Introduction

I will be delving in the world of Air Traffic Management (ATM) and Control (ATC) in the near future1, and I will try to document my learning process in this blog.

'ATC is all the systems that contribute towards managing air traffic and the airspace that the air traffic flies in. The main purpose of ATC is to make sure that all this is done safely, efficiently and economically.'
- The International Civil Aviation Organization

Let’s break this down, to understand what this really means.

The different aspects of Air Traffic

The most well-known and obvious aspects of air traffic are those directly involved in the flights and the airports. These are the ones that we can see and hear, and the ones that we are most familiar with. We can classify them into three groups:

  • Air traffic service providers: these are the people that ensure the safety of the flights. They are the ones that control the air traffic, and they are the ones that provide the information to the pilots. They are the ones that we can hear in the radio.

  • Airspace management: this is the part that deals with the airspace. It is the part that defines the different airspaces, and the rules that apply to them. It is the part that defines the routes that the flights can take, and the rules that apply to them.

  • Air traffic flow management: is the regulation of air traffic in order to avoid exceeding airport or air traffic control capacity in handling traffic, and to ensure that available capacity is used efficiently.

But there are other aspects of air traffic that are not so obvious, and that are not so well-known. These are the ones that we cannot see or hear, and the ones that we are not so familiar with. For instance, there are several systems that are needed for the previous three groups to work properly. For instance, the most important ones are:

  • Communication systems: these are the systems that allow the different actors to communicate with each other. They are the ones that allow the pilots to communicate with the air traffic controllers, as well as the the air traffic controllers to communicate with each other.

  • Navigation systems: these are the systems that allow the pilots to know where they are. They allow the pilots to know the position and the direction of the aircraft.

  • Surveillance systems: these are the systems that allow the air traffic controllers to know where the aircraft are. They are the ones that allow the air traffic controllers to see the aircraft, and they are the ones that allow the air traffic controllers to know the position of the aircraft.

And even given all these systems, there are yet more important systems that aid the previous ones:

  • Meteorological systems: these are the systems that allow the air traffic controllers to know the weather conditions.

  • Aeronautical information systems: these are the systems that allow the air traffic controllers to know the information about the flights.

  • Rescue systems: these are special systems that are used when an aircraft goes missing or if there is a need for a rescue operation after an accident.

This whole ecosystem is what we call Air Navigation Services (ANS). It is the whole system that allows the aircraft to fly safely and efficiently. Air Traffic Control is the most visible part of Air Traffic Management and sits in the Air Traffic Service Providers group.

Why do we need ATC?

Let’s go through a few situations, so that we can understand the need for ATC.

  1. Picture an aircraft going from one airport to another. This aircraft is alone in the world. In this situation, there is no danger that the aircraft will collide with another plane.

  2. Now, imagine the same aircraft going from one airport to another. Just before takeoff, the pilot sees another plane approaching the airport. Should the pilot take off anyway? He would probably asks himself questions like:

    • What is the speed of the coming plane?
    • What is its exact direction?
    • Will it land in the same airstrip I need for takeoff?
    • Is this plane in some kind of trouble or emergency landing? Without an answer to these questions, taking off is a bet which could cost tons of money, materials and, the worst of all, lives.

If the pilots of the two aircraft were able to communicate with each other, they could probably come to an agreement, and decisions could be made safely.

  1. However, that is still a fictional scenario. Reality is thousands of aircrafts are going in and out hundreds of airports. It is not rare at all to have a situation in which 15 aircrafts want to land in the same airport, while another 6 are passing through the same aerial zone to go somewhere else. In addition, there might be a couple helicopters patrolling the area, as well as another 12 airplanes on the ground, in the wait to go to different places: some of them will take off soon, some others will park the plane in their designated area, or must be moved to the area where the hangars are.

Not only this, but the aircrafts are of different types. They can be big long haul passenger carrying aircrafts, or training aircrafts, or short hop commuter aircrafts.

A passenger aircraft
A big long haul passenger carrying aircraft2.

A training aircraft
A training aircraft3.

A commuter aircraft
A short hop commuter aircraft4.

This situation can rapidly become chaotic and it would be bold to think that the pilots could solve any possible issue, and coordinate with each other effectively in such a complex scenario, just by talking to one another.

There is the need for a mechanism that can ensure:

  • Planes don’t collide.
  • Planes arrive to where they want to go.
  • With the side objective to minimize the delay.

