Air Traffic Management Modernization: Promise and Challenges
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This entry provides a broad overview of how air traffic for commercial air travel is scheduled and managed throughout the world. The major causes of delays and congestion are described, which include tight scheduling, safety restrictions, infrastructure limitations, and major disturbances. The technical and financial challenges to air traffic management are outlined, along with some of the promising developments for future modernization.
KeywordsAir traffic management air traffic control airport capacity airspace management flight safety
Introduction: How Does Air Traffic Management Work?
This entry focuses on air traffic management for commercial air travel, the passenger- and cargo-carrying operations with which most of us are familiar. This is the air travel with a pressing need for modernization to address current and future congestion. Passenger and cargo traffic is projected to double over the next 20 years, with growth rates of 3–4 % annually in developed markets such as the USA and Europe and growth rates of 6 % and more in developing markets such as Asia Pacific and the Middle East.
In most of the world, air travel is a distributed, market-driven system. Airlines schedule flights based on when people want to fly and when it is optimal to transport cargo. Most passenger flights are scheduled during the day; most package carrier flights are overnight. Some airports limit how many flights can be scheduled by having a slot system, others do not. This decentralized schedule of flights to and from airports around the world is controlled by a network of air navigation service providers (ANSPs) staffed with air traffic controllers, who ensure that aircraft are separated safely.
The International Civil Aviation Organization (ICAO) has divided the world’s airspace into flight information regions. Each region has a country that controls the airspace, and the ANSP for each country can be a government department, state-owned company, or private organization. For example, in the United States, the ANSP is the Federal Aviation Administration (FAA), which is a government department. The Canadian ANSP is NAV CANADA, which is a private company.
Each country is different in terms of the services provided by the ANSP, how the ANSP operates, and the tools available to the controllers. In the USA and Europe, the airspace is divided into sectors and areas around airports. An air traffic control center is responsible for traffic flow within its sector and rules and procedures are in place to cover transfer of control between sectors. The areas around busy airports are usually handled by a terminal radar approach control. The air traffic control tower personnel handle departing aircraft, landing aircraft, and the movement of aircraft on the airport surface.
Air traffic controllers in developed air travel markets like the USA and Europe have tools that help them with the business of controlling and separating aircraft. Tower controllers operating at airports can see aircraft directly through windows or on computer screens through surveillance technology such as radar and Automatic Dependent Surveillance-Broadcast (ADS-B). Tower controllers may have additional tools to help detect and prevent potential collisions on the airport surface. En route controllers can see aircraft on computer screens and may have additional tools to help detect potential losses of separation between aircraft. Controllers can communicate with aircraft via radio and some have datalink communication available such as Controller-Pilot Datalink Communications (CPDLC).
Flight crews have tools to help with navigating and flying the airplane. Autopilots and autothrottles off-load the pilot from having to continuously control the aircraft; instead the pilot can specify the speed, altitude, and heading and the autopilot and autothrottle will maintain those commands. Flight management systems (FMS) assist in flight planning in addition to providing lateral and vertical control of the airplane. Many aircraft have special safety systems such as the Traffic Alert and Collision Avoidance System, which alerts the flight crew to potential collisions with other airborne aircraft, and the Terrain Avoidance Warning Systems (TAWS), which alert the flight crew to potential flight into terrain.
Causes of Congestion and Delays
Congestion and delays are caused by multiple reasons. These include tight scheduling, safety limitations on how quickly aircraft can take off and land and how closely they can fly, infrastructure limitations such as the number of runways at an airport and the airway structure, and disturbances such as weather and unscheduled maintenance.
Tight scheduling is a major contributor to congestion and delays. The hub and spoke system that many major airlines operate with to minimize connection times means that aircraft arrive and depart in multiple banks during the day. During the arrival and departure banks, airports are very busy. As mentioned previously, passengers have preferred times to travel, which also increase demand at certain times. At airports that do not limit flight schedules by using slot scheduling, the number of flights scheduled can actually exceed the departure and arrival capacity of the airport even in best-case conditions. One of the reasons that airlines are asked to report on-time statistics is to make the published airline schedules more reflective of the average time from departure to arrival, not the best-case time.
Aircraft themselves are also tightly scheduled. Aircraft are an expensive capital asset. Since customers are very sensitive to ticket prices, airlines need to have their aircraft flying as many hours as possible per day. Airlines also limit the number of spare aircraft and flight crews available to fill in when operations are disrupted to control costs.
Safety restrictions contribute to congestion. There is a limit to how quickly aircraft can take off from and land on a runway. Sometimes runways are used for both departing and arriving aircraft; at other times a runway may be used for departures only or arrivals only. Either way, the rule that controllers follow for safety is that only one aircraft can occupy the runway at one time. Thus, a landing aircraft must turn off of the runway before another aircraft can take off or land. This limitation and other limitations like the ability of controllers to manage the arrival and departure aircraft propagate backwards from the airport. Aircraft need to be spaced in an orderly flow and separated no closer than what can be supported by airport arrival rates. The backward propagation can go all the way to the departure airports and cause aircraft to be held on the ground as a means to regulate the traffic flow into a congested airport or through a congested air traffic sector.
