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M6A Seiran (晴嵐) submarine-launched attack floatplane designed for the IJN during World War II.It was designed and produced by Aichi Aircraft and was designated M6A1. It was developed for strategic bombing by the I-400 submarine, and was a small, light, dive bomb capable submarine-borne seaplane (classified by the Navy as a special attack aircraft). The first aircraft was completed in November 1943. However, even in September 1944, it was still in the experimental flight stage. It was operated by the 631st Air Group (formed on 15 December 1944).

Development history

After the First World War, the Imperial Japanese Navy built submarines equipped with small surface reconnaissance aircraft. In the early Showa period, the IJN generally established the use of “submarines for monitoring the enemy fleet and for engaging part of its fleet” . The installation of small reconnaissance scouts on submarines led to the strengthening of the reconnaissance capability of submarines.
In January 1942, after the outbreak of the Pacific War, Yoshio Suzuki, Director General of the Second Department of the Military Command, made an enquiry to the Warship Administration Headquarters about a “new type of submarine”. In May of the same year, a “special type submarine” with a range of 33,000 nautical miles, two floatplane attack aircraft (revised to three at the beginning of 1944), and a continuous operational period of more than four months was decided upon. This special type submarine was the I-400, and the attack aircraft was the Seiran. It was intended to operate on the east coast of the United States, and according to Kameto Kuroshima, the second director of the Military Command, the idea itself was attributed to Isoroku Yamamoto, then commander-in-chief of the Combined Fleet. The plan was revised on 30 June of the same year, and included the construction of 18 special submarines (standard displacement 3,530 tons, speed 19.6 knots) . However, due to the worsening of the war situation, the number of special type submarines (Type I-400) was reduced to five by a military ordinance dated October 15, 1943. In the end, only three special type submarines ( I-400, I-401, I-402) were completed . In the later part of the Pacific War, the number of Seiran aircraft carried by Type I-400 was increased from two to three . At the same time, it was decided to change the number of aircraft on board the I-13 submarines from “one reconnaissance aircraft” to “two attack aircraft”, and a conversion plan was executed.
The Seiran was a special attack aircraft designed to be catapulted from a surfaced submarine and used for strategic strikes, using the I-400 submarine (and later the I-13) as its mother ship. The aircraft had a maximum speed of 474 km/h (250 knots) and 560 km/h when jettisoning floats, a low-wing, monoplane, twin-seat configuration, a range of 642 nautical miles (at 166 knots), one 13 mm machine gun and one 250 kg bomb. In November 1943, the first prototype was completed and was capable of carrying a aerial torpedo, or 250 kg bombs (up to four bombs could be carried), or 800 kg bombs for horizontal and dive bombing. In the event of an attack, due to the power of the engine, floats were not fitted in the case of large bombs, and after the attack the aircraft would either land on the surface of the sea close to the ship, or the crew would be parachuted in and only the crew would be taken on board. In this case, recovery of the aircraft would be impossible. The submarine, on the other hand, was equipped with spare torpedoes and bombs and could be re-launched depending on its condition. However, on her only and last sortie, she was scheduled for a suicide mission.
In order to fit into the I-400’s hangar, the main wings can be rotated 90 degrees backwards under the leading edge by removing one pin (like a bird attaching its wings to its body when walking), the horizontal tail fins can be folded downwards, and the top of the vertical tail fins can be folded to the right side. The floats are removed, but placed close to the fuselage so that they can be fitted in a short time. In addition, instead of warming up, heated oil and cooling water can be injected, and it is said that the aircraft can be launched within about three minutes after starting work. The Type I-400 was capable of carrying three Seiran, and was already armed with bombs and torpedoes when it was loaded onto the submarine . However, since the maintenance space on the submarine deck is limited for the No. 3 aircraft, which is stored at the far end of the aircraft hangar, preparations for launching the aircraft are started after the maintenance and ejection of the No. 1 and No. 2 aircraft are completed. For this reason, the second aircraft was scheduled to be ejected 20 minutes after its launch. Even though the first aircraft could be launched in 3 minutes, it really depended on the skill of the crew and mechanics. According to the crew member Atsushi Asamura, at first it took more than 20 minutes to complete the launch of the three planes, but in the end it was reduced to more than ten minutes. The timing of the launches was particularly difficult because the mothership itself was moving up and down (if the aircraft were launched when the bow was pointing downwards, the aircraft would plunge into the sea), and the ejection commander had to make careful decisions. Because of the danger involved in launching the aircraft, the crew were given a risk allowance of 6 yen per launch. The starting salary for a university graduate at that time was 60 yen. It was a very advanced aircraft, with a folding structure for submarine use and high performance, and a gyroscope to enable it to be used all over the world (even in the North and South Poles) depending on its mission. It was said to be the equivalent of 50 A6M. As mentioned below, there are no regulations that can positively prove that this aircraft was adopted as a naval weapon.
A land-based version of the Seiran was also produced, and was known as the ” Shisei-Seiran Kai” or “Nanzan” (M6A1-K). In October 1944, Takahashi piloted the Nanzan in a successful torpedo test. This convinced Takahashi that the Seiran was a suitable aircraft for torpedo attacks. A total of 28 Seiran and Nanzan aircraft were built. At the planning stage, more than 36 aircraft were planned to be produced, but even the aircraft for training could not be secured, so two small Type Zero floatplane reconnaissance aircraft were borrowed from the Air Technical Arsenal to train the 631st Group. The crews complained that they had to train with toy-like planes . Therefore, they borrowed a E16A from the 634th to conduct training. However, both the Seiran and Nanzan were treated as experimental aircraft with the title of “test production”, and the Ministry of the Navy did not enforce the regulation to adopt them as weapons, regardless of the recognition of the implementing units. In addition, it is said that in the “Naval Current Aircraft Performance Summary List” prepared by the Naval Aviation Headquarters in July 1945, the aircraft names of “Trial SEIRAN” and “Trial SEIRAN Kai” were written respectively. Takahashi, who served as a test pilot of the Seiran in the 631st, stated that the aircraft was formally adopted after a receipt flight on November 24, 1944. In addition, the inventory of weapons delivered to the 631st Naval Air Squadron after the end of the war lists eight Seiran Type 11 aircraft, three of which were damaged.