Note also that the problem is not just happening at the surroundings of the airports, but this kind of complex scenario can occur in the sky, where many aircrafts can be flying from one place to another, at speeds of more than 1000 km/h and with limited visibility.

A mechanism that can provide a solution for all these problems needs to have a big picture of what all aircrafts are doing, so that coordination can be done effectively and safely.

This immense responsibility lies on the shoulders of air traffic controllers. To this end, they must:

  1. Ensure that the aircrafts are safely separated from one another both in the air and the ground.
  2. Enable the aircrafts to meet their objectives, orderly, and with the least amount of delay possible.

In the characterization of air navigation services that we did before, we saw that the ATC is part of a wider system, the Air Traffic Services. Apart from ATC, this system is formed by the Flight Information System, the Advisory Services and the Alerting Services, which we will cover later. It is worth noting that an air traffic controller can provide one or more of these services at the same time, depending of where they work, how large the system is, how complex the air space is,…

Airspace Structures and Classification

The airspace is any specific three-dimensional portion of the atmosphere5. It can be understood in terms of substructures, and, at the most basic level, it can be divided into two categories:

  • Controlled airspace: The controlled airspace refers to the segments of airspace that are monitored and controlled by air system control systems.
  • Uncontrolled airspace: refers to the rest of the airspace, that is not controlled.

ATC takes several factors into account to work out where and what service needs to be provided:

  • Number of aircrafts using that segment of space.
  • The activities that these aircrafts are performing (landing, takeoff,…).
  • Types of aircrafts involved.
  • Weather conditions.
  • Terrain.
  • …

When ATC has determined where a control service is needed, the next step is to define how high, how wide, and what shape should those segment in the airspace be. Once this is done, these segments are designated as controlled airspace.

In the following figure, I depict a 2D analogy:

A 2D airspace definition example
A 2d airspace definition example.

In this analogy, we can see the Earth, and above it, the airspace. In the left, the airspace is completely uncontrolled, and in the right, we can see a possible definition for a controlled airspace, after all the considerations explained before are taken into account.

Now, how are these segments or chunks in real life? Around airports, the chunks usually start at ground level, and go up to 4000 ft (~1.2 km), but in some cases they can go up to 10000 ft (~3 km). This kind of airspace around an airport is called a control zone.

Sitting just in top of the control zone, there is another chunk of space, called a terminal control area. These areas may have more than one airport below it and it is in this kind of airspace where aircrafts are climbing out of the airports to get to their cruising levels, or descending to land and will be positioned into a landing sequence by ATC.

Joining the terminal control areas together, we find corridors of airspace, called airways. The airways are around 10 nautical miles (~18.5 km) wide.

The airways, together with the terminal control areas, are part of what is called a control area.

Sometimes, ATC recognizes that a very large segment of airspace needs to be controlled, and instead of having airways joining the different terminal control areas, they declare the entire upper airspace between the airports as control area.

No matter how these chunks of airspace are configured, they are all contained within a larger chunk of airspace: a flight information region. Sometimes, these regions are so big that a horizontal split is made, and the top part is called the upper information region, while the bottom part keeps the name flight information region. This split is made for practical reasons, so that the upper part can be managed by an ATC unit, and the lower part by another ATC unit.

ATS Routes

To help pilots and controllers identify how an aircraft plans to go from point A to point B, Air Traffic Service routes (ATS routes) are defined, joining together several locations. These are like the highways of the sky. There are hundreds of ATS routes defined, and states publish the structure of their airspace and the ATS in a document called the Aeronautical Information Publication, and they make it available in aeronautical charts.

Control services

At this point, we have defined the chunks of airspace that are controlled, but we still need to determine what kind of control the controllers will be providing to the aircrafts that are flying inside this airspace. To this end, we need to know the different ways to fly:

  • When students pilots first learn to fly, they navigate from one place to another by using a map and looking at the cockpit window, identifying landmarks along their route. They can only fly if the weather is good enough for them to see a fair distance out the window. This type of flying is known as flying under VFR (visual flight rules). It is not just student pilots who do this, but many general aviation pilots fly like this when the weather allows it.

  • When a pilot flies mainly using the navigational instruments in the aircraft, they are flying under IFR (instrument flight rules). Every airline pilot must be qualified to fly under these conditions.

