Mitsubishi A6M Zero

The Mitsubishi A5M was only just reaching front-line units in early 1937 when the Imperial Japanese Navy began looking for its replacement. On 5 October 1937 it sent Nakajima and Mitsubishi the planning requirements for a prototype 12-shi carrier-based fighter, and a few months later, having absorbed the lessons of the A5M over China, it tightened those requirements to a point that seemed self-contradictory: 500 km/h at 4,000 m, a climb to 3,000 m in nine and a half minutes, two hours of endurance at normal power and six to eight hours at economical cruise with drop tanks, an armament of two 20 mm cannon, two 7.7 mm machine guns and two 60 kg bombs, a full radio set with a direction finder for long-range navigation, manoeuvrability no worse than the A5M's, and a wingspan under 12 m so the aircraft could work from carrier decks. Nakajima's team judged the package unachievable and withdrew from the competition in January.

Mitsubishi's chief designer, Jiro Horikoshi, reached the opposite conclusion: the figures could be met, but only if the aeroplane were built as light as physics allowed. Born in 1903 near Fujioka, in Gunma Prefecture, Horikoshi had trained at the newly founded aviation laboratory in the engineering faculty of the University of Tokyo and had spent his whole career at the Nagoya works that would become part of Mitsubishi Heavy Industries. The project handed to him in 1937 was known internally as Prototype 12, after the twelfth year of the Shōwa era.

Every decision on the 12-shi was subordinated to saving grams. The structure was built almost entirely from a secret aluminium alloy developed by Sumitomo Metal Industries in 1936, "extra super duralumin", lighter, stronger and more ductile than contemporary alloys but so vulnerable to corrosion that it turned brittle; the cure was a zinc chromate coating applied after fabrication. No armour was fitted for the pilot, the engine or any critical point of the airframe, and the self-sealing tanks then becoming standard among the other belligerents were left out. The result was an exceptionally light and agile fighter with a radius of action unmatched among single-engine types of the war, able to seek out an enemy hundreds of kilometres from its base or carrier, fight, and return. The price was written in from the first day: a lucky hit in the tanks was enough to set it ablaze.

Aerodynamically it was as modern as anything then operating from a flight deck: a low-wing cantilever monoplane with wide-track retractable gear and an enclosed cockpit. The high-lift, low-speed wing gave very low wing loading and a stalling speed below 60 knots, and from that came the agility that let it out-turn any Allied fighter of the period. Early aircraft were given servo tabs on the ailerons because pilots complained that control forces stiffened above 300 km/h; they were dropped on later models once it became clear that lighter controls let pilots overstress the wing in violent manoeuvres.

Two A6M1 prototypes were finished in March 1939, powered by a 780 hp Mitsubishi Zuisei 13 driving a two-blade propeller. The first flight came on 1 April and testing was passed remarkably quickly: by September the aircraft had been accepted for Navy trials, the only notable change being a three-blade propeller to cure a vibration. While the first two machines were still being tested, the Navy suggested fitting the third with the 940 hp Nakajima Sakae 12. Mitsubishi was reluctant, having its own engine of that class in the Kinsei, but the first A6M2, completed in January 1940, cleared the specification comfortably on the Sakae's additional power.

The full Japanese designation was Rei shiki Kanjō sentōki, Type 0 carrier fighter, taken from the last digit of Imperial year 2600, equivalent to 1940, when it entered service. In Japan it was called both Rei-sen and Zero-sen, the latter being the usual form among pilots. In the short designation A6M, "A" meant carrier fighter, "6" that it was the sixth such model built for the Navy, and "M" identified Mitsubishi. The Allies assigned it the code name "Zeke" in 1942, part of the first batch chosen by Captain Frank T. McCoy at the Allied technical air intelligence unit in Australia, who wanted labels that were quick and easy to remember. Two variants got names of their own: the A6M2-N floatplane was "Rufe", and the A6M3-32 was first called "Hap" until General Arnold, commander of the US Army Air Forces, objected to the nickname and it became "Hamp".

