The Aichi M6A Seiran, the Submarine-Launched Floatplane at Udvar-Hazy, and the Attack on the Panama Canal That Never Happened

The Aichi M6A Seiran hanging in the main hall of the Steven F. Udvar-Hazy Center in Chantilly, Virginia is the last surviving example of its kind on earth. It was designed to fold down inside a submarine, reassemble on the open ocean in darkness, and strike the Gatun Locks of the Panama Canal - a blow that would have severed Allied logistics in the Pacific. The attack never happened, but the planning, the training, and the hardware were entirely real.

The Problem That Created the Seiran

By the fall of 1942, the Imperial Japanese Navy had just survived the Battle of Midway - barely. The loss of four fleet carriers in a single engagement shifted the strategic momentum of the Pacific War. Japan still held vast territory, but the admirals who were honest with themselves understood the long-term picture had changed.

The challenge was operational: how do you strike a target on the other side of the Pacific when your surface fleet can no longer guarantee safe passage and land-based aircraft lack the range? The answer required thinking that had no precedent in naval aviation. Not a submarine that carried a small scout floatplane for reconnaissance. Something more radical - a submarine that functioned as an aircraft carrier.

The I-400 Class: The Submarines Built Around an Idea

The I-400 class submarines were constructed at the Kure Naval Arsenal and were extraordinary by any standard of the era. They were nearly 400 feet long and displaced more than 6,500 tons when submerged. They carried enough fuel to reach any point on earth and return to Japan without refueling.

Forward of the conning tower, flush with the deck, sat a watertight cylindrical hangar approximately 115 feet long and 12 feet in diameter - large enough to hold three aircraft, folded and nested inside. Three boats were eventually completed. The I-400 and I-401 were configured as aircraft carriers. The I-402 was converted to a supply role and never carried aircraft. The resources committed to these two submarines, in a war where Japan was desperately short of nearly everything, were staggering.

Engineering a Floatplane for a Submarine

The Seiran - the name translates roughly as mountain haze or clear sky after a storm - was not simply a floatplane. It was an aircraft engineered from the first day of its design to be disassembled, stored in a tube, reassembled by a submarine crew on a pitching deck in darkness, and launched by catapult in conditions that would have grounded conventional operations.

The wings folded back along the fuselage. The tail unit folded down. The floats disconnected and stowed separately. An aircraft with a roughly 40-foot wingspan could be collapsed into a shape that fit through a 12-foot hatchway.

The powerplant was the Aichi Atsuta, a liquid-cooled 12-cylinder inverted-V producing approximately 1,400 horsepower, derived from the German Daimler-Benz DB 601 and built under license for Japanese production. In a floatplane application the inline engine was unusual, but it gave the Seiran a clean nose profile and performance numbers that matched or exceeded some contemporary fighters - a top speed of around 295 mph, combat range of about 730 miles, and a crew of two: pilot and observer.

The floats were also designed to be detachable. For a conventional floatplane, the floats are the landing gear. Removing them produces an aircraft with no way to land on water and no way to land on a runway. The Seiran’s designers built that option in deliberately. The pilots who trained on it understood exactly what detachable floats meant about mission planning - and about what came after the target.

The Training Program

The training program began in the summer of 1944 at Ominato Naval Base in the cold north of Japan. The operation was compartmented tightly enough that most of the men involved did not know the full scope of what they were preparing for.

What they did was drill. Assembly and disassembly until they could work in near darkness without sound. Catapult launch procedures. Low-level navigation over water to defeat radar. Practiced crews working in coordinated rhythm could have a Seiran assembled, fueled, armed, and sitting on the catapult in just over six minutes per aircraft - three aircraft, three crews working simultaneously, on a rolling deck in the Pacific.

Operation PX: The Panama Canal Target

The original target was the Gatun Locks of the Panama Canal.

In 1944 and 1945, the Panama Canal was the logistical hinge of the American war in the Pacific. The Gatun Locks lift ships approximately 85 feet from the Atlantic side up to Gatun Lake. Without functioning locks, nothing moves. The repair timeline for seriously damaged lock chambers - given the concrete dimensions and engineering complexity - would be measured in years.