Now that we know the different kinds of flight rules under which a flight can fly, we can delve into what services do controllers provide inside the controlled airspace chunks. To do this, a classification of each chunk is done:

  • Class A airspace: only IFR flights are allowed to fly in that chunk, and the controllers will make sure that they are all safely separated.
  • Class B airspace: both IFR and VFR flights are allowed into the airspace chunk, and they will all be separated from each other by ATC.
  • Class E airspace: both IFR and VFR flights are allowed into the airspace chunk, but only IFR flights will be separated from other IFR flights by ATC, while VFR flights must keep themselves separated from the rest of the flights. ATC will only tell IFR flights about VFR when they are near to them.
  • Uncontrolled airspace (Class G): in the uncontrolled airspace, aircrafts are allowed to fly VFR and IFR, but it is the responsibility of the flight crew to ensure their separation from other aircrafts. The Air Traffic Services does not (of course!) completely ignore them, and pilots are provided by information that might be useful for their safety, such as weather information, traffic in their surroundings that may interfere with them, or conditions at the airport in which they plan to land.

The following table6 summarizes the different airspace classes:

Class Type of Flight Separation Provided Service Provided
A IFR only All aircraft Air traffic control service
B IFR All aircraft Air traffic control service
  VFR All aircraft Air traffic control service
C IFR IFR from IFR, IFR from VFR Air traffic control service
  VFR VFR from IFR 1) Air traffic control service for separation from IFR 2) VFR/VFR traffic information service (and traffic avoidance advice on request)
D IFR IFR from IFR Air traffic control service, traffic information about VFR flights (and traffic avoidance advice on request)
  VFR Nil IFR/VFR and VFR/VFR traffic information (and traffic avoidance advice on request)
E IFR IFR from IFR Air traffic control service and, as far as practical traffic information about VFR flights
  VFR Nil Traffic information as far as practical
F IFR IFR from IFR as far as practical Air traffic advisory service; flight information service
  VFR Nil Flight information service
G IFR Nil Flight information service
  VFR Nil Flight information service

How does ATC work?

We have seen how that the airspace is divided into chunks, that are classified and somehow ensemble into a structure that is called the Air Traffic Services route network. We have also seen that the ATC provides different services depending on the airspace class. In addition to this structure, there are several systems that ATC needs to use to provide the services to the aircrafts. These systems are:

  • Navigational systems
  • Surveillance systems
  • Communication systems

In addition, there are several rules that ATC must follow to ensure the safety of the flights, and the basic rules are defined in the International Civil Aviation Organization (ICAO) with international agreements. These rules define things like the minimum separation between aircrafts, the minimum altitude that an aircraft must fly at, and the minimum visibility that must be present for a flight to take place, among others.

Nonetheless, all this does not explain how ATC works. To understand this, we need to know the different types of activities that ATC does:

  1. Controllers need to know in advance all the information regarding the flights that they want to control: source and destination, route, type of aircraft, etc. For this, pilots or the airlines operators must file a flight plan. This flight plan is sent to all the ATC units that will be involved in the flight, and it is used to coordinate the flights between the different ATC units. The flight plan is also used to coordinate the flights with the airports, so that the airport knows when to expect the flight, and the ATC knows when the flight will be arriving to the airport.

  2. Controllers must monitor the progress of the flights that they are controlling in real time. If the controller is in a tower, this activity is done by keeping visual contact with the aircrafts. At busier airports, this can be supported by a ground movement radar. Monitoring of airborne aircrafts is done by using a radar or other types of surveillance systems. The radar is used to determine the position of the aircraft, and the controller can use this information to ensure that the aircrafts are separated from each other. The radar can also be used to provide information to the pilots about the weather conditions in their surroundings. In some parts of the world, the radar is not available, and the controllers must rely on the information provided by the pilots, puting this information in a flight progress strip. This strip is a piece of paper that contains all the information about the flight, and it is used to keep track of the flight progress.

  3. Controllers have to use the information about where the aircraft is and where it intends to go, in order to work out if it will come into conflict with any other aircraft.

  4. Controllers issue clearances and instructions to pilots to ensure that the aircrafts are separated from each other. These clearances and instructions are sent to the pilots via the communication systems. For this activity, it is necessary that pilots and controllers remain in two-way communication, so that the pilots can ask for clarification if they do not understand the instructions, or if they need to change the instructions. This is primarily achieved by using radio communication.

