An aircraft registration is a unique alphanumeric string that identifies an aircraft, in similar fashion to a licence plate on an automobile. In accordance with the Convention on International Civil Aviation all aircraft must be registered with a national authority (such as the FAA or Transport Canada), and furthermore, they must carry proof of this registration in the form of a legal document called a Certificate of Registration at all times when in operation. Most countries also require the aircraft registration to be imprinted on a permanent fireproof plate mounted on the fuselage for the purposes of post-fire/post-crash aircraft accident investigation.

Because airplanes typically display their registration numbers on the aft fuselage just forward of the tail, in earlier times more often on the tail itself, the registration is often referred to as the "tail number". In the United States, the registration number is also referred to as an "N-number", as it starts with the letter N.

Although each aircraft registration is unique, some, but not all countries allow it to be re-used when the aircraft has been sold, destroyed or retired. For example N3794N is assigned to a Mooney M20F. It had been previously assigned to a Beechcraft Bonanza (specifically, the aircraft in which Buddy Holly was killed). Also note that an individual aircraft may be assigned different registrations during its existence. This can be either the aircraft changes ownership, state of registration changes or in some countries like the United States for vanity reasons.

Certificate of Registration

The Certificate of Registration contains contact information used by national authorities for enforcement purposes, and for the purposes of disseminating Airworthiness Directives to aircraft owners. Most national authorities require that the aircraft owner update said contact information immediately or as soon as possible any time there is a change in the same.

International standards

The first use of aircraft registrations was based on the radio callsigns allocated at the London International Radiotelegraphic Conference in 1913. This was modified by agreement by the International Bureau at Berne and published on April 23 1913. Although initial allocations were not specifically for aircraft but for any radio user, the International Air Navigation Convention held in Paris in 1919 made allocations specifically for aircraft registrations based on the 1913 callsign list. The agreement stipulated that the nationality marks were to be followed by a hyphen then a group of four letters that must include a vowel (and for the convention Y was considered to be a vowel).

At the International Radiotelegraph Convention at Washington in 1927 the list of markings was revised and adopted from 1928, these allocations are the basis of the currently used registrations. The marking have been amended and added to over the years and the allocations and standard are managed by the International Civil Aviation Organization (ICAO).