8 Air navigation services
Merriam-Webster defines the term airspace as “the space lying above the earth or above a certain area of land or water”. According to the Chicago Convention on International Civil Aviation of 1944, every state “has complete and exclusive sovereignty over the airspace above its territory”. Horizontally, a state’s airspace extends over its entire territoriy, which also include territorial waters. According to the United Nations Convention on the Law of the Sea, territorial waters extend up to 12 nautical miles (22.2 km) from a state’s coastline. Vertically, a state’s airspace begins at the earth’s surface and ends at an altitude of 100 km above mean sea level, which is also known as the Kármán-Line. The Kármán-Line - as a legal differentiation - separates the Earth’s atmosphere from space.
According to the Chicago Convention and its subsidary framework, next to the soverignty clause, each State has the responsibility to establish an Air Navigation Service (ANS) under Article 28(a) of the convention. For this purpose, States will introduce an airspace organisation , associated rules of the air, and assign the provision of these services to appropriate entities.
In order to manage an airspace, a state usually commissions a so-called Air Navigation Service Provider (ANSP), which is a public or private entity that offers so-called air navigation services.
Air navigation services facilitate the ‘safe, efficient, and orderly flow of air traffic’ and comprise of the following five groups of services:
- Aeronautical Information Management (AIM)
- Air Traffic Management (ATM)
- Meteorological Services (MET)
- Communication, Navigation, Surveillance (CNS)
- Search and Rescue (SAR)
For the remainder of this section, the components of air navigation services are described in more detail.
8.1 Aeronautical Information Management
Aeronautical Information Management (AIM) is considered with the collection, assembly, publication, exchange, and dissemination of quality-assured, timely and digital aeronautical data in collaboration with all relevant stakeholders. Aeronautical data includes aeronautical information publications (AIP), notice to airmen (NOTAM), preflight information bulletins (PIB), and aeronautical information circulars (AIC).
Aeronautical Information Publications (AIP), which contain “aeronautical information of a lasting character essential to air navigation”, see ICAO Annex 15, are issued by a state or by an authority of the state, such as an ANSP. Since their structure is standardized by ICAO Annex 15, AIP usually consist of three distinctive parts:
- GEN - General information: As the name implies, GEN contains general information, such as the authority responsible for the AIP, locally applied units of measure, abbreviations used in the AIP, locally applicable time zones and daylight saving time regulations, airspace charges regulations, etc.
- ENR - Information referring to en-route: ENR contains information that is of importance during the flight, i.e., while aircraft are en-route. ENR is divided into the following 6 parts: (i) information on general rules and procedures for visual flight (VFR) and instrument flight (IFR) or the airspace classes applied, (ii) description of the vertical and horizontal spread of airspaces, (iii) description of airways, (iv) description of radio navigation systems and infrastructures, (v) navigation warnings such as military airspaces, known aviation obstacles, danger areas, etc., and (vi) en-route chart material.
- AD - Information referring to aerodromes: AD includes information on both aerodromes and heliports and is divided into 3 parts. The first part contains an index of all aerodromes and helipads of a state as well as a description of the classification system of aerodromes applied in the state. Parts 2 and 3 of AD include detailed information on individual aerodromes or heliports, such as opening times, aerodrome operators, available service facilities, dimensions and alignment of runways, runway and approach lights, relevant aeronautical obstructions, etc. detailed maps are also provided, which contain information on ground-based infrastructure, i.e., aprons, taxiway, runways, etc., as well as flight procedures, i.e., standard instrument departures, standard terminal arrival routes, approach procedures.
To keep AIPs up to date, they are revised in an internationally standardized cylce, which is known as the AIRAC cycle. This AIRAC cycle follows a fixed 28-day rhythm, which is internationally standardised. The publication dates are known years in advance and are published, for example, by Eurocontrol.
Notice to airmen (NOTAM) A NOTAM contains information that is of importance to personnel involved in flight operations. While AIPs tend to contain static information that is valid over long periods of time and is known well in advance, NOTAMs include information that is dynamic in nature as it is not known far enough in advance to be published in any other way. Indeed, according to ICAO Annex 15, NOTAMs contain “information concerning the establishment, condition or change in any aeronautical facility, service, procedure or hazard…”.
Prefilght information bulletins (PIB) A PIB consists of a compilation of a number of NOTAMs which are important for the execution of a flight. Consequently, a PIB contains NOTAMs for the origin, destination and alternate airports, as well as NOTAMs for the airspace that the flight is likely to use.
