Chapter 4

Into Action

1940 was the year of transition for the Japanese Navy’s fighter force, with 153 A5M4s being produced against 98 of the new A6M2s. The figures are very modest for both types considering the combat involvement in China but they make reflective reading. In the first four months of the year just one A6M2 per month appeared compared with A5M4 figures of 20, 19, 17 and 14 respectively. In the next three months A5M4 production remained steady at fourteen machines per month, while in May four of the new fighters were produced, with three in June and nine in July. August saw A6M2 production exceed A5M4 numbers for the first time, but only six of the latter and eight of the former appeared. September was similar with just five A5M4s against nine A6M2s. The last three months saw a continuation of the trend with the A5M4 tailing off to eleven, nine and ten only while the A6M2 totals were nineteen, twenty-three and nineteen respectively for October, November and December.¹

Meanwhile, knowledge of the new Navy fighter had long circulated among the veterans at the front over the three years of development and their demands for its earliest introduction led, as we have seen, to the despatch of fifteen preproduction Rei-sens. There was strong opposition from many in the Navy against this move, which was felt to be highly premature. Although impressed by the aircraft’s performance many felt that further flying experience was necessary as minor problems were still being experienced. Even top-notch pilots needed to get accustomed to such a powerful new machine before being pitched into combat. Teams of highly qualified pilots were being assembled and were undergoing intense training and these had encountered difficulties. Problems were experienced during the firing of the 20mm cannon, already suspect by the China Group as we have seen, when they tended to jam at high acceleration. The fault was found to be inadequate clearance in the cartridge ejector tube, which had to be modified. Such a blockage might very well have proved fatal if encountered in actual aerial combat. A different problem occurred on 25 June when Lieutenant-Commander Manbeye Shimokawa’s group of six aircraft was conducting climb and altitude testing. One aircraft had a sudden oil pressure malfunction at 25,590.6ft (7,800m) altitude and a second reached 33,793ft (10,300m) before suffering engine difficulties. Analysis of the problems found that due to the A6M’s good, if not exceptional, climb rate, fuel cooled extremely rapidly and vaporized, chocking the pipe. The solution was found to be the use of a newly introduced aviation fuel with a 92-octane rating. Another fuel-related problem concerned the new streamlined long-range ventral drop tank. This was designed for quick-release when combat was imminent but in practice it sometimes refused to detach unless the aircraft’s speed was below 205mph (330km/h). Fortunately this unintentional retention was found not to impair the A6M’s actual aerial performance unduly when it occurred. Finally, cylinder overheating problems were encountered during dog-fighting practice or when climbing under full-power. This glitch was unresolved when the units were suddenly ordered to China.

Despite these teething problems the urgency from the front was such that these aircraft flew out with their picked, but still not yet fully familiar, pilots, in two groups, six under the command of an experienced combat pilot from the carrier Sōryū, Lieutenant Tamotsu Yokoyama of the Omura Kōkūtai, and nine under Lieutenant Saburo Sugindo of the Yokosuka Kōkūtai. Initially these A6Ms were allocated to operate under the 12 Rengo Kōkūtai (Joint Naval Air Corps) commanded by Captain Kiichiro Hasegawa, based at Hankow on July 21—which was ten days before the IJN officially accepted the A6M2 into service. Overall command was vested in Admiral Shigetarō Shimada of the China Air Area Fleet. The move also necessitated the despatch of ten newly trained A6M ground servicing teams, plus specialist technicians to work on the faults while in the field; these included armament specialist Sotoji Inzei, engine specialist Lieutenant Osamu Nagano and Lieutenant Shoichi Takayama to oversee airframe problems.²