There is a limit on how close aircraft can fly. Aircraft produce a wake that can be dangerous for other aircraft that are following too closely behind. Pilots are aware of this limitation and space safely when doing visual separation. Rules that controllers apply for separation take into account wake turbulence limitations, surveillance limitations, and limitations on how well aircraft can navigate and conform to the required speed, altitude, and heading.
The human is a safety limitation. Controllers and pilots are human. Being human, they have excellent reasoning capability. However, they are limited as to the number of tasks they can perform and are subject to fatigue. The rules and procedures in place to manage and fly aircraft take into account human limitations.
Infrastructure limitations contribute to congestion and delays. Airport capacity is one infrastructure limitation. The number of runways combined with the available aircraft gates and capacity to process passengers through the terminal limit the airport capacity.
The airspace itself is a limitation. The airspace where controllers provide separation services is divided into an orderly structure of airways. The airways are like one-way, one-lane roads in the sky. They are stacked at different altitudes, which are usually separated by either 1,000 ft. or 2,000 ft. The width of the airways depends on how well aircraft can navigate. In the US domestic airspace where there are regular navigation aids and direct surveillance of aircraft, the airways have a plus or minus 4 NM width. Over the ocean, airways may need to be separated laterally by as much as 120 NM since there are fewer navigation aids and aircraft are not under direct control but separated procedurally. The limited number of airways that the airspace can support limits available capacity.
The airways themselves have capacity limitations just as traditional roads do. There are special challenges for airways since aircraft need a minimum separation distance, aircraft cannot slow down to a stop, and airways do not allow passing. So, although it may look like there is a lot of space in which aircraft can fly, there are actually a limited number of routes between a city pair or over oceanic airspace.
The radio that is used for pilots and controllers to communicate is another infrastructure limitation. At busy airports, there is significant radio congestion and pilots may need to wait to get an instruction or response from a controller.
Weather is a significant disturbance in air traffic management. Weather acts negatively in many ways. Wet or icy pavement affects the braking ability of aircraft so they cannot vacate a runway as quickly as in dry conditions. Low cloud ceilings mean that all approaches must be instrument approaches rather than visual approaches, which also reduces runway arrival rates. Snow must be cleared from runways, closing them for some period of time. High winds can mean that certain approaches cannot be used because they are not safe. In extreme weather, an airport may need to close. Weather can block certain airways from use, requiring rerouting of aircraft. Rerouting increases demand on nearby airways, which may or may not have the required additional capacity, so the rerouting cascades on both sides of the weather.
Why Is Air Traffic Management Modernization So Hard?
Air traffic management modernization is difficult for financial and technical reasons. The air traffic management system operates around the clock. It cannot be taken down for a significant period of time without a major effect on commerce and the economy.
Financing is a significant challenge for air traffic management modernization. Governments worldwide are facing budgetary challenges and improvements to air travel are one of many competing financial interests. Local airport authorities have similar challenges in raising money for airport improvements. Airlines have competitive limitations on how much ticket prices can rise and therefore need to see a payback on investment in aircraft upgrades that can be as short as 2 years.
Another financial challenge is that the entity that needs to pay for the majority of an improvement may not be the entity that gets the majority of the benefit, at least near term. One example of this is the installation of ADS-B transmitters on aircraft. Buying and installing an ADS-B transmitter costs the aircraft owner money. It benefits the ANSPs, who can receive the transmissions and have them augment or replace expensive radar surveillance, but only if a large number of aircraft are equipped. Eventually the ANSP benefit will be seen by the aircraft operator through lower operating costs but it takes time. This is one reason that ADS-B transmitter equipage was mandated in the USA, Europe, and other parts of the world rather than letting market forces drive equipage.
All entities, whether governmental or private, need some sort of business case to justify investment, where it can be shown that the benefit of the improvement outweighs the cost. The same system complexity that makes congestion and delays in one region propagate throughout the system makes it a challenge to accurately estimate benefits. It is complicated to understand if an improvement in one part of the system will really help or just shift where the congestion points are. Decisions need to be made on what improvements are the best to invest in. For government entities, societal benefits can be as important as financial payback, and someone needs to decide whose interests are more important. For example, the people living around an airport might want longer arrival paths at night to minimize noise while air travelers and the airline want the airline to fly the most direct route into an airport. A combination of subject matter expertise and simulation can provide a starting point to estimate benefit, but often only operational deployment will provide realistic estimates.
It is a long process to develop new technologies and operational procedures even when the benefit is clear and financing is available. The typical development steps include describing the operational concept; developing new controllers procedures, pilot procedures, or phraseology if needed; performing a safety and performance analysis to determine high level requirements; performing simulations that at some point may include controllers or pilots; designing and building equipment that can include software, hardware, or both; and field testing or flight testing the new equipment. Typically, new ground tools are field tested in a shadow mode, where controllers can use the tool in a mock situation driven by real data before the tool is made fully operational. Flight testing is performed on aircraft that are flying with experimental certificates so that equipment can be tested and demonstrated prior to formal certification.
