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Aircraft Type Designator
From SKYbrary Wiki
Throughout SKYbrary, aircraft types are described by the ICAO Type Designator.
In principle, ICAO allocates an Aircraft Type Designator to all aircraft heavier than ultra/micro light. A designator consists of not more than four characters and is primarily be intended for use in flight plans and associated air traffic services messages. Each designator is, in principle, be derived from the manufacturer’s model number or model name or, from a common military type number. Only one designator is assigned per aircraft type - a different designator for a variant or subtype will only be allocated when a difference in a performance is significant for air traffic services, or when no shared designator can be assigned.
For further information see ICAO Doc 8643: Aircraft Type Designators