Aichi M6A
The Aichi M6A Seiran ("Mountain Haze") was a submarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

-launched attack floatplane
A floatplane is a type of seaplane, with slender pontoons mounted under the fuselage; only the floats of a floatplane normally come into contact with water, with the fuselage remaining above water...

 designed for the Imperial Japanese Navy
Imperial Japanese Navy
The Imperial Japanese Navy was the navy of the Empire of Japan from 1869 until 1947, when it was dissolved following Japan's constitutional renunciation of the use of force as a means of settling international disputes...

 during World War II. It was intended to operate from I-400 class
I-400 class submarine
The Imperial Japanese Navy submarines were the largest submarines of World War II and remained the largest ever built until the construction of nuclear ballistic missile submarines in the 1960s. They were submarine aircraft carriers able to carry three Aichi M6A Seiran aircraft underwater to their...

 submarines whose original mission was to conduct aerial attacks against the United States.

Design and development

From the late 1920s, the Imperial Japanese Navy had developed a doctrine of operating floatplanes from submarines to search for targets.

In December 1941, however, Commander-in-Chief of the Japanese Combined Fleet, Admiral Isoroku Yamamoto
Isoroku Yamamoto
was a Japanese Naval Marshal General and the commander-in-chief of the Combined Fleet during World War II, a graduate of the Imperial Japanese Naval Academy and a student of Harvard University ....

, proposed constructing a large fleet of submarine aircraft carrier
Submarine aircraft carrier
Submarine aircraft carriers are submarines equipped with fixed wing aircraft for observation or attack missions. These submarines saw their most extensive use during World War II, although their operational significance remained rather small...

s (also designated STo or sen-toku — special submarine) whose purpose was to mount aerial attacks against American coastal cities. The submarines would surface to launch their aircraft by catapult
Aircraft catapult
An aircraft catapult is a device used to launch aircraft from ships—in particular aircraft carriers—as a form of assisted take off. It consists of a track built into the flight deck, below which is a large piston or shuttle that is attached through the track to the nose gear of the aircraft, or in...

, submerge to avoid detection, then surface again to retrieve the aircrews who would ditch their planes nearby. By June 1942, the plan was to build a fleet of eighteen such subs. This was later cut to nine, then five and finally just three as Japan's wartime fortunes declined.
To equip the submarine aircraft carriers, the Imperial Japanese Navy Air Service
Imperial Japanese Navy Air Service
The Imperial Japanese Navy Air Service was the air arm of the Imperial Japanese Navy during World War II, the organization was responsible for the operation of naval aircraft and the conduct of aerial warfare in the Pacific War.It was controlled by the Navy Staff of the Imperial Japanese Navy and...

 requested that Aichi
Aichi Kokuki
was a Japanese aircraft manufacturer which produced several designs for the Imperial Japanese Navy.The company was established in 1898 in Nagoya as Aichi Tokei Denki Seizo Co., Ltd. . Aircraft production started in 1920, and the company relied initially on technical assistance from Heinkel, which...

 design a folding attack aircraft with a range of 1500 km (809.9 nmi) and a speed of 555 km/h (283.2 kn). Aichi was already manufacturing under license, the D4Y1
Yokosuka D4Y
The D4Y Navy Type 2 Carrier Dive bomber was operated by the Imperial Japanese Navy. Its Allied reporting name was "Judy". The D4Y was one of the fastest dive-bombers of the war, and only the delays in its development hindered its service, while its predecessor, the slower fixed gear Aichi D3A...

 Suisei (Judy), a relatively small single-engined carrier dive bomber with exceptionally clean lines and high performance. Detailed engineering studies commenced in an effort to modify the Susei for use aboard the I-400 submarines but the difficulties in doing so were eventually judged insurmountable and a completely new design was initiated.
Aichi's final design, designated AM-24 by Aichi and given the military designation
Japanese military aircraft designation systems
The Japanese military aircraft designation systems for the Imperial period are rather difficult to keep track of, primarily because multiple designation systems were in use by each armed service...

 M6A1, was a two-seat, low-winged monoplane
A monoplane is a fixed-wing aircraft with one main set of wing surfaces, in contrast to a biplane or triplane. Since the late 1930s it has been the most common form for a fixed wing aircraft.-Types of monoplane:...

 powered by a 1050 kW Aichi AE1P Atsuta 30
Aichi Atsuta
The Aichi Kokuki KK AE1A Atsuta was a Japanese licensed version of the German Daimler-Benz DB 601A 12-cylinder liquid-cooled inverted-vee aircraft engine. The Atsuta powered only two models of Imperial Japanese Navy Air Service aircraft aircraft in World War II...

 engine (a licence-built copy of the Daimler-Benz DB 601
Daimler-Benz DB 601
|-See also:-Bibliography:* Mankau, Heinz and Peter Petrick. Messerschmitt Bf 110, Me 210, Me 410. Raumfahrt, Germany: Aviatic Verlag, 2001. ISBN 3-92550-562-8.* Neil Gregor Daimler-Benz in the Third Reich. Yale University Press, 1998-External links:...

 liquid-cooled V12 engine
V12 engine
A V12 engine is a V engine with 12 cylinders mounted on the crankcase in two banks of six cylinders, usually but not always at a 60° angle to each other, with all 12 pistons driving a common crankshaft....

