control System Command Center (ATCSCC). Headquartered in Herndon, Virginia, the ATCSCC has been operational since 1994 and is located in one of the largest and most sophisticated facilities of its kind. The ATCSCC regulates air traffic at a national level when weather, equipment, runway closures, or other conditions place stress on the NAS. In these instances, traffic management specialists at the ATCSCC take action to modify traffic demands in order to remain within system capacity.

They accomplish this in cooperation with:

• Airline personnel.
• Traffic management specialists at affected facilities.
• Air traffic controllers at affected facilities.

Efforts of the ATCSCC help minimize delays and congestion and maximize the overall use of the NAS, thereby ensuring safe and efficient air travel within the U.S. For example, if severe weather, military operations, runway closures, special events, or other factors affect air traffic for a particular region or airport, the ATCSCC mobilizes its resources and various agency personnel to analyze, coordinate, and reroute (if necessary) traffic to foster maximum efficiency and utilization of the NAS.

The ATCSCC directs the operation of the traffic management (TM) system to provide a safe, orderly, and expeditious flow of traffic while minimizing delays. TM is apportioned into traffic management units (TMUs), which monitor and balance traffic flows within their areas of responsibility in accordance with TM directives. TMUs help to ensure system efficiency and effectiveness without compromising safety, by providing the ATCSCC with advance notice of planned outages and runway closures that will impact the air traffic system, such as NAVAID and radar shutdowns, runway closures, equipment and computer malfunctions, and procedural changes. [Figure 1-7]

Takeoffs and Landings

According to the FAA Administrator’s Fact Book for March 2005, there were 46,873,000 operations at airports with FAA control towers, an average of more than 128,000 aircraft operations per day. These figures do not include the tens of millions of operations at airports that do not have a control tower. User demands on the NAS are quickly exceeding the ability of current resources to fulfill them. Delays in the NAS for 2004 were slightly higher than in 2000, with a total of 455,786 delays of at least 15 minutes in 2004, compared to 450,289 in 2000. These illustrations of the increasing demands on the NAS indicate that current FAA modernization efforts are well justified. Nothing short of the integrated, systematic, cooperative, and comprehensive approach spelled out by the OEP can bring the NAS to the safety and efficiency standards that the flying public demands.

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.

Runway Incursion Statistics

While it is difficult to eliminate runway incursions, technology offers the means for both controllers and flight crews to create situational awareness of runway incursions in sufficient time to prevent accidents. Consequently, the FAA is taking actions that will identify and implement technology solutions, in conjunction with training and procedural evaluation and changes, to reduce runway accidents. Recently established programs that address runway incursions center on identifying the potential severity of an incursion and reducing the likelihood of incursions through training, technology, communications, procedures, airport signs/marking/lighting, data analysis, and developing local solutions. The FAA’s initiatives include:

• Promoting aviation community participation in runway safety activities and solutions.
• Appointing nine regional Runway Safety Program Managers.
• Providing training, education, and awareness for pilots, controllers, and vehicle operators.
• Publishing an advisory circular for airport surface operations.
• Increasing the visibility of runway hold line markings.
• Reviewing pilot-controller phraseology.
• Providing foreign air carrier pilot training, education, and awareness.
• Requiring all pilot checks, certifications, and flight reviews to incorporate performance evaluations of ground operations and test for knowledge.
• Increasing runway incursion action team site visits.
• Deploying high-technology operational systems such as the Airport Surface Detection Equipment3 (ASDE-3) and Airport Surface Detection Equipment-X (ASDE-X).
• Evaluating cockpit display avionics to provide direct warning capability to flight crew(s) of both large and small aircraft operators.

Statistics compiled for 2004 show that there were 310 runway incursions, down from 332 in 2003. The number of Category A and Category B runway incursions, in which there is significant potential for collision, declined steadily from 2000 through 2003. There were less than half as many such events in 2003 as in 2000. The number of Category A incursions, in which separation decreases and participants take extreme action to narrowly avoid a collision, or in which a collision occurs, dropped to 10 per year.

Accident Rates

The NTSB released airline accident statistics for 2004 that showed a decline from the previous year. Twenty- nine accidents on large U.S. air carriers were recorded in 2004, which is a decrease from the 54 accidents in 2003.

Accident rates for both general aviation airplanes and helicopters also decreased in 2004. General aviation airplane accidents dropped from 1,742 to 1,595, while helicopter accidents declined from 213 to 176. The number of accidents for commuter air services went up somewhat, from 2 accidents in 2003 to 5 in 2004. Air taxi operations went from 76 accidents in 2003 to 68 accidents in 2004. These numbers do not tell the whole story. Because the number of flights and flight hours increased in 2004, accident rates per 100,000 departures or per 100,000 flight hours will likely be even lower. 

CFIT Reduction

Among the top priorities for accident prevention are CFIT and ALAR. Pilots can decrease exposure to a CFIT accident by identifying risk factors and remedies prior to flight. [Figure 1-5] Additional actions on the CFIT reduction front include equipping aircraft with state-of-the art terrain awareness and warning systems (TAWS), sometimes referred to as enhanced ground proximity warning systems (EGPWS). This measure alone is expected to reduce CFIT accidents by at least 90 percent. With very few exceptions, all U.S. turbine powered airplanes with more than six passenger seats were required to be equipped with TAWS by March 29, 2005. 

Added training for aircrews and controllers is part of the campaign to safeguard against CFIT, as well as making greater use of approaches with vertical guidance that use a constant angle descent path to the runway. This measure offers nearly a 70 percent potential reduction. Another CFIT action plan involves a check of ground- based radars to ensure that the minimum safe altitude warning (MSAW) feature functions correctly.

Like CFIT, the ALAR campaign features a menu of actions, three of which involve crew training, altitude awareness policies checklists, and smart alerting technology. These three alone offer a potential 20 to 25 percent reduction in approach and landing accidents. Officials representing Safer Skies—a ten-year collaborative effort between the FAA and the airline industry—believe that the combination of CFIT and ALAR interventions will offer more than a 45 percent reduction in accidents.