ICAO 24 bit Aircraft Addresses

The Civil Aviation Authority is responsible for the management and assignment of 24-bit Aircraft Addresses in the UK. This page gives information on how we achieve this.

24-bit Aircraft Addresses

The provision of Air Traffic Services (ATS) in an SSR Mode S environment relies on a unique ICAO 24-bit Aircraft Address (24-bit AA) for selective interrogation of individual aircraft.  The 24-bit AA is also an essential element of the airborne collision and avoidance system, ACAS II. 

Civil Aircraft

An ICAO 24-bit aircraft address is automatically allocated to every UK registered aircraft as part of the process to register the aircraft regardless of whether the individual aircraft is fitted with a Mode S transponder.  Applicants will be advised of their allocated aircraft address in accordance with the Aircraft Registration service standards, following receipt of the application to register their aircraft.  For details of how to  register an aircraft please refer to the Register an Aircraft page of the Aircraft Registration section of the CAA website.

Automatic allocation of ICAO 24-bit aircraft addresses simplifies administration and saves the operator from having to make separate applications to the Applications & Approvals Department solely for the assignment of addresses. 

All aircraft currently registered that have not previously requested the issue of an address have now had an address assigned to them.  Details of allocated addresses are available via the G-INFO UK Register database.

If an operator needs an ICAO 24-bit aircraft address for a civil aircraft in the UK that is not required to be UK registered (such as Annex II gliders operated through the British Gliding Association scheme) please contact the Aircraft Registration Section for further details and include details of the aircraft concerned.

020 7453 6666 – tel
020 7453 6670 – fax         

State Aircraft/Other Uses

The CAA is responsible for the policy and overall management of 24-bit AAs and also deals with all other requests for 24-bit AAs, such as:

  • Assignment to State aircraft
  • SSR interrogator site monitors (Far Field Monitors and Ground-based transponders)
  • Multilateration systems (airfield and off-shore)
  • Satellite Communications Base Stations (Inmarsat)
  • Test & Development (T&D)

All requests for 24-bit AAs in the UK that are NOT for civil aircraft should be addressed to:

020 7453 6536 – tel
020 7453 6565 – fax

24-bit AAs and Aircraft Identification Reporting

There are strict requirements regarding what information is entered in to a transponder and broadcast as aircraft identification (Ac ID), sometimes referred to as Flight Identity (Flt ID).  The correct setting of Ac ID is essential to ensure safety and for the correlation of radar tracks with flight plan data in ATM and Airport operator ground systems.  Aeronautical Information Circular AIC 101/2007 (Yellow 250) details how 24-bit AAs must be entered/hardwired into an aircraft transponder and what information and in what format must be entered as Ac ID/Flt ID in the transponder.  This information is also provided in the En Route (ENR) section, paragraph 1.6.2, of the UK Aeronautical Information Publication (AIP).

Emergency Locator Transmitters (ELTs)

24-bit AAs assigned to civil aircraft for use with a Mode S transponder may also be used in aircraft Emergency Locator Transmitter (ELT) systems.  Further details are available in AIC 57/2003 (Pink 55). However, it should be noted that operators who choose to use their Mode S transponder 24-bit AA as the unique identification for their ELT are still required to register it separately with the Maritime and Coastguard Agency in accordance with the AIC:

01326 211569 – tel
01326 319264 – fax