). The original specification dispensed with a traditional undercarriage but it was later decided to fit the aircraft with detachable twin floats to increase its versatility. If conditions permitted, these would allow the aircraft to alight next to the submarine, be recovered by crane and then re-used. The floats could be jettisoned in flight to increase performance or left off altogether for one-way missions.

The Seiran's wings rotated 90 degrees and folded back hydraulically against the aircraft's fuselage (with the tail also folding down) to allow for storage within the submarine's 3.5 m (11.5 ft) diameter cylindrical hangar. Armament was a single 850 kg (1,873.9 lb) torpedo or an equivalent weight in bombs. One 13 millimetre Type 2 machine gun
Type 2 machine gun
The Type 2 machine gun was developed for aerial use for the Imperial Japanese Navy during World War II. It was an adaptation of the German MG 131 machine gun.-Installations:* Aichi B7A* Mitsubishi A6M5c* Mitsubishi G4M* Nakajima B6N* Yokosuka P1Y...

 was mounted on a flexible mounting for use by the observer.
As finalised, each I-400 class submarine had an enlarged watertight hangar capable of accommodating up to three M6A1s. The Seirans were to be launched via a 26 m (85.3 ft) compressed-air catapult mounted on the forward deck. A well-trained crew of four men could roll a Seiran out of its hangar on a collapsible catapult carriage, attach the plane's pontoons and have it readied for flight in approximately 7 minutes.
In order to shorten the launching process and eliminate the need for time-consuming engine warm-ups, the Seirans were to be catapulted from a cold start. This necessitated heating the engine oil for each plane to approximately 60 °C (140 °F) in a separate chamber and pumping it, as well as hot water, back through the engine just prior to launch while the planes were still in the hangar. In this way, the aircraft's engine would be at or near normal operating temperature immediately upon getting airborne. The idea was borrowed from the Germans who planned on using a similar launch method for the aircraft of their unfinished carrier Graf Zeppelin
German aircraft carrier Graf Zeppelin
German aircraft carrier Graf Zeppelin was the lead ship in a class of two carriers ordered by the Kriegsmarine. She was the only aircraft carrier launched by Germany during World War II and represented part of the Kriegsmarine's attempt to create a well-balanced oceangoing fleet, capable of...

The first of eight prototype Seirans was completed in October 1943, commencing flight testing in November that year. A problem with overbalance of the auxiliary wings was eventually solved by raising the height of the tail fin. Further testing was sufficiently successful for production to start in early 1944. In order to aid pilot conversion to the Seiran, two examples of a land based trainer
Trainer (aircraft)
A trainer is a class of aircraft designed specifically to facilitate in-flight training of pilots and aircrews. The use of a dedicated trainer aircraft with additional safety features—such as tandem flight controls, forgiving flight characteristics and a simplified cockpit arrangement—allows...

 version fitted with a retractable undercarriage
Conventional landing gear
thumb|The [[Piper PA-18|Piper Super Cub]] is a popular taildragger aircraft.thumb|right|A [[Cessna 150]] converted to taildragger configuration by installation of an after-market modification kit....

 were built. These were given the designation M6A1-K Nanzan ("Southern Mountain"). Besides the difference in landing gear, the vertical stabilizer
Vertical stabilizer
The vertical stabilizers, vertical stabilisers, or fins, of aircraft, missiles or bombs are typically found on the aft end of the fuselage or body, and are intended to reduce aerodynamic side slip. It is analogical to a skeg on boats and ships.On aircraft, vertical stabilizers generally point upwards...

's top portion, which was foldable on the Seiran, was removed.

Operational history

The first production examples of the Seiran were completed in October 1944. Deliveries were slowed by an earthquake near Nagoya on 7 December 1944, and by an American air-raid on 12 March 1945. Construction of the STo submarines was stopped in March 1945, after two submarine aircraft carriers had been completed and a third finished as a fuel tanker. These were supplemented by two smaller Type AM submarines, originally designed as command submarines carrying reconnaissance floatplanes, but capable of carrying two Seirans. Owing to the reduced carrier submarine force, production of the Seiran was halted, with a total of 28 completed (including the prototypes and the M6A1-Ks).

The new submarines and aircraft were assigned to the 1st Submarine Flotilla
IJN 6th Fleet
The was a fleet of the Imperial Japanese Navy during World War II, primarily responsible for command of submarine operations.-History:The IJN 6th Fleet was formed on 15 November 1940, and was assigned general control of all Japanese submarine operations...

, comprising the two STo submarines, the I-400 and the flagship I-401
Japanese submarine I-401
The Sen Toku-class I-401 was once the largest submarine in the world. It was commanded by Lieutenant Commander Nobukiyo Nambu of the Imperial Japanese Navy during World War II...