Under the IJN organization each Daitai (unit) comprised a base unit with between eighteen and twenty-seven operational aircraft and up to twelve reserve machines. The entire A6M complement of a carrier division was termed the Kōkū-sentai. Each type of aircraft, fighter, dive-bomber, torpedo-bomber, had its own hikōtai (squadron) with a hikōtaichō (squadron leader) under overall command of a rear-admiral, and was composed of hikōtai (squadrons) under the command of a lieutenant (j.g) or warrant officer. Each had a fukuchō (executive officer or XO). Normally nine aircraft comprised a chūtai (division), which normally had nine aircraft on strength plus three reserves. There were usually three sub-divisions, shōtai, with three aircraft apiece, usually deployed in inverted ‘V’ formation and the leading aircraft stepped-down a little, described by one RAF pilot as a ‘… the loose trailing formation …’. Later, from 1944 onward, a different formation was adopted following American practice and became based on the two-plane buntai. The shōtai became four-plane formations and chūtai eight with the daitai reduced to sixteen, while the hikōtai became uniform and adopted a two-or three-numerical designation.³

In fact the initial fifteen Model 11 A6M2s⁴ that were sent over to China for combat evaluation, were hand-made pre-production prototypes. Immediately upon arrival at Hankow there were high expectations among veterans of air fighting conditions over China, chiefly Rear-Admiral Takijirō Ōnishi, 2 Rengo Kōkūtai 2 Joint Air Corps) commander and Rear-Admiral Tamon Yamaguchi, 1 Rengo Kōkūtai (1 Joint Air Corps) that they could go straight into action. Both men were ‘fire-eaters’ who wished to get stuck into the enemy, but Yokoyama had to tactfully disabuse his two superiors and explain that the Zero was not quite ready. Even so he was under considerable and constant pressure to get his units into battle. A suitable ‘fix’ was found for the cylinder overheating problem and meanwhile nine further A6Ms flew in under Lieutenant Saburo Shindo and were similarly modified. The Zero was finally ready for action.

Although pronounced fit to fight, actually achieving any combat contact with an increasingly reluctant Chinese enemy was itself a frustrating experience as they once again stubbornly refuse to engage. Whether forewarned by spies at Hankow or not, the Chinese policy was to avoid all air action. The first mission of the A6M took place on 19 August and saw Lieutenant Yokoyama lead twelve of them as escort for fifty-four Mitsubishi G3M Rikko Navy bombers⁵ for a bombing attack on Chungking. For the A6M this mission, involving as it did a round-trip of over 1,118 miles (1,800km) by a single-engined aircraft was a validation of its long-range credentials. The hitherto immunity of distant enemy targets abruptly ceased with the arrival of the Zero. Thirty enemy fighters were reported in the air and waiting for them; however, whether or not the Chinese had been forewarned, they once more refused the bait and absolutely no aerial opposition was encountered over the target. The bombers discharged at will with no reaction and all returned to base unscratched. The same lack of reaction was encountered the following day when Lieutenant Shindo carried a similar escort mission of G3M bombers to the same target. All the A6Ms could do was make a strafing attack on Chonmachow airfield on their journey back to base.

The Navy was about to terminate the bulk of its involvement in China, planning to withdraw all its aircraft save a few selected units and this inability to draw up the Chinese defenders and administer a drubbing before they left was frustrating. It therefore was decided to leave the special group of Zeros and a group of bombers to continue operations for a further period. The Japanese tried again on 12 September, with Lieutenant Yokoyma leading twelve A6Ms escorting twenty-seven bombers this time. Accompanying this force was a Tachikawa Ki-36 Type 98 Direct Cooperation Reconnaissance aircraft. A monoplane (later codenamed ‘Ida’), this machine had a service ceiling of 26,740ft (8140.4m) and she circled the Chungking area at high altitude and took aerial photographs while the attack was underway. Once again no air combat resulted but the Japanese were able to analyse that thirty-two Chinese fighter aircraft identified on the ground prior the attack, took off and sought sanctuary in the mountains during the raid. Once the Japanese aircraft had withdrawn these fighters re-appeared and after a brief sortie landed once more.

Accordingly, Yokoyama and Shindo laid a simple trap to surprise these reluctant warriors into battle. On 13 September yet another air raid was mounted against Chungking and thirteen A6Ms participated in two groups, commanded respectively by Lieutenant Saburo Shindo in person and Lieutenant (j.g.) Ayao Shirane. The formation was again observed from on high by the Tachikawa Ki-36. The fighters stationed themselves above and behind the bombers during the actual attack and, as before, departed in the same formation on completion of the bombing run. However, the reconnaissance aircraft maintained a watch above the city and was duly rewarded when Chinese aircraft were once more seen returning to the area from their south-westerly loitering point. The scout immediately radioed this information to Shindo, and the thirteen A6Ms reversed course once more taking up a watching position to the north of the city, an area of the sky in which they were obscured from the approaching Chinese, which were deployed in nine sections each of three fighters, twenty-seven machines in total. The enemy were identified as Soviet-built and supplied Polikarpov I-15 and I-16 fighters.