The Navy did not wait for testing to finish: it ordered fifteen pre-production machines and sent them to China. They reached Manchuria in July 1940 and went into action with the 12th Rengō Kōkūtai as part of the bombing campaign against Chongqing and Chengdu. The first documented air combat came on 13 September 1940, when thirteen A6M2s led by Lieutenant Saburo Shindo, escorting twenty-seven G3M medium bombers over Chongqing, met thirty-four Polikarpov I-15 and I-16 fighters of the Chinese Nationalist air force. The Japanese pilots claimed all twenty-seven Chinese fighters they identified destroyed without loss to themselves; on the Chinese side ten pilots were killed and eight wounded, and Major Louie Yim-qun brought an I-15 home with forty-eight bullet holes in it. At most four Zeros took damage in the half-hour engagement. By the time the units were pulled out of the Chinese theatre a year later they were credited with 99 Chinese aircraft shot down, and up to 266 according to other sources.

The effect was immediate: the Chinese air command issued instructions to avoid combat with the new fighter. They were not always obeyed, and engagements such as the one over Chengdu on 14 March 1941, which cost eight Chinese pilots their lives, confirmed the mismatch. The last recorded dogfight between Chinese fighters and Navy Zeros took place on 11 August 1941, shortly before every unit equipped with the type left China to prepare for the operation against Pearl Harbor. Early and accurate Allied reports did exist: two A6M2s fell into Chinese hands more or less intact, the wreck of V-110 examined in September 1940 by a team that included Western volunteers, and V-173 after a forced landing on 17 February 1941, the latter repaired and flown extensively by a team that included Gerhard Neumann. The reports were met with scepticism, since most American officers did not believe Japan capable of building such an aircraft.

At the time of the attack on Pearl Harbor there were 521 Zeros active in the Pacific, 328 of them in first-line units. The type the Americans met was the carrier-borne Model 21, whose radius of action of over 2,600 km let it appear far further from its carriers than anyone expected and led Allied commanders to credit the Japanese with several times the number of aircraft actually in service. The combination of manoeuvrability and firepower disposed easily of the fighters sent against it in 1941, and the A6M proved a difficult opponent even for the Supermarine Spitfire: without being faster, it out-turned the British fighter, sustained a very steep climb, and could stay airborne three times as long.

The Allied answer came along two lines, technical and tactical. On 4 June 1942, during a raid on Dutch Harbor, anti-aircraft fire hit an A6M; Flight Petty Officer Tadayoshi Koga tried an emergency landing on Akutan Island, some 32 km to the north-east, and the aircraft flipped over on soft ground. Koga died instantly and his wingmen, hoping he had survived, refrained from destroying the machine as doctrine required. An American salvage team found it more than a month later and shipped it to Naval Air Station North Island, where test flights of the repaired Zero revealed both its virtues and its limits. The evaluators measured about 2,360 kg fully loaded, roughly 1,260 kg less than the F4F Wildcat then standard in the US Navy; they described an airframe built like a fine watch, with flush rivets and guns flush with the wings, and an instrument panel of exemplary simplicity. What struck them most was that fuselage and wings were built in one piece, a slower method than the American one but one that produced a very strong structure. Test pilots found the controls very light at 320 km/h but stiffening above 348 km/h to protect the wing, established that the aircraft could not follow Allied types in high-speed manoeuvres, that its low never-exceed speed made it vulnerable in a dive, and that it did not roll to the right as quickly as to the left, a detail that could be exploited.

The tactical answer was the diving pass: come down from above at high speed, fire a short burst and climb away, never entering a sustained turn. That was how the F4Fs fought in the defence of Guadalcanal, backed by a warning network of coastwatchers and radar, and how the P-40s of the American Volunteer Group fought in the China-Burma-India theatre, trained by Claire Lee Chennault to exploit their aircraft's ruggedness, armament and diving speed. To this was added the defensive formation devised by Lieutenant Commander John S. Thach, the Thach weave: two fighters flying about 60 m apart which, as soon as a Zero settled on the tail of one of them, turned towards each other, bringing the pursuer into the wingman's sights. Thach first tested it in combat at Midway in June 1942, when a squadron of Zeros attacked his flight of four Wildcats, and the manoeuvre became standard in the Navy and was adopted by Army fliers as well; it was used to good effect again over the Solomons and at the Battle of the Santa Cruz Islands.