, each carrying three Seirans together with two type AMs, the I-13
Japanese submarine I-13
Japanese submarine I-13 was an AM type submarine. The submarine was built at the Kawasaki shipyard in Kobe....

 and I-14
Japanese submarine I-14
The submarine I-14 was a large, seaplane-carrying submarine of the Imperial Japanese Navy of the AM type. 400 feet long and 40 feet high, it didn't see action in World War II. With a range of 21,000 nmi, it could carry two or three Aichi M6A Seiran bombers, whose wings and tail could be folded to...

. The 1st Submarine Flotilla commenced training with the Seirans in January 1945, the crews gradually learning how to handle the submarines and aircraft. Launching all three Seirans took longer than expected: 30 minutes if floats were fitted, although this could be reduced to 14.5 minutes if the floats were not used.

The first mission of the Seirans was to be a surprise air strike on the Gatun locks
Panama Canal Locks
The Panama Canal Locks, which lift ships up 25.9 m to the main elevation of the Panama Canal, were one of the greatest engineering works ever to be undertaken at the time, eclipsed only by other parts of the canal project. No other concrete construction of comparable size was undertaken...

 of the Panama Canal
Panama Canal
The Panama Canal is a ship canal in Panama that joins the Atlantic Ocean and the Pacific Ocean and is a key conduit for international maritime trade. Built from 1904 to 1914, the canal has seen annual traffic rise from about 1,000 ships early on to 14,702 vessels measuring a total of 309.6...

, to cut the main supply line for US forces in the Pacific. When the force was finally ready to set off on their mission against Panama, however, Japan's increasingly desperate situation lead to a change in plan, with the target for the attack, called Operation Hikari (Splendour) being switched to the American base at Ulithi Atoll where forces, including aircraft carriers, were massing in preparation for attacks on the Japanese Home Islands. The flotilla departed Japan on 23 July 1945 and proceeded towards Ulithi. However, on 16 August, the flagship I-401 received a radio message from headquarters, informing them of Japan's surrender
Surrender of Japan
The surrender of Japan in 1945 brought hostilities of World War II to a close. By the end of July 1945, the Imperial Japanese Navy was incapable of conducting operations and an Allied invasion of Japan was imminent...

 and ordering them to return to Japan. All six Seirans on board the two submarines, having been disguised for the operation as American planes in violation of the laws of war, were catapulted into the sea with their wings and stabilizers folded (for the I-401) or pushed overboard (for the I-400) to prevent capture.


Prototypes powered by Atsuta 30 or 31 1,044 kW (1,400 hp) engine and removable floats, 8 built.

M6A1 Seiran (Shisei-Seiran)
Special Attack Bomber, 18 built.

M6A1-K Nanzan (Shisei-Seiran Kai)
Prototypes of training version, retractable wheeled landing gear, 2 built.

Prototype, equipped with one 1,163 kW (1,560 hp) Mitsubishi Kinsei
Mitsubishi Kinsei
|-See also:-Bibliography:* Matsuoka Hisamitsu, Nakanishi Masayoshi. The History of Mitsubishi Aero Engines 1915-1945. Miki Press, Japan, 2005. ISBN 4-89522-461-9...

 MK8P 62 of driving a three-blade, constant-speed metal propeller, 1 built.


  • Imperial Japanese Navy Air Service
    Imperial Japanese Navy Air Service
    The Imperial Japanese Navy Air Service was the air arm of the Imperial Japanese Navy during World War II, the organization was responsible for the operation of naval aircraft and the conduct of aerial warfare in the Pacific War.It was controlled by the Navy Staff of the Imperial Japanese Navy and...


A single M6A1 has been preserved and resides in the Udvar-Hazy Center of the Smithsonian's National Air and Space Museum
National Air and Space Museum
The National Air and Space Museum of the Smithsonian Institution holds the largest collection of historic aircraft and spacecraft in the world. It was established in 1976. Located in Washington, D.C., United States, it is a center for research into the history and science of aviation and...

. It is located in the Washington, DC suburb of Chantilly, VA near Dulles International Airport. The Seiran was surrendered to an American occupation contingent by Lt Kazuo Akatsuka of the Imperial Japanese Navy, who ferried it from Fukuyama
-Places:*Fukuyama, Hiroshima, a city in Japan*Fukuyama, Kagoshima, a former town in Japan, now part of Kirishima city*Fukuyama, Japanese word for Mount Fuji, meaning "mountain of wind"-People:*Francis Fukuyama, an American philosopher and political economist...

 to Yokosuka. The US Navy donated it to the Smithsonian Institution
Smithsonian Institution
The Smithsonian Institution is an educational and research institute and associated museum complex, administered and funded by the government of the United States and by funds from its endowment, contributions, and profits from its retail operations, concessions, licensing activities, and magazines...

 in November 1962. Restoration work on the Seiran began in June 1989 and was completed in February 2000.

Specifications (M6A1)

See also

External links

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