ANNEX 11 to the Convention on International Civil Aviation
Air Traffic Services

Control of air traffic was almost unknown in 1944. Today, air traffic control, flight information and alerting services, which together comprise air traffic services, rank high among the indispensable ground support facilities which ensure the safety and efficient operation of air traffic throughout the world. Annex 11 to the Chicago Convention defines air traffic services and specifies the worldwide Standards and Recommended Practices applicable in the provision of these services.

The world's airspace is divided into a series of contiguous flight information regions (FIRs) within which air traffic services are provided. In some cases, the flight information regions cover large oceanic areas with relatively low air traffic density, within which only flight information service and alerting service are provided. In other flight information regions, large portions of the airspace are controlled airspace within which air traffic control service is provided in addition to flight information and alerting services.

The prime objective of air traffic services, as defined in the Annex, is to prevent collisions between aircraft, whether taxiing on the maneuvering area, taking off, landing, en route or in the holding pattern at the destination aerodrome. The Annex also deals with ways of expediting and maintaining an orderly flow of air traffic and of providing advice and information for the safe and efficient conduct of flights and alerting service for aircraft in distress. To meet these objectives, ICAO provisions call for the establishment of flight information centres and air traffic control units.

All aircraft fly in accordance with either instrument flight rules (IFR) or visual flight rules (VFR). Under IFR, the aircraft fly from one radio aid to the next or by reference to self-contained airborne navigation equipment from which the pilot can determine the aircraft's position at all times. IFR flights are conducted through all but the severest of weather conditions, while aircraft flying under VFR must remain clear of cloud and operate in visibility conditions which will permit the pilot to see and avoid other aircraft. Chapter 3 specifies the types of service to be provided to these flights - for example, IFR flights are provided with air traffic control service when operating in controlled airspace. When operating in uncontrolled airspace, flight information service, which includes known traffic information, is provided and the pilot is responsible for arranging the flight to avoid other traffic. Control service is normally not provided to VFR flights, unless in specific areas, in which case VFR flights are separated from IFR flights but no separation service is provided between VFR flights, unless specifically required by the ATC authority. However, not all aircraft are provided with air traffic services. If an aircraft is operating entirely outside of controlled airspace in an area where a flight plan is not required, the flight may not even be known to air traffic services.

Safety is the overriding concern of international civil aviation and air traffic management contributes substantially to safety in aviation. Annex 11 contains an important requirement for States to implement systematic and appropriate air traffic services (ATS) safety management programmes to ensure that safety is maintained in the provision of ATS within airspaces and at aerodromes. Safety management systems and programmes will serve as an important contribution toward ensuring safety in international civil aviation.

Air traffic control service consists of clearances and information issued by air traffic control units to achieve longitudinal, vertical or lateral separation between aircraft, in accordance with the provisions set out in Chapter 3 of the Annex. This chapter also deals with the contents of clearances, their coordination between ATC units and the co-ordination of transfer of responsibility for control as a flight progresses from the area of one control unit to another. An orderly transfer process requires that an aircraft must be under the control of only one air traffic control unit at any one time.

Air traffic control units are sometimes faced with a traffic demand beyond the capacity of a particular location or area, as occurs at busy aerodromes during peak periods.

Annex 11 provides for ATC units to specify restrictions to the traffic flow, when required, for the purpose of avoiding excessive delays to aircraft in flight.

Annex 11 also specifies the requirements for coordination between the civil air traffic control units and military authorities or other agencies responsible for activities that may affect flights of civil aircraft. Military units are provided with flight plan and other data concerning flights of civil aircraft to assist in establishing identification in the event that a civil aircraft approaches or enters a restricted area.

Flight information service is provided to aircraft operating in controlled airspace and to others known to the air traffic services units. The information includes significant meteorological (SIGMET) information, changes in the serviceability of navigation aids and in the condition of aerodromes and associated facilities and any other information likely to affect safety. IFR flights receive, in addition, information on weather conditions at departure, destination and alternate aerodromes, collision hazards to aircraft operating outside of control areas and control zones and, for flight over water, available information on surface vessels. VFR flights also receive information on weather conditions which would make visual flight impractical. Annex 11 also contains specifications for operational flight information service (OFIS) broadcasts, including automated terminal information service (ATIS) broadcasts.

Chapter 5 of Annex 11 is concerned with the alerting service, which provides for the alerting of rescue coordination centres when an aircraft is believed or known to be in a state of emergency, when it fails to communicate or to arrive on time or when information is received that a forced landing has been made or is imminent. Alerting service is automatically provided to all aircraft receiving air traffic control service and, as far as is practicable, to all other aircraft whose pilots have filed a flight plan or are otherwise known to air traffic services. It is also provided to aircraft known or believed to be subject to unlawful interference. The effect of the alerting service is to set in motion all appropriate rescue and emergency organizations which can provide assistance when and where required.

Subsequent chapters of the Annex cover ATS requirements for air-ground communications and for communications between ATS units and between those units and other essential offices. These chapters also specify the information required to be supplied to each type of air traffic services unit. Air-ground communications should permit direct, rapid and continuous static-free two-way radiotelephony communication, whenever practicable, while those between ATS units should permit exchange of printed messages and, in the case of air traffic control units, direct voice communications between controllers. Because of the importance of the information transmitted over air-ground radio channels and that received from other units and offices, Annex 11 recommends that such communications should be recorded.

An Appendix to the Annex spells out the principles governing the identification of air traffic services routes to allow both pilots and ATS to make unmistakable reference to any route without resorting to geographical references. Another Appendix specifies the requirements for designators for significant points marked by a radio aid as well as those not marked by a radio aid. Annex 11 also contains a series of attachments with guidance material on a variety of subjects, from airspace organization to ATS requirements for air-ground channels to the establishment and naming of standard arrival and departure routes.

Contingency planning is an important responsibility of all States that provide air navigation services. An Attachment to Annex 11 contains concise guidance to assist States in providing for the safe and orderly flow of international air traffic in the event of disruptions of air traffic services and related supporting services and in preserving the availability of major world air routes in the event of disruptions.

The sky may be limitless but not for air traffic. As more aircraft fill the crowded air routes, air traffic control concepts, procedures, equipment and rules will continue to evolve as will the provisions of this Annex.