  5. Controllers must coordinate with other ATC units to ensure that the aircrafts are safely transferred from one unit to the next. This is done by using the flight plan, and by using the communication systems.

Types of ATC

There are different types of ATC, and the type of ATC that is used depends on the type of airspace and the type of flight. We are going to see the different types of ATC, and the services that they provide.

Aerodrome Control

Those controllers that provide control services at an aerodrome or close to it are called Tower Controllers. This area is usually contained in a control zone, and the controllers make sure that the flow of traffic in and out of the aerodrome is safe and efficient, that aircrafts flying in the vicinity of the aerodrome are separated from each other, and that aircrafts are separated from vehicles and other obstacles on the ground.

At larger airports, the control tasks are split into:

  • Clearance Delivery: This controller is responsible for issuing the initial clearance to the pilots, and for coordinating the departure of the aircrafts with the aerodrome controllers.
  • Ground Control: This controller is responsible for the aircrafts that are on the ground, and for the vehicles that are moving on the ground.
  • Air Control: This controller is responsible for anything happening on the runways, and for the aircrafts that are in the vicinity of the aerodrome. These controllers will need to take into account the wake turbulence that is generated by the aircrafts, and that can be dangerous for other aircrafts. A smaller aircraft could be severely affected by the wake turbulence of a larger aircraft, and this is why the controllers need to keep the aircrafts separated from each other, to ensure that the turbulence dissipates before the next aircraft arrives. A possible solution we might think of is to let smaller aircrafts depart first, and then the larger ones. However, as larger aircrafts are faster, it would also be needed to ensure that the larger aircrafts do not catch up with the smaller ones, and this would be very inefficient.

Tower controllers work in a control tower inside the aerodrome, which provides 360º visibility of the aerodrome. The tower is usually located in a central position, so that the controllers can see all the runways and taxiways. The tower is also equipped with a ground movement radar. This radar is used to monitor the vehicles that are moving on the ground, and to ensure that they do not interfere with the aircrafts.

Recently, remote towers have been developed. These remote towers are equipped with cameras that provide a 360º view of the aerodrome, and the controllers can see the images in a screen.

Approach Control

The controllers that provide control services to departing and arriving airplanes in the vicinity of an aerodrome are called Approach Controllers. They usually work in terminal control areas or part of the control zone. Their job is to set up the arrival sequence for an aerodrome, before handing the aircrafts over to the aerodrome controllers. They also provide control services to aircrafts that are departing from the aerodrome, until they are handed over to the en-route controllers.

When they use a surveillance system, they are called approach surveillance controllers. When they do not use a surveillance system, they are called approach procedural controllers.

Their job is usually split into:

  • One controller manages the departing aircrafts.
  • One controller manages the arrival sequence.

Approach controllers use different techniques to set up the arrival sequences and to decide how far apart the aircraft should be on final approach. They need to take into account the performance of the aircrafts, including the speed, the anticipated amount of time they would take to get out of the runway, and the wake turbulence that they would generate. They also need to take into account the weather conditions, and the traffic that is arriving to the aerodrome from other directions.

Approach controllers may be assisted by planning controllers. These controllers are responsible for coordinating with all the other units that interface with the approach control unit.

Area Control

Those controllers that control aircrafts flying en route along airways or between airports are the Area Controllers. When an aircrafts leaves the terminal control area, the approach controller hands it over to the area controller. The area controller will then control it as it climbs to its cruising altitude, and will hand it over to the approach controller of the destination aerodrome.

They can also be classified as area procedural controllers or area surveillance controllers.

As the traffic demand has increased, large air spaces are divided into smaller sectors, and each sector is controlled by one controller. The divisions might be vertical, horizontal, or both. Error controllers have a broad view of the air traffic and they can predict when a conflict might occur with 10-20 minutes of anticipation. They can then take action to prevent the conflict from happening. For example, they can instruct an aircraft to climb or descend, or to change its speed. Oceanic controllers provide control services to aircrafts that are flying over the ocean, where there is limited communication, minimal surveillance and no ground-based navigation aids. They use a combination of surveillance and procedural control.

Flight Information Service

The Flight Information Service is a service that provides information and advice useful for the safe and efficient conduct of flights. It is provided by air controllers. They provide information about the weather, the state of the aerodrome, the condition of the runways, the traffic in the vicinity of the aerodrome, and any other information that might be useful for the pilots. In controlled areas, controllers provide both ATC and FIS services. In uncontrolled areas, they only provide FIS services.