Unwittingly then the Chinese Air Force had seemingly obligingly played to the A6M’s strengths and paid the penalty. The A6Ms had the advantage of altitude and surprise and attacked immediately before the enemy could flee once more. Within the space of ten minutes every one of the Chinese fighters were destroyed without loss to the Japanese. In addition to their speed, which enabled the Japanese aircraft to easily overtake the fleeing enemy, Horikoshi was told that the smashing power of the 20mm cannon was decisive; as he later explained, ‘… if the shells hit the wing root, they destroyed the entire wing. This is understandable if one remembers that the wing of an aircraft is under heavy load when it is going at high speed, and under such circumstances the wing will come off very easily when hit.’ Those not destroyed in the one-sided dog-fight were wrecked by crashing after three of their pilots prematurely baled out, while two others went in trying to evade combat.⁶ Two more were torched on the ground at Baishiyei field. The only damaged received by the Japanese aircraft was that one, piloted by Ensign Yoshio Oki, was hit in the fuel tank, which failed to release, but did not explode, and three others who had trivial damage.

So pleased was the Navy at the magnificent debut that what were considered to be three main contributors, the Mitsubishi design team, the Dai-Nihon Heiki (Japanese Weapons Company) for the 20mm cannon and Nakajima Aircraft for the Sakae engine, were summoned directly to Tokyo on 14 September for a special commendation ceremony at the Navy Ministry itself overseen by Vice-Admiral Teijiro Toyata, Chief of Naval Aeronautics. However, in order to conceal the performance of the new fighter from potential enemies the terms Zero or Navy Type O were, for the time being, censored from the press who referred to the victorious fighters and their crews by the euphemism ‘Sea Hawks’.

Clearly the A6M had proved a great success and rapid expansion was ordered. In common with the usual existing IJN practice at this time the Navy extended the production to include the rival Nakajima Company. Mitsubishi was paid for the design and then transferred all the necessary documentation over from Nagoya to Nakasaki’s Koizumi plant. This ensured a smooth transition and production was thus almost seamless, with the first Koizumi Zero being accepted as early as September 1941.

None of the subsequent aerial fighting exceeded this stunning debut, but the operational qualities of the A6M impressed both friend and foe alike and from then on the Japanese dominated the air war in China, while the Chinese found fewer and fewer sanctuaries. The next mission they were involved in was mounted on 16 September, and again the target was Chungking, but they could only find a single enemy aircraft, which was summarily dispatched. The skies were clear and it was plain that even longer-range sorties would be necessary to flush their foes out of hiding. Accordingly, on 4 October, eight A6Ms in two groups of four, under Lieutenants Shirane and Yokoyama, flew cover for twenty-seven G3M bombers against Chengtu, Szechwan Province. The flight involved a refuelling stop at Ichang. Only one enemy fighter was encountered near the target and this was promptly destroyed by Petty Officer Matsuo Hagiri. The fighters went to Taipingsi airfield where they located and destroyed five I-16Bs and a Soviet Tupolev SB-2 medium bomber before machine-gunning and destroying nineteen aircraft on the ground, damaging four more and many of the airfield facilities. Not content with this, and to show their contempt for the enemy, four A6M pilots, Warrant Officer Ichirō Higashiyama and Petty Officers Matsuo Hagiri, Masataka Nakaishi and Hideo Oishi, deliberately landed their aircraft on the runway. Their plan was to leave their aircraft engines running and by hand, personally destroy remaining enemy machines. This was a foolhardy and quite dangerous idea, typical of the aggressive mindset of the Japanese of course, but could have left four of Japanese most secret fighter aircraft at the mercy of the enemy. Strong defensive fire from the Chinese troops guarding the airfield forced this mad scheme to be aborted promptly but all four got away safely. Subsequently two I-16s and a Soviet-supplied Tupolev SB twin-engined bomber were destroyed. Only two A6Ms received slight damage and the unit received yet another special commendation for the mission.

