Definition

Aircraft address. A unique combination of 24 bits available for assignment to an aircraft for the purpose of air-ground communications, navigation and surveillance.

Source: ICAO Annex 10: Aeronautical communications

Description

The 24-bit aircraft address is a unique identifier of a particular aircraft. It is normally expressed as an alphanumerical code, consisting of six hexadecimal characters (each hexadecimal character represents 4 bits). In theory, 24 bits allow for 16 777 214 unique identifiers. The address is only used for identification and does not provide any specific information such as e.g. aircraft type, operator, etc.

At any given time, a particular address can only be assigned to a single aircraft and any aircraft can be assigned only one address. If a removable transponder is shared between several aircraft, it must have the capability to update the address and this must be done every time the equipment is installed in a different aircraft. The address can be changed but only under exceptional circumstances but in any case never during a flight.

Aircraft adresses are assigned by the ICAO member states as stipulated in Annex 10, Volume III, Part I, Appendix to Chapter 9. The first 4/6/9/12/14 bits of the aircraft address represent the member state and the remaining 20/18/15/12/10 are the number within that state. A State should choose the method used to assign addresses in a way that ensures efficient use of the entire address block allocated to that State. A request for additional aircraft addresses can be made when at least 75% of the allocated addresses have been assigned to aircraft. When an aircraft changes its state of registry, the new state assigns an address from its own address block. The old address (from the previous state) is released for re-use.

Aircraft addresses are used in applications which require the routing of information to or from individual aircraft such as:

• Aeronautical telecommunication network (ATN) (the CPDLC implementation used in e.g. the European Union)
• Mode S
• Airborne collision avoidance system (ACAS)

The 24-bit aircraft address is filled in during flight plan completion in order to link the aircraft to a particular flight. The information is entered in field 18 (other information), under the "CODE/" designator (e.g. "CODE/F00001"). This step is crucial for the correct operation of ATN as it ensures the uplinked clearances are received by the intended aircraft.

Aircraft are not the only entities that are given 24-bit addresses. Transponders installed on aerodrome vehicles, obstacles or fixed Mode S target detection devices for surveillance and/or radar monitoring purposes are also assigned such identifiers. Under such specific conditions, the meaning of the term "aircraft" is expanded to include "aircraft (or pseudo-aircraft) or vehicle (A/V)". It is possible to have the same address assigned to more than one vehicle provided that these operate at aerodromes spaced by at least 1 000 km.

Further Reading

• ICAO Annex 10, Vol. III, Part I, Chapter 9
• Guidelines for ICAO 24 Bits Adresses Assignment