Avionics need to be certified before operational use to meet the rules established to ensure that a high safety standard is applied to air travel. To support certification, standards are developed. Frequently the standards are developed through international cooperation and through consensus decision-making that includes many different organizations such as ANSPs, airlines, aircraft manufacturers, avionics suppliers, pilot associations, controller associations, and more. This is a slow process but an important one, since it reduces development risk for avionics suppliers and helps ensure that equipment can be used worldwide.
Once new avionics or ground tools are available, it takes time for them to be deployed. For example, aircraft fleets are upgraded as aircraft come in for major maintenance rather than pulling them out of scheduled service. Flight crews need to be trained on new equipment before it can be used, and training takes time. Ground tools are typically deployed site by site, and the controllers also require training on new equipment and new procedures.
Promise for the Future
Despite the challenges and complexity of air traffic management, there is a path forward for significant improvement in both developed and developing air travel markets. Developing air travel markets in countries like China and India can improve air traffic management using procedures, tools, and technology that is already used in developed markets such as the USA and Europe. Emerging markets like China are willing to make significant investments in improving air traffic management by building new airports, expanding existing airports, changing controller procedures, and investing in controller tools. In developed markets, new procedures, tools, and technologies will need to be implemented. In some regions, mandates and financial incentives may play a part in enabling infrastructure and equipment changes that are not driven by the marketplace.
The USA and Europe are both supporting significant research, development, and implementation programs to support air traffic management modernization. In the USA, the FAA has a program known as NextGen, the Next Generation Air Transportation System. In Europe, the European Commission oversees a program known as SESAR, the Single European Sky Air Traffic Management Research, which is a joint effort between the European Union, EUROCONTROL, and industry partners. Both programs have substantial support and financing. Each program has organized its efforts differently but there are many similarities in the operational objectives and improvements being developed.
Airport capacity problems are being addressed in multiple ways. Controllers are being provided with advanced surface movement guidance and control systems that combine radar surveillance, ADS-B surveillance, and sensors installed at the airport with valued-added tools to assist with traffic control and alert controllers to potential collisions. Datalink communications between controllers and pilots will reduce radio-frequency congestion, reduce communication errors, and enable more complex communication. The USA and Europe have plans to develop a modernized datalink communication infrastructure between controllers and pilots that would include information like departure clearances and the taxiway route clearance. Aircraft on arrival to an airport will be controlled more precisely by equipping aircraft with capabilities such as the ability to fly to a required time of arrival and the ability to space with respect to another aircraft.
Domestic airspace congestion is being addressed in Europe by moving towards a single European sky where the ANSPs for the individual nations coordinate activities and airspace is structured not as 27 national regions but operated as larger blocks. Similar efforts are undergoing in the USA to improve the cooperation and coordination between the individual airspace sectors. In some countries, large blocks of airspace are reserved for special use by the military. In those countries, efforts are in place to have dynamic special use airspace that is reserved on an as-needed basis but otherwise available for civil use.
Oceanic airspace congestion is being addressed by leveraging the improved navigation performance of aircraft. Some route structures are available only to aircraft that can flight to a required navigation performance. These route structures have less required lateral separation, and thus more routes can be flown in the same airspace. Pilot tools that leverage ADS-B are allowing aircraft to make flight level changes with reduced separation and in the future are expected to allow pilots to do additional maneuvering that is restricted today, such as passing slower aircraft.
Weather cannot be controlled but efforts are underway to do better prediction and provide more accurate and timely information to pilots, controllers, and aircraft dispatchers at airlines. On-board radars that pilots use to divert around weather are adding more sophisticated processing algorithms to better differentiate hazardous weather. Future flight management systems will have the capability to include additional weather information. Datalinks between the air and the ground or between aircraft may be updated to include information from the on-board radar systems, allowing aircraft to act as local weather sensors. Improved weather information for pilots, controllers, and dispatchers improves flight planning and minimizes the necessary size of deviations around hazardous weather while retaining safety.
Weather is also addressed by providing aircraft and airports with equipment to improve airport access in reduced visibility. Ground-based augmentation systems installed at airports provide aircraft with the capability to do precision-based navigation for approaches to airports with low weather ceilings. Other technologies like enhanced vision and synthetic vision, which can be part of a combined vision system, provide the capability to land in poor visibility.
Air traffic management is a complex and interesting problem. The expected increase in air travel worldwide is driving a need for improvements to the existing system so that more passengers can be handled while at the same time reducing congestion and delays. Significant research and development efforts are underway worldwide to develop safe and effective solutions that include controller tools, pilot tools, aircraft avionics, infrastructure improvements, and new procedures. Despite the technical and financial challenges, many promising technologies and new procedures will be implemented in the near, mid-, and far term to support air traffic management modernization worldwide.