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Unit Structure Of IJA Air Force
This essay is part of a larger work on the Japanese Nakajima Ki-43 "Oscar" fighter airplane, soon to be published in Polish by AJ Press in the "Monografie Lotnicze" book series. An English translation will hopefully follow soon. This chapter on the structure and nomenclature of the Japanese Army Air Force units is an attempt to provide a quick guide to a subject that often leads to confusion and missquotations. I hope this "sneak preview" will please fellow aviation enthusiasts and modellers / MK
53rd Sentai: a briefing
Photo: Briefing before a mission, 53rd Hiko Sentai, Nov.1944. (in background: Type 2 / Ki-45 Kai "Nick") © Model Art Co.Ltd 1992

From the very beginning, Japanese military aviation was split under the direct command of the two main services, the Army and the Navy, creating their own strategies and becoming hard competitors in the process. It remained as such practically until the end of the Pacific War. Not surprisingly, the Japanese Naval Air Force was influenced by Great Britain, while the Japanese Army Air Force looked upon the major land powers of the time: France, Russia and Germany.
          This article describes the structure of the Army Air force. For many obvious reasons, the structure of the naval air units was quite different and deserves a note of its own.
The early period
At the outset, shortly before WWI, the main tactical unit of the JAAF was the Koku Daitai (air battalion), consisting of two Chutai (squadron) of nine aircraft each plus three a/c of the reserve and three a/c of the Daitai HQ, making a total of 27 machines. On May 5, 1925, the size of the basic unit was increased and its name changed to Hiko Rentai (air regiment). It now comprised two, three or, usually, four Chutai grouped into two Daitai.
          The typical Hiko Rentai, as well as the older Hiko Daitai, was a mixed-purpose unit consisting of, for example, two reconnaissance Chutai forming the 1st Daitai and two fighter Chutai forming the 2nd Daitai. With the China-conflict escalating into full war around 1937, many Hiko Rentai operated it's Daitai as self-dependent units, thus initiating the use of smaller units in response to field conditions, gradually developing into full independence, with their own unit markings. These new Hiko Daitai, not to be confused with the pre-1925 nomenclature, as well as the even smaller Dokuritsu Hiko Chutai (independent flying squadrons), operated under the direct command of the brigade or army HQ.
1938 - 1944
Combat experience proved the operative and command advantages of smaller units, although in many cases the size of a Hiko Daitai was almost too small. Therefore, in the months of July & August of 1938, a second major change in unit organization of the JAAF was carried out.
          All operating Hiko Rentai and Hiko Daitai received the new designation of Hiko Sentai, while the name of the smaller Dokuritsu Hiko Chutai remained unchanged and the habit of forming new, similiar units was encouraged according to the needs.
Name confusions
The correct translations for the Japanese expressions Chutai, Daitai and Rentai are "squadron", "battalion" and "regiment". Same words are also being used in the context of other, land units of the Japanese Army. Although the new Sentai was equal to the old Rentai in numbers, it lacked the proper regimental structure of its predecessor and had a simplified system of command, in adaptation to air operations. Therefore, the often used term "regiment" is both incorrect and misleading and the expression "air group" seems more appropriate, strenghtened by the fact that the exact Japanese meaning of Sentai is "combat group".
          The Japanese word Hiko, here meaning "flying", used in full designations of some units, is ofted deleted in casual descriptions in foreign languages for reasons of convenience and we will hereon not use it the context of units such as Sentai or smaller.
The Sentai
A typical Sentai was a single-purpose unit consisting of three Chutai divided into three Shotai (flight) of three a/c each. Together with the machines of the Sentai Hombu (HQ flight) and the reserve a/c, nominally amounting to some 1/3 of the Sentai strength, the total number of aircraft in a regular fighter Sentai amounted to 45.
          A few figter Sentai consisted of four Chutai, while most bomber or reconnaissance Sentai numbered only two and totalled up to some 30 aircraft.
Larger Structures
Two or more Sentai formed a Hikodan (air brigade), two or more Hikodan, together with some base and support units and, often, some independent Dokuritsu Chutai formed a Hikoshudan (air corps). In 1942 this name was changed to Hikoshidan (air division) but the structure remained the same. At the highest level of the organization, acting under the command of the Koku Hombu (JAAF HQ), was the Kokugun (air army), consisting usually of two Hikoshidan.
          Similar to most units at lower levels, some Kokugun contained irregularities, such as independent Hikodan, sometimes even Sentai, operating directly under the Kokugun command, without the Hikoshidan intermediary. During the Pacific War the JAAF consisted of six Kokugun. In April of 1945 the four Kokugun operating over China, Korea and the Japanese home islands were united under the parasol Kokusogun (supreme air army), but this was, however, of little practical importance.
Last Changes
In April of 1944 the structure of a Sentai was redesigned. This far, a Sentai was a pure air unit, while the maintenance and other ground services were conducted by other, separate units. From now on, they were all incorporated into the Sentai which, in turn, was divided into an air section called Hikotai (air unit) and a ground section called Seibutai (maintenance unit).
          The Hikotai itself was divided into Kogekitai (assault units). It was no more than a new, fancy, morale-boosting name of the old Chutai. It didn't stick among the ranks and the Chutai - label remained in use throughout the war. The only practical consequence of this last structural change was the fact that the Sentaicho (the Sentai commander) was now in charge of a larger unit, while the new functions of a Hikotaicho and a Seibitaicho were instated for the command of the air and ground sections.
The Graph
The scheme below shows the basic outline of the JAAF organization. Please note that the independent units of various kinds could operate at any level, bypassing one or more steps on the command ladder.
JAAF table

Outside of the structure outlined above, there were other units encountered in the JAAF organization, such as:
Dokuritsu Hikodan (independent air brigade, see above): a Hikodan operating under the direct command of a Kokugun, in some cases directly under the supervision of the Koku Hombu (in fact, the exact meaning of dokuritsu is "direct command").
Dokuritsu Hikotai Hombu (independent HQ air unit): sometimes when a Sentai operated it's Chutai as independent units, often with individual unit insignia, also the Sentai HQ flight became independent, keeping, however, the administrative supervision of the other Chutai's. This system was most common among the reconnaissance units.
Chokkyo Hikotai (support air unit): independent units of a size of a Chutai or smaller, used for liaison or for ground support of army land units. In October of 1944 all such units were renamed as Dokuritsu Hiko Chutai.
Flight, maintenance and communication schools acted as separate units with their own specific insignia and names, under the direct command of the Koku Hombu (JAAF HQ). Separate training or second-line units, and also later in the war combat units, were sometimes created within the schools. They were gradually incorporated into the reglar structure on a proper level of command, keeping the original names.
Finally, with the introduction of massive use of special attack tactics, many new units were formed, most often within the existing ones, for ramming attacks and other self-destructive actions. These often short-lived units received own insignia and names including expressions such as Shinten or Shinbutai, meaning more or less "special attack unit", or other phrases derived from Japanese mythology or tradition.

Mark Kaiser 1997/1998


Many Thanks to Bruce D.Fraites and James F. Lansdale for research & enthusiasm and to Gavin Johns for editorial assistance.
Mark Kaiser mark@marksindex.com