The 24 bit Mode S address is issued by the national CAA where the aircraft is registered.
If it is sold to another country, with another CAA, is the Mode S address maintained or is a new one assigned?
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A new Mode S address will have to be assigned by the new authority:
Aircraft addresses will be assigned in keeping with the following principles:
- [...] when an aircraft changes its State of Registry, the previously assigned address shall be relinquished and the new registering authority shall assign it a new address; [...]
Until the new authority has assigned a new address, ICAO may assign a temporary address to the aircraft:
Temporary addresses will be assigned to aircraft in special circumstances when operators have been unable to obtain an address from their States of Registry or private common mark registering authorities on a timely basis. ICAO will assign temporary addresses from the ICAO block shown in the appendix to this paper [...]