The plan, designated Operation PX, called for the I-400 boats to approach the Caribbean side of Panama running submerged, surface offshore in darkness, and prepare the aircraft through the night. Before first light, the catapults would fire. Pilots and observers would climb away into the Caribbean predawn, thread their way to the canal, and strike the locks with bombs or torpedoes. The floats would have been left on the deck.

Whatever came after the strike would come after the strike.

Why the Attack Was Called Off

In the spring of 1945, the Panama Canal strike was cancelled. The exact reasoning is not fully preserved in surviving records. By 1945, the strategic situation had deteriorated enough that the logic of a long-range infrastructure strike had shifted. The Philippines were gone. The American bombing campaign against Japan’s home islands was already in motion. The window for that kind of strategic disruption had narrowed.

The new target became the American fleet anchorage at Ulithi Atoll in the Caroline Islands - briefly one of the most important naval bases in the world, with more than 700 American warships anchored there simultaneously at its peak. Strike the fleet at anchor, and you disrupt the logistics of the Pacific campaign at its most critical moment.

Then the target shifted again, to Okinawa. Then August 6, 1945. Then August 9. On August 15, Emperor Hirohito delivered the surrender broadcast.

The End of the Program

The I-400 and I-401 were somewhere in the Pacific, en route to their target, when the surrender order arrived. The commander of Submarine Squadron One - the officer who had overseen the entire I-400 program - chose not to survive the surrender. His crew committed his body to the sea.

The remaining crews received the order to stand down. Weapons went over the side. Some Seiran aircraft were prepared for ditching to deny them to the Americans, though accounts differ on how many were actually lost that way. American forces took possession of both submarines.

Naval architects and intelligence officers examined both boats carefully. At least one Seiran was test-flown at Naval Air Station Patuxent River in Maryland. Test pilots came away noting its performance with something that sounded like respect.

The Cold War was already beginning before the hot war officially ended. With Soviet observers demanding access to captured Japanese military hardware under postwar agreements, the U.S. Navy made a decision. In May 1946, the I-400, the I-401, and several other captured Japanese submarines were scuttled in deep water off Pearl Harbor. The technology went to the bottom of the Pacific.

The Seiran at Udvar-Hazy Today

The Seiran that had been tested at Patuxent River eventually entered the Smithsonian collection. When the Steven F. Udvar-Hazy Center opened in December 2003, the Seiran was placed on display in the main hall.

The floats are reattached for the display, restoring the floatplane silhouette it would have worn on the catapult - though on the one-way mission, those floats would have been left on the deck. The lines are clean and almost elegant for a wartime aircraft. The inline engine gives it a fighter-like nose profile that stands out from the radial-engined floatplanes of the same era.

Most visitors walk past it. Some stop at the placard. Very few stand there long enough to work through what they’re actually looking at: an aircraft built for a concept that had no precedent - a submarine running submerged across the Pacific with three folded aircraft inside a tube, surfacing in darkness off a hostile coast, a crew assembling those aircraft by muscle memory in silence, a catapult firing them into the predawn dark toward a target on another continent.

It’s in Chantilly, Virginia. Go find it.

Key Takeaways

The Aichi M6A Seiran is the only surviving example of a submarine-launched strike aircraft; it is on permanent display at the Udvar-Hazy Center in Chantilly, Virginia.
The I-400 class submarines were the largest submarines built in World War II, each carrying a watertight aircraft hangar capable of housing three folded Seirans.
The Seiran was engineered with detachable floats, deliberately designing the aircraft for one-way strike missions with no planned return.
The original target was the Gatun Locks of the Panama Canal under Operation PX; the mission was cancelled in early 1945 as the strategic situation shifted.
Both I-400 class submarines were scuttled by the U.S. Navy in May 1946 off Pearl Harbor to prevent Soviet access to the technology.
Practiced crews could assemble, fuel, arm, and position a Seiran on the catapult in just over six minutes - working on a rolling deck, at night, in near silence.