Japanese military aircraft designation systems

The Japanese military aircraft designation systems for the Imperial period (pre-1945) had multiple designation systems for each armed service. This led to the Allies' use of code names during World War II, and these code names are still better known in English-language texts than the real Japanese names for the aircraft. A number of different schemes were simultaneously in use.

The Imperial Japanese Navy Air Service (Japanese: 大日本帝國海軍航空隊, Dai-Nippon Teikoku Kaigun Koukuu-tai) (IJNAS) used several different aircraft designation systems simultaneously. Between 1931 and 1945, aircraft had Shi numbers designating the specification they were designed to. They also had a long form of Type and Model Number system used between 1920 and 1943, a short designation system akin to that of the United States Navy in use between the late 1920s and 1945, a system of popular names introduced to replace type numbers from 1943 through 1945.

Japanese Navy specifications from 1931 were given an experimental, or Shi number, based on the year of the Emperor's reign the specification was issued in. Since multiple specifications could be issued in a year, the number was disambiguated with the aircraft purpose.

During the period this designation system was in use, the Emperor in question was Hirohito, the Showa Emperor, thus the years of Showa were those used, which began in 1926.

Thus, the Mitsubishi Zero was designed to meet the 1937 specification called 12-shi carrier fighter.

After 1929, aircraft types were given a type number based on the last two digits of the year as counted from the mythical founding of Japan in 660BC by Emperor Jimmu. Added to this was a brief description of the aircraft's function. The Mitsubishi Zero was so-called because entered service in 1940 which was the Japanese year 2600, thus it was designated Type 0 Carrier Fighter.

Model numbers were added to show subtypes. By the late 1930s these were two digits, the first being airframe revisions, the second engine revisions.

The system was abandoned in 1943, when it was decided that it gave away too much information about the aircraft.

In the late 1920s a short designation scheme was adopted, which was strikingly similar to the 1922 United States Navy aircraft designation system. This scheme used a letter or two letters to designate a type of aircraft, a number to indicate the number in series of that type of aircraft, and finally a letter to designate the manufacturer. Unlike the US Navy system, the Japanese system did not have a different number series for each manufacturer, and did not omit the number "1".

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