Aeronautical information circulars (AIC) ICAO Annex 15 defines an AIC as “a notice containing information that does not qualify for the origination of a NOTAM of for inclusion in the AIP, but which relates to flight safety , air navigation, technical, administrative or legislative matters.” For example, AICs are issued to communicate a strategy in which a state explains how it aims to modernise its ground-based navigational aids, or how a certain new procedure will be implemented in the future.
8.2 Air Traffic Management
The term air traffic management (ATM) is defined in ICAO Document 4444 as “the aggregation of the airborne functions and ground-based functions […] required to ensure the safe and efficient movement of aircraft during all phases of operations”, which, in practice, is enabled in practice by the following services: (i) air traffic services, (ii) airspace management, and (iii) air traffic flow management.
8.2.1 Air Traffic Services
Air Traffic Services (ATS) is concerned with the control, regulation, and assistance of aircraft and flight crews in real time. To this end, ATS has the following four main objectives:
- The avoidance of collisions between aircraft.
- The provision of advice for the safe and efficient conduction of flights.
- The conduction and maintainance of an orderly flow of air traffic.
- The notification and assistance of concerned organisations in case of search and rescue operations.
To achieve these main objectives, ATS provides the following main services: (i) air traffic control (ATC), (ii) air traffic advisory services, (iii) flight information services, and (iv) alerting services.
Air Traffic Control (ATC): According to ICAO Annex 11, ATC is a service, which aims at the prevention of collisions between aircraft being airborne as well as between obstructions and aircraft being on the maneouvring area of an aerodrome. Besides that, ATC ensures the expedition and maintenance of an orderly flow of air traffic.
operational limits –> rules of the air to operate in such airspaces –> ultimately requiring pilot/aircrews to operate a respectively equipped aircraft in accordance with these flight rules and associated meteorological condistions, i.e. IMC or VMC –> To ensure that the airspace is managed efficiently and effectively by ATC, flights are categorized and segmented according to the flight rules they are following. Basically, flights are thereby either following so-called visual flight rules (VFR) or instrument flight rules (IFR). As the name implies, VFR flights follow the principle of “see and avoid”. That is, by looking out of the window, flight crew of VFR flights are responsible for ensuring that they are always sufficiently spatially separated from other flights and obstacles. Therefore, VFR flights can only be conducted when meteorological conditions permit. For instance, VFR flights can only operate in an airspace when the visibility is better than a certain minimum value. Moreover, pilots of VFR flights are prohibited from flying closer than a certain distance to clouds, which implies requirements regarding clouds. These condistions are referred to as Visual Meteorological Conditions (VMC). In contrast, flight crew operating under IFR are not obliged to look out of their cockpit windows in order to “see and avoid” other flights and obstalce. Rather, IFR flights are separated from other aircraft and obstacles by the ATC or procedurally (e.g. departure procedure ensuring freedom of obstacles when departing from an aerodrome). Therefore, the aforementioned meteorological requirements in terms of visibility and distance to clouds do not exist.
The associated consditions are referred to as Instrument Meteorological Conditions (IMC). With the exemption of so-called special VFR (SVFR)* operations, it follows that when the VMC are not met, flights operate in IMC.
Besides differentiating between VFR and IFR flights, the airspace managed by ATC is segmented into a number of smaller sub-segments or airspace volumes to ensure safe and expeditious flow of air traffic. These airspace volumes are further assigned a specific airspace class, which defines (i) which types of flight may use the airspace, (ii) under which circumstances these flights may enter this airspace, and (iii) which services are offered to aircraft and/or aircraft crews being present in this airspace block.
In [ICAO Annex 11][Annex_11, a total of seven distinct airspace classes, called Class A to Class G, are defined. In a first distinction, a distinction can be made between controlled and uncontrolled airspace, which, as indicated in Table XX, refers to Class A to E, and Class F to G, respectively. In this regard, the term controlled airspace indicates that ATC offers and provides services to aircraft being present in this airspace block, while no services are provided uncontrolled airspace blocks.