Advisory Service

The Advisory Service is a service which is somehow a lighter version of ATC. It is provided by air traffic controllers, and it is used to provide advice and information useful for the safe and efficient conduct of flights. The controllers give advice on what actions the pilots should take, but the pilots are not obliged to follow the advice.

Alerting Service

The Alerting Service is a service that is provided by air traffic controllers, and that is used to notify appropriate organizations regarding aircrafts that are in need of search and rescue aid, and to assist such organizations as required. The controllers are responsible for notifying the appropriate organizations when an aircraft is overdue, or when it is known or believed to be in a state of emergency. They are also responsible for notifying the appropriate organizations when an aircraft is known or believed to be lost, or when it is known or believed to have been involved in an accident.

There are three types of alerts:

  • INCERFA: This alert is used when an aircraft is overdue, or when it is known or believed to be in a state of emergency. The controller will notify the appropriate organizations, and will provide them with all the information that is available. Usually, the aircraft is just now reporting to a different ATC unit and the alert is cancelled.
  • ALERFA: When an aircraft is not found after a certain amount of time, the INCERFA alert is upgraded to an ALERFA alert. The controller will notify the appropriate organizations, and will provide them with all the information that is available.
  • DETRESFA: When an aircraft is not found after a certain amount of time or information is received that indicates that the aircraft is in distress, the ALERFA alert is upgraded to a DETRESFA alert. The controller will notify the appropriate organizations, and will provide them with all the information that is available.

Airspace Management

There are many different types of airspace users, like commercial airlines, business jets, cargo operators, private pilots, and others including balloons, drones, and sport para-dropping. All these users need to have a safe place to do their activities. Airspace management is about making sure everyone can use the airspace safely.

There is also the military, which is another major airspace user. In the past, the military had their own airspace that civilians couldn’t fly in. This meant that civilian aircraft had to take long detours around the military airspace, even when there was nobody using it. But now things have changed. The military allows civilians to use their airspace when they are not using it themselves. This is called flexible use of airspace.

The Flexible Use of Airspace (FUA) considers airspace a single continuum, and it allows the airspace to be used by both civil and military users. FUA is based on user’s requirements and adaptable airspace structures and procedures, suited to temporary allocation and utilisation. It realies on National Airspace Management Cells (NAMCs), which are responsible for the management of the airspace in their country. They are responsible for the coordination of the use of airspace, and for the allocation of airspace to civil and military users. They are also responsible for the coordination of the use of airspace with other countries. Advanced FUA will further enhance both FUA operations and network performance by introducing new airspace design principles, real-time information sharing, common situational awareness, and collaborative decision making, as well as interoperability between civil and military systems.

Air Traffic Flow Management

The objectives of ATFM are to:

  • Protect ATC from over-delivery of traffic.
  • Optimize the utilization of the available capacity of the ATC system.

Here, capacity is the number of flights that can be handled safely and efficiently in a sector during a given time period, while the demand is the number of flights that intend to fly in a sector during a given time period. Therefore, ATFM is about making sure that the demand doesn’t exceed the capacity. If the demand exceeds the capacity, then the ATC system will be overloaded, and this will lead to delays and congestion.

Air Traffic Flow Management is done in three phases:

  1. Strategic planning: This happens months ahead of time. It involves things like scheduling flights, changing flight paths to avoid crowded areas, and predicting where and when problems might occur.

  2. Pre-tactical planning: This is done a few days before the actual operation. It’s like fine-tuning the original plan and deciding what specific actions will be taken on the day of operation.

  3. Tactical operations: This takes place on the day of operation. Actions are adjusted in real-time based on the traffic situation. If needed, delays or restrictions are put in place to make sure the airspace doesn’t get too crowded and all aircraft stay safe.

Flight Plan

A Flight Plan is a document that contains information about a flight. It is used by air traffic controllers to provide air traffic services. It is also used by pilots to plan their flights. The flight plan contains information about the aircraft, the pilot, the route, the destination, and other information that is useful for the flight. The flight plan is filed by the pilot well before the flight. The format of the flight plans is defined by the International Civil Aviation Organization (ICAO). Flight Plans are usually compulsory for flights operating under IFR and for flights operating in controlled airspace. They can be optional for flights operating under VFR and for flights operating in uncontrolled airspace. If the flight crosses international borders, then the flight plan is compulsory in all cases, as well as if the flight crew wants the ATC to report them overdue if they don’t arrive at their destination within a certain amount of time.