On 5 October a raid was made on Chengtu’s Fenghuangshan airfield with just seven A6Ms under Lieutenant Fusata Lida; ten aircraft were destroyed along with fourteen decoys.

Meanwhile, from 5 September Japan’s southern expansion had continued when the pro-Nazi Vichy Regime in French Indo-China agreed to permit Japanese forces to move into French air bases in the Saigon area, Thu Dau Moi, Soc Tranaga, Phu Quoc and others. Into these bases flew one hikōtai of the 12 Rengo Kōkūtai (12 Joint Air Corps) A6Ms to conduct long-range escort missions. Their first success was during an attack on Kunming when seven A6Ms and nine A5Ms escorted Aichi D3A1 dive-bombers (later codenamed ‘Val’). During an aerial melee there thirteen Chinese aircraft were shot down. A similar mission against Siangyun on 12 December saw seven A6Ms involved and they destroyed twenty-two Chinese aircraft caught on the ground, while on 8 October to 31 December 1940, they flew a total of twenty-two sorties but only managed to find and destroy two enemy aircraft.

Meanwhile the IJN recalled all its G3M bombers back to Japan leaving just the remaining thirty A6Ms at Hankow to continue to fly missions against the familiar targets of Chunking and Chengtu. No enemy were flushed on 10 October, but on the 26th of that month eight A6Ms caught and shot down ten Chinese aircraft over Chengtu, while on 30 December eight Zeros made a fourth attack over that cities airfields, strafing Fenghuangshan, Taipingssu, Shuanglin and Wenchiang, during which thirty-two enemy planes were destroyed as well as wrecking ground facilities, all without loss, although a pair of A6Ms were slightly damaged by flak.

By the end of 1940 the A6M’s combat record was impressive. Not a single Zero had been lost, and just thirteen slightly damaged, mainly by ground fire, while in return they had mounted 153 sorties, and achieved 212 ‘kills’ both aerial and on the ground. When aerial fighting resumed targets remained scarce but, on 14 March, again over Chengtu, the A6Ms led by Yokoyama managed to intercept a strong Chinese squadron and destroyed two dozen of them, with three probables. The G3M bombers returned in May along with the new Mitsubishi G4M (late code-named ‘Betty’) and the A6M’s resumed long-range escort missions once more.

On 20 May thirty Zeros yet again strafed installations and parked aircraft at Shuanaglin and Taipingssu fields at Chengtu. Here the A6Ms’ incredible good fortune finally ran out and they suffered their first casualty. The aircraft of Petty Officer Eichi Kimura was heavily hit by ground fire and it crashed, killing the pilot. Three other Zeros suffered damage on this day, an indication that the German-trained and controlled flak gunners were getting their eyes in. Kimura’s aircraft wreckage was located and examined and a fairly accurate report on its armament and other details was compiled by the Chinese. This information was subsequently passed on to Singapore via Hong Kong and thence forwarded to London. As we have recorded the British Air Attaché at Chungking also sent details of the estimated performance figures for the machine, which he stated was ‘Good, by any standard’, which the Combined Air Intelligence Bureau also forwarded but it failed to make much impression. The RAF’s Air Vice-Marshal Conway Walter Heath Pulford stated on 15 October 1941 to assembled pressmen that the ‘Navy O’ fighter was ‘… on a par with our Buffalo …’.⁷

Attacks continued, 22 May seeing nineteen A6Ms deployed against Chentgtu, where they shot down a pair of Chinese machines and burnt ten more on the deck, but they suffered five of their numbers hit by antiaircraft fire in return. On 26 May nine A6Ms hit Nancheng where they engaged twenty Chinese machines and shot down five of them without loss to themselves before strafing and destroying another eighteen aircraft on the ground. On 27 May twenty A6Ms were over Lanchow but the Chinese refused combat, although a pair were torched on the runway. It was back to Chungking once more on 7 June when four Zeros shot down one of a pair of Chinese fighters. Another large group, totalling twenty A6Ms again sortied against Lanchow but encountered no aerial opposition, but again fierce flak damaged two of their number.

