Aircraft Designations, Japan

The Imperial Japanese Army Air Force (IJAAF) used a rather complicated designation system after 1932 which, based on the US system, was different than that of the Navy. All aircraft designed to an IJAAF requirement were assigned a Kitai (airframe) number followed by a dash and the model number. This model number, preceded by the Kitai prefix "Ki", was sequential and was completely unrelated to the manufacturer or the function of the aircraft. Thus the Ki-21 designates the twenty-first airframe model designed for the IJAAF. An exception was made later for gliders, which were given the "Ku" (Guraida) prefix and numbers.

Aircraft variants were assigned a roman numeral after the model number. These were separated by a dash. Initial production variants were usually given the numeral I. Thus, the Ki-21-I was the first production variant of the 'Sally' bomber. Lesser modifications were indicated by a sub-variant type, this could be written in one of two ways. The first way was using the original Japanese designation system which assigned a sequence of suffixes which were Ko, Otsu, Hei, Tei, Bo, Ki, Ko, Shin, Jin, and Ki (in practice only the first four were generally used). These had no real alphabetic or numerical significance and more closely represented the western concepts of direction (north, south, east, west). This would mean that the Ki-84-I-Ko was the initial variant of the 'Frank'. Western sources often simplified this system by substituting the suffixes for a lowercase letter in the order of the original sequence. Thus, Ko would be "a", Otsu would be "b", etc. Rewritten, this same version of the 'Frank' would be designated Ki-84-Ia. For simplicity, aircraft on this site use this system.

Major modifications, however, used a different system, this included assigning a Kaizo symbol to the designation, usually abbreviated as KAI. These variants usually involved a change in the function of the aircraft. The Ko-Ki sequence of suffixes was then appended to the KAI. An example of this designation is the Ki-102-KAI-Ko which was the initial heavy-fighter variant of the 'Randy' while the ground attack version was referred to as the Ki-102-KAI-Otsu. As before, the Ko-Ki sequence was often substituted by letters in the West, with the former known as the Ki-102-KAIa and the latter as the Ki-102-KAIb. Again, for simplicity, the western system is used in this site.

Finally, IJAAF aircraft were given a Type number and function for their naming system. The Type number was based on the last digits of the Japanese year when that aircraft was introduced. Before the Japanese year 2599 (1939) the last two digits were used, in the year 2600 (1940) the Type number became 100 and after 2601 (1941) only the last digit was used. To differentiate multiple aircraft being introduced in the same year, the aircraft function was included in the name. For example, in 1937 both the Ki-21 'Sally' and the Ki-21 'Nate' were introduced. The 'Sally' was officially known as the Ki-21 Army Type 97 Heavy Bomber, while the 'Nate' was known as the Ki-27 Army Type 97 Fighter. Type numbers are generally not included in Western designations.

The Type number system was eventually discontinued after 1942 since it proved to reveal too much information about the aircraft. In its place, aircraft were given arbitrary names usually involving animals or mystical creatures and places. An example is the Ki-84 Hayate. Nevertheless, the Allies assigned a code name to almost every Japanese aircraft which flew during World War II much like NATO did to Soviet aircraft during the Cold War. Most Japanese aircraft during World War II from both the IJAAF and the IJN are known to Western readers by these code names.

Beginning in the late 1920's, the Imperial Japanese Navy (IJN) employed an almost similar designation system as the US Navy, with designation indicating function, model number and manufacturer. Using the famous Zero as an example, the base designation for this aircraft is the A6M. Unlike the IJAAF, the sequential model number was related to the function, thus, in the case of the Zero, it would mean it was the sixth carrier-borne fighter to be designed for the IJN. Using the chart below, we see that the A refers to a carrier-borne fighter, and the M to Mitsubishi. The IJN, like the USN, often had aircraft produced by multiple companies due to the demands of the war. However, unlike the USN, the Japanese system always pointed at the original designer of the aircraft regardless of who actually manufactured it.

Major aircraft variants included a numerical suffix immediately following the manufacturer letter (like the USN but without the hyphen). This number was generally sequential also, thus, the A6M1 was the initial production variant of the Zero. Often a sub-variant designation was necessary, this was done by appending a lower-case letter suffix like in the case of the A5M1a. Should the same aircraft model be designed with different roles in mind, the secondary function would be added at the ended, preceded by a hyphen. This function letter followed the same system as the main function, thus, from the chart we can see that the N1K1-J 'George' was specifically a day fighter variant of what had originally been a seaplane fighter design. Sub-variant suffixes were in this case appended to the function, not the major variant so that the N1K1-Ja indicates a version of the 'George' armed with a particular bomb load.

The IJN naming system was almost identical to the IJAAF, involving a Type number and function. The major difference was that those aircraft introduced in the Japanese year 2600 (1940) were given the Type number 0 instead of 100. Thus, the Zero, introduced in 1940, was officially called the A6M Navy Type 0 Carrier-borne Fighter meaning it actually did not have a proper name. Furthermore, each aircraft was also assigned with a Model number. IJN designations specified that this double-digit number indicated airframe in the first digit, and engine in the second. Initial models therefore, were designated Model 11. A change in the airframe would result in the Model 21 whereas a change in the engine resulted in the Model 12. A change in both led to the Model 22. The full name of one of the more popular Zero models was thus A6M5 Navy Type 0 Carrier-borne Fighter Model 52 which was the fifth airframe model and second engine model compared to the initial version. The Zero is perhaps the only exception of a Japanese aircraft known primarily in the West by its Type number, rather than its Allied code name ('Zeke').

Like with the IJAAF, the IJN eventually discontinued the use of these Type numbers and function in an aircraft's naming system since they were too revealing. A similar naming system as the IJAAF was substituted instead resulting in aircraft like the N1K Shiden. The Allies also applied identical code names to IJN aircraft as explained above.

Most Japanese aircraft in the post-war period have been of US origin and have retained their original designations. Generally, variants designed specifically for the JASDF (and often license-produced by Mitsubishi) have been given a J suffix either replacing the original suffix as is the case with the F-15J, or added to it as can be seen in the F-4EJ and F-15DJ. Major modifications mertied a Kai symbol, as in the F-4EJ Kai, rather than a change in the letter suffix.

The few indigenous designs have largely mirrored the US post-1962 designation system, using the same function code (with modified mission code if applicable) and a sequential number for each. Examples are the C-1 (the first transport design), and F-2 (the second fighter design). However, certain Japanese designs have no direct US equivalent in terms of their function and modified mission combination. These include the OH-1 observation/attack helicopter, and the PS-1/US-1 amphibian. Civilian designs used for millitary purposes are often given a non-sequential number that makes reference to its original company number. Examples of these are the Gulfstream IV (U-4), BAe 125 (U-125), and Falcon 400 (T-400). One notable exception to these rules is the YS-11, a civilian airliner used for various transport, training, and electronic warfare roles and which has retained its company designation.