The International Civil Aviation Organization (ICAO) requires each aircraft to have a unique alphanumeric identifier. Because this number is displayed prominently on the fuselage or tail of the aircraft, it is also known as an airplane tail number. A complete database of all registrations can be found here.
An airplane registration number has 2 parts: a 1- or 2-letter country code prefix, and a suffix consisting of a group of between 3 and 5 letters or numbers. As a tail number, the prefix and suffix are usually separated with a hyphen. In the United States, which uses the prefix N, the hyphen is usually dropped.
Country code prefixes
In general, countries with the most domestic and international air traffic have single letter country code prefixes, while countries which have less air traffic have double letter country code prefixes. A complete list of country codes can be found here.
Some countries have more than one country code prefix. For example, Canada uses both C- and CF-.
The first letter of the suffix may also depend on the country of registration. Very high air traffic countries may have up to 3 primary variations. The United States uses N(1 to 99999), N(1A to 9999Z), and N(1AA to 999ZZ).
A few small countries and colonies are assigned prefixes which consist of a letter and a number. For example, Aruba uses the prefix P4. A few colonies, such as the Netherlands Antilles (PJ), use the same first letter as their governing country but a different second letter (Netherlands – PH).
Some non-countries also are assigned unique prefixes and registrations. The prefix for the United Nations is 4U, with a suffix ranging from AAA to ZZZ.
Most suffixes are assigned alphanumerically by time of registration. However, most colonies use the prefix of their governing country, along with a suffix belonging to a limited range. Some suffixes are also restricted to specific types of aircraft, such as helicopters, gliders, ultralights, or balloons. A few countries reserve some suffixes for historical or experimental aircraft.
Airplane tail numbers can be reused in some countries, but not in others. If a number is reused, the original aircraft must be out of commission.
Like a license plate, an aircraft registration number can change over the life of the aircraft. It may change if ownership changes, if the state of registration changes, or even if the owner asks for a specific combination of numbers and letters.
Not all aircraft are required to have registration numbers. Because the ruling applies only to aircraft over a certain weight, some ultralight aircraft are exempt, while others must be registered. Gliders do have registration numbers, just the same as powered aircraft.