On 22 June nine Zeros were over Chengtu again, where they found a few targets on the ground, which they strafed. At the same time a further six had better fortune when they engaged six Chinese fighters over Kanyuan, and shot down half of them without loss. Seven further A6Ms raided Tienshui with seven aircraft and destroyed a solitary enemy plane there, while on 27 June it was the turn of Ipin, where six A6Ms destroyed two and damaged five other Chinese aircraft.

The second combat loss of a Zero occurred on 23 June. She was part of a three-plane escort for a pair of Type Mitsubishi C5M2 Type 98 Command Reconnaissance aircraft (later code-named ‘Babs’) at low-altitude in the area of Lanchow and Yuncheng. This group was bracketed by extremely accurate anti-aircraft barrage and the mount of Kichiro Kobayashi was heavily hit and crashed, killing the pilot.

Not until 11 August did the A6Ms manage to bring the enemy to battle once more; this time the engagement took place above Chengtu. Seven of the new G4M bombers were under escort of sixteen Zeros this day when the Chinese decided at last to offer resistance. While defensive fire from the bombers claimed two enemy fighters, the Zeros’ 20mm cannon destroyed three more before the fighters disengaged, without loss to the Japanese force. Seven further enemy machines were destroyed on the ground and nine damaged to varying degrees.

Finally the A6M’s range was tested as never before with a raid planned against Sung-p’an (Songpan, Sichuan Province). Due to its remote location and barred by mountain ranges, this city had only been attack once before. The plan was for five of the best Zero pilots to mount the operation being led through the difficult approaches by a pair of Tachikawa Ki-36 98 reconnaissance aircraft. The mission was mounted on 31 August, but the weather conditions were too intense and all seven aircraft were forced to abort. This proved to be the final Zero sortie of the ‘China Incident’ for now the war clouds were lowering over the Pacific and the Navy was withdrawing its eagles to their eyries in readiness for more serious opposition.

The operations of 1941 had seen the A6M conduct 354 sorties and they had destroyed forty-four Chinese aircraft in aerial combat and damaged sixty-two more. All this cost the Zero just two of their number, both destroyed by flak, while twenty-six had minor damage.

The final commander of the A6M detachment in China had been Ayao Shirane and the unit had built up a fine record of expertise and skill during its time in China, among the recognized outstanding pilots were Koshiro Yamashita with five confirmed kills; Kihei Fujiwara and Yoshio Ohki each with four kills; Tsutomu Iwai and Toraichi Takatsuka both with three kills; Saburo Kitabatake, Masayuki Mitsumasa, Kazuki Mikami and Hatsuyama Yamaya all with two kills apiece, and Masaharu Miramoto, Ayao Shirane and Toshiyuki Sueda, each with a single kill to their credit. By this time Stalin had ordered the Russian ‘volunteers’ back home, leaving the skies to the Japanese until the much later arrival of the American Volunteer Group (AVG or ‘Flying Tigers’) equipped with Curtiss P-40B Warhawks.

At this period Mitsubishi was producing about one A6M each day at Kagamigahara, which were then flown to Yokosuka but as reports from the front came in minor changes were being incorporated all the time to tweak and improve the aircraft. As a result the strengthening of the rear wing spar was deemed desirable and this was carried out on the twenty-second A6M2 onward when the fin and rudders were enhanced. Minor amendments included the shifting of the engine exhaust tubes back from the fourth to the fifth cowl flap from airframe #35 onward. The oil cooler and carburettor intake remained combined as in the Model 11 for the initial trio of the new model, used solely as trial aircraft but subsequent to those three airframes the cooler and carburettor were separate and each had it own individual air intake. From the sixty-seventh aircraft the ‘pinched’ shape of the chin scoop changed to a full depth intake. Another problem was the re-occurrence of complaints, first encountered with the initial Prototype, of the heaviness of the ailerons at certain speeds and in certain configurations. The ‘fix’ here was invented by Technical Establishment’s Lieutenant Isao Takayama who introduced balance tabs on the trailing aileron edges, which kicked in once the undercarriage was fully retracted. This