The Japanese ace Saburō Sakai recorded the other factor: the structural toughness of Grumman aircraft, which in his account absorbed hundreds of 7.7 mm rounds without falling, something unthinkable in a Zero. Armour, protected tanks and new tactics together exacted a heavy price among veteran Japanese airmen. Aircrew losses in the Coral Sea, at Midway, in the Eastern Solomons and at the Santa Cruz Islands, all in 1942, cost the Japanese carrier air arm a core of experience it never rebuilt, and the steady decline in average pilot quality became a decisive factor in Allied success.

The arrival in theatre of the Lockheed P-38 Lightning, the Grumman F6F Hellcat and the Vought F4U Corsair changed the terms of the problem. Against faster, better-protected aircraft armed with six .50-calibre machine guns, or four plus a 20 mm cannon in the P-38's case, a fighter with neither armour nor self-sealing tanks had little margin. Worse, the A6M could not be improved: its engine and airframe had no capacity to absorb the weight that protection and heavier armament demanded, and adding them would have destroyed the very manoeuvrability that justified the design. From 1943 it remained lethal in competent hands, and units based at Rabaul and Lae with experienced pilots fought Corsairs, Lightnings and Hellcats over New Guinea to something close to parity for months. In late 1943, with Japan turning to the defensive, the naval staff at Rabaul discussed for the first time giving the Zero bullet protection: the type's purely offensive qualities, beginning with its range, had ceased to be necessities. That same staff still regarded it as the best fighter in the theatre, while conceding that the Corsair's speed in level flight made it a match.

By the time the war reached the Marianas the decay was irreversible. At the Battle of the Philippine Sea in June 1944, a patrolling A6M found Task Force 58 at 05:50 and was shot down shortly afterwards; the aircraft based on Guam took off to attack and thirty-five of them were destroyed for the loss of a single Hellcat. The first wave from the Japanese carriers, 68 aircraft strong, lost twenty-five machines within minutes for one American aircraft and sixteen more immediately after, and none reached the enemy carriers. A fourth wave, misdirected, was intercepted while landing on Guam, where thirty of forty-nine aircraft were shot down and the rest damaged beyond repair. American aviators christened the day the Great Marianas Turkey Shoot; the imbalance is generally attributed to an abundance of well-trained Allied pilots, to superior tactics and to the new anti-aircraft proximity fuze, set against Japanese replacement crews with few flying hours and almost no combat experience.

In the defence of the home islands the A6M remained the most numerous Japanese fighter, but the P-51 Mustang outclassed it in the altitude band where the raids operated, even if at low and medium level the Zero could still out-turn or out-climb it. On 29 May 1945, 150 A6Ms tried to intercept a daylight incendiary attack on Yokohama flown by 517 B-29s escorted by 101 P-51s: five B-29s fell and a further 175 returned damaged, while the Mustangs claimed twenty-six kills and twenty-three probables for three fighters lost. The 454 B-29s that reached the target destroyed 18 km² of the city's main business district, and over a thousand people were killed.

The family evolved without ever escaping its original compromise. After the initial Model 11, folding wingtips were introduced from the sixty-fifth aircraft onward to give the Model 21, the emblematic version of the war's first half, with a 520 l wing tank and a 320 l drop tank; 740 were completed by Mitsubishi and another 800 by Nakajima. On that basis Nakajima built the A6M2-N "Rufe" floatplane, and Hitachi and the Sasebo naval air arsenal delivered 508 A6M2-K two-seat trainers. The Sakae 21, with a two-speed supercharger and 1,130 hp, flew in a Zero on 15 July 1941 and produced the A6M3 Model 32: to offset the shift in centre of gravity the engine mountings were cut back by 185 mm, which reduced the main fuselage tank from 518 to 470 l, and the wing was shortened by deleting the folding tips. The result gained barely 11 km/h and gave up nearly 1,000 km of range, a serious error in the Solomons campaign, where Zeros from Rabaul flew close to their limit to reach Guadalcanal and return; only 343 were built. The correction came with the Model 22, which restored the folding tips and raised internal fuel to 570 l: some 560 aircraft were ready by December 1942, before the type had even been formally accepted.