Operational history

The Seiran squadron was established on 15 December 1944 as the 631st Naval Group (attached to the 6th Fleet). In addition, the Seiran was deployed to the 1st Submarine Squadron ( Commander Ryunosuke Ariizumi), which consisted of the I-400 , I-401, I-13, and I-14 submarines . Commander Ariizumi also served as Commander of the 631st Group. Only Second Lieutenant Takahashi and Second Lieutenant Takano had experience in submarine aircraft operations. Commander Ariizumi, who had long experience as a submarine captain, had no experience in aerial combat, Flight Commander Fukunaga had no actual combat experience in either air or submarine combat, Squadron Leader Asamura had no experience in submarines, and Squadron Leader Yamamoto had no actual combat experience, making the squadron’s training problematic. As of February, the 631st had only six Seirans and five Zuiuns. In January 1945, Commander Ariizumi ordered a study of a torpedo attack on the Panama Canal, and in late March and early April, a study of the operation proceeded. On 25 April, a plan for a night attack on the Panama Canal by the entire 1st Submarine Squadron and 10 M6A (2 torpedoes, 8 bombs) was announced to the officers. At this stage, the plan was for a conventional attack, but Captain Fukunaga insisted, “It is impossible for the Seiran to launch a conventional attack alone when all planes are under suicide attack. All planes were to be used in suicide attacks,” and he ordered that the bombs not fall from the dropers. In the end, all planes were equipped with 800kg bombs and became a special attack force. On 25 June, the Commander-in-Chief of the Navy, Jisaburo Ozawa, ordered the 1st Submarine Squadron of the 6th Fleet to carry out the following operation. The unit was named the “Shinryu Special Attack Force”. At a pep rally before the sortie, Commander-in-Chief of the Sixth Fleet, Lieutenant General Daigo Tadashige, presented the aircrews with daggers. This dagger was meant for the special strike force. Nambu (submarine commander I-401) said: “Neither Commander Ariizumi nor I (Captain Nambu) officially ordered this operation to be a suicide mission, and at least I intended to take the means to survive to the end. But what about the airmen? Also, in violation of international wartime law, the M6A was marked with a USAAF stars and painted silver, the same colour as US aircraft. On 20 July, I-400 and I-401 departed Maizuru, and arrived at Ominato on 21 or 22 July, and sailed from Ominato on 23 July, with an attack date of 17 August. She sailed from Ominato on 23 July and continued her voyage with an attack date of 17 August. On 14 August, Iyakuichi (commanded by Ariizumi) took a circuitous course through the eastern Marshall Islands. On 14 August, she reached the rendezvous point with Iyakuichi, but could not find her, and remained there on 15 August. On the other hand, Iyakuichi also could not find her companion ship and spent 15 August waiting at the rendezvous point. This difference was due to the fact that Iyaku did not receive the telegram sent by Commander Ariizumi (Iyakuichi) to change the meeting point (Captain Nanbu had no memory of the telegram sent by Iyakuichi), and Iyaku was ahead of the meeting point as planned. On 16 August, the Commander-in-Chief of the Sixth Fleet and the Commander-in-Chief of the Maritime Command issued an order to cancel the operation. Seirans was launched unmanned with engines running and wings folded, and was dropped at sea . On the I-400, three aircraft were assembled in just ten minutes . It is said that at the request of the crew, the stars were painted out and the Japanese markings was painted on. On 26 August, the I-401 dumped the planes, ammunition, secret documents and other items. Commander Ariizumi committed suicide aboard the ship.