Global Challenges of Air Traffic Management

Global air traffic has been growing rapidly, doubling in size roughly every 15 years since 1977. This growth continues despite economic downturns. As air traffic increases, air traffic management (ATM) faces several challenges:

  1. Safety: ATM prioritizes safety and has a strong safety record. However, with more diverse traffic and new users, extra efforts are needed to maintain safety standards.

  2. Efficiency/Capacity: The demand for air travel is exceeding the available capacity, leading to congestion and delays that slow down traffic flow.

  3. Cost Reduction and Economic Growth: There is pressure on ATM to reduce costs for airspace users. The challenge is to create a globally viable air traffic management system that considers the varying economic development levels worldwide, while promoting sustainable growth and improvements.

  4. Security: ATM systems rely on networks to share information, which improves efficiency and decision-making. However, sharing information also increases the risk to network security and the aviation community it serves. The challenge is to ensure an acceptable level of security while sharing networked information.

  5. Environment: Aviation currently contributes to adverse environmental effects. The global challenge for ATM is to minimize the negative impacts caused by civil aviation activities and work towards environmental sustainability.

Functional Airspace Blocks

One solution to many of the previously mentioned problems in Europe is being attacked using the concept of functional airspace block (FAB)7. FABs are a key mechanism of the Single European Sky (SES) initiative, which aims to reorganize air traffic management (ATM) in the European Union (EU) in a more efficient and rational manner. FABs are established regardless of state boundaries and are designed to be performance-driven and optimized.

The problem FABs aim to solve is the fragmentation of air traffic management in the EU. Currently, every time a plane enters the airspace of a Member State, it is serviced by a different air navigation service provider (ANSP) based on different rules and operational requirements. Each service provider procures tailored equipment and most maintain their own training schools and all other support functions. This fragmentation impacts safety, limits capacity, and adds to cost.

FABs are important because they are designed to improve capacity and efficiency, enhance safety, and lower costs for air navigation services through enhanced cooperation and integration across borders. They are expected to change the landscape of ATM service provision and provide an invaluable tool for ANSPs in reaching binding performance targets. They also facilitate civil-military coordination in airspace and air traffic management.

FABs are also a tool to develop a Single European Sky. They are vital for reducing airspace fragmentation and are necessary to accommodate the steadily growing traffic, as well as to minimize delays by managing the traffic more dynamically. Objectives for enhancing current safety standards and overall efficiency can best be achieved by increasing the scale of operations, regardless of national borders.

As of the time of the article, all nine FABs have been declared, established, and notified to the European Commission. However, the implementation is still far too slow for almost all FABs. Delays in delivering operational FABs are holding back the implementation to a significant degree, which in turn generates inefficiencies in the entire European air traffic management system. This results in extra costs of close to € 5 billion a year which are passed on to airlines and their customers in addition to increased journey times, delays, and emissions.

Conclusion

In this post, we have explored the world of Air Traffic Management, focusing on Air Traffic Control. We have seen how the airspace is divided into controlled and uncontrolled zones, and how the different controlled zones are classified based on the type of traffic they handle. We have also seen how the Air Traffic Control system works, and how the different types of controllers work together to ensure the safety of the aircraft and the passengers. Finally, we have seen how Air Traffic Flow Management is done, and how it is used to make sure that the demand doesn’t exceed the capacity of the ATC system.

Moreover, we have addressed the global challenges of Air Traffic Management, and how the concept of Functional Airspace Blocks is being used to solve some of these problems in Europe.

I hope you have enjoyed this post and that you have learned something new. If you have any questions or comments, feel free to reach out to me. I would love to hear from you!

References

  1. I will start this journey via the free courses delivered by Eurocontrol Learning Zone. ↩

  2. Passenger Aircraft Image: source ↩

  3. Training Aircraft Image: source ↩

  4. Commuter Aircraft Image: source ↩

  5. Visit Airspace in Wikipedia. ↩

  6. Table adapted from this blog in SkyBrary ↩

  7. See the Eurocontrol post about Functional Airspace Blocks ↩