The A6M5 Model 52, with a shortened, square-cut wing, revised ailerons and flaps and a new exhaust system that contributed residual thrust, is often considered the most effective variant: it reached 565 km/h at 6,000 m and climbed to that altitude in seven minutes and one second. It flew in August 1943 and was built mainly at Nakajima's Koizumi plant, with a chain of refinements identifiable by production number: carbon dioxide fire extinguishers in the wing tanks from 4274, the Model 3 radio from 4354, shortened lower exhaust stacks from 4551, and from 4651 a belt-fed Type 99-2 Mark 4 cannon carrying 125 rounds per gun, replacing the earlier drum-fed weapon and its 100. The sub-variants gradually added what the original design had refused: the 52b traded a 7.7 mm gun for a 13.2 mm weapon and added 45 mm armoured glass, while the 52c added 13.2 mm guns in the wings, 8 mm of armour behind the seat, 55 mm armoured glass at the headrest and rails for rockets; it gained so much weight that it was used chiefly to intercept B-29s and for special attack. There were also night-fighter conversions with a 20 mm cannon behind the pilot firing obliquely upward, ineffective for want of radar, and fighter-bombers carrying a 250 kg bomb in place of the ventral drop tank.

The last rungs of the ladder were never finished. Only one prototype of the A6M6, with a Sakae 31a and water-methanol injection, was built. The A6M7 Model 62/63, with a reinforced fin and a rack for 250 or 500 kg bombs, entered production in May 1945 and was the last variant in service, also used for suicide attacks. The A6M8 replaced the Sakae, by then out of production, with a 1,560 hp Mitsubishi Kinsei 62, sixty per cent more powerful than the A6M2's engine, which forced a complete redesign of cowling and nose: two prototypes were completed and flown in April 1945, but nothing came of the planned programme of 6,300 aircraft. Nor did the successor arrive. The A7M Reppū, ordered from Horikoshi in 1942 with a requirement to exceed 639 km/h above 6,000 m while keeping the A6M3's manoeuvrability, was crippled by its powerplant: the first prototype, with a 2,000 hp Nakajima Homare 22, was underpowered and only eight A7M1s were built, and the design never got past prototypes, although Saburō Sakai, who tested it at Nagoya in June 1945, came away impressed.

The type's final role was one no designer had foreseen. When Vice Admiral Takijirō Ōnishi organised the first special attack unit in the Philippines in October 1944, the 1st Air Fleet based at Manila had just 41 aircraft, 34 of them A6M fighters. It was those machines, carrying bombs, that made up the four original subunits — Shikishima, Yamato, Asahi and Yamazakura, names taken from a poem by the classical scholar Motoori Norinaga — with which the kamikaze campaign began.

The A6M stayed in production until 1945, which had never been the plan and which is explained by shortages of high-powered engines and by the failure of its successor. Totals differ by source: the United States Strategic Bombing Survey report gives 10,934 aircraft, of which 10,094 were A6M fighters proper, 323 A6M2-N floatplanes and 517 trainers; Francillon raises the total to 11,291, with 10,449 A6Ms, 327 A6M2-Ns, 508 A6M2-Ks and 7 A6M5-Ks. On either count it was the most-produced Japanese fighter of the war.

Its reputation outlived its obsolescence. Eric Brown, chief naval test pilot of the Royal Navy, summed up the Allied verdict when he recalled never having flown a fighter that could match the Zero's rate of turn, and that until mid-1943 it was the finest fighter in the world. The A6M remains the symbol of Japanese naval air power in 1941-1942 and, at the same time, the textbook case of a design pushed so far in one direction — lightness, range, manoeuvrability — that it left no room to evolve once the war changed shape. Horikoshi, who after the Zero produced the J2M Raiden and the A7M Reppū, was permanently associated with this aircraft. A few examples survive: the only remaining Model 32 is displayed at the Tachiarai Peace Memorial Museum in Fukuoka, and an A6M5 Model 52 is part of the collection of the Smithsonian's National Air and Space Museum.