Besides that, airspace classes differ in they type of flight allowed to enter and operate therein. While IFR flights are allowed to operate in all airspace classes, VFR flights are prohibited from using airspace Class A. Moreover, flights require an ATC clearance to enter certain airspace classes. If a clearance is required, pilots must contact ATC and request clearance before entering the airspace. As such, IFR and (if applicable) VFR flights require an ATC clearance for airspace Classes, A, B, C, and D. In airspace Class E, only IFR aircraft require a clearance, while VFR flights are exempt from this obligation. Please note: Aircraft operating as a special VFR flight1, which refers to aircraft conducting VFR flights under lower weather minima than those of conventional VFR flights.
| Airspace Class | IFR-Flights | VFR-Flights | ATC-Clearance for IFR-Flights | ATC-Clearnace for VFR-Flights |
|---|---|---|---|---|
| A | Allowed | Prohibited | Required | n/a |
| B | Allowed | Allowed | Required | Required |
| C | Allowed | Allowed | Required | Required |
| D | Allowed | Allowed | Required | Required |
| E | Allowed | Allowed | Required | Only for SVFR-flights required |
| F | Allowed | Allowed | Not required | Not required |
| G | Allowed | Allowed | Not required | Not required |
The above mentioned airspace classes are valid in all states that have ratified the Chicago Convention on International Civil Aviation. This does not mean, however, that every airspace class is applied in practice by every State. Rather, each state is free to decide on the structure of its entire airspace and the categorisation of the resulting airspace blocks. For example, Italy applies airspace classes A, C, D, E and G, while Germany, Austria and Switzerland apply airspace classes C, D, E and G.
To prevent collisions between aircraft, ATC uses so-called separation minima. As such, the term separation refers to the vertical and lateral spatial spacing between aircraft. As a target for separations, ATC uses specified minimum separations, which define exactly which vertical and lateral minimum distances must be maintained at all times. In practice, not all aircraft are separated from each other, as separations are only (i) provided in certain airspace classes and (ii) not offered to all flights. With regard to flights, a distinction is made between aircraft operating under Instrument Flight Rules (IFR) and aircraft operating under Visual Flight Rules (VFR). The following table summarises for all airspace classes and types of flight, whether or not separation is provided by ATC. Note, the table must be read as follows: the first column indicates in which airspace class a particular flight is located. The second column indicates whether this flight is operating under IFR or VFR. The third column then indicates which separation services are offered to this flight.
| Airspace Class | Flight Rules | Separation provided to this flight |
|---|---|---|
| A | Only IFR flights allowed | Between all IFR flights |
| B | IFR, VFR | Between all other flights |
| C | IFR | Between all other flights |
| C | VFR | Between other IFR flights |
| D | IFR | Between other IFR flights |
| D | VFR | No separations |
| E | IFR | Between other IFR flights |
| E | VFR | No separations |
| F | IFR | Between other IFR flights, as far as practicable |
| F | VFR | No separations |
| G | IFR, VFR | No separations |
A key aspect for the application of separation minima is the supporting infrastructure in terms of navigation and surveillance. For surveillance purposes, ATS use radar - both primary and secondary . The primary driver for the associated minimum is linked to the type of radar and its technological characteristics.
In certain airspaces only long-range radars are used. Here typical separation minima range from 10 to 20 Nautical Miles (NM) Within the European core-area, with a sufficient highly coverage of a mix of primary and secondary radars, the separation minima is 5 NM unless wake-turbulence categorisation requires a wider lateral spacing.
Closer to aerodromes radars with a higher revolution are dployed. This increases the accuracy of measurement within the arrival and departure airspace. This allows for a reduction of the separation minima. Within Europe and the United States, the required minimum is 3NM unless wake-turbublence categorisation requires a wider separation distance.
Within the close proximity of airports and given the deployment of so-called precision radars, the separation minima can be further reduced to 2 1/2 or 2 NM.
Advisory services
Flight Information Services
Alerting services
According to ICAO Annex 11 - Air Traffic Services - alerting services refers the notification of the appropriate organisations about aircraft in need of search and rescue or assist such organisations as required.
Alerting services are part of air traffic services (ATS) and typically provided by the ATS units involved. Accordingly, this service applies to all aircraft under the control of an ATS unit, or processing the associated flight plan and therefor know to the unit or in support of adjacent units, or information about aircraft - known or believed to be - subject to unlawful interference.
8.2.2 Airspace Management
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8.2.3 Air Traffic Flow Management
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8.3 Meteorological Services (MET)
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8.5 Search and Rescue (SAR)
List of Acronyms
- ANS
- Air Navigation Service
- ANSP
- Air Navigation Service Provider
Special VFR (SVFR) flights are only allowed in controlled airspace, in which (i) the minimum visibility must be at least 1500m, (ii) ground must be visible at all times, and (iii) aircraft must be clear of clouds. Aircraft operating SVFR flights must be equipped as if they conducted an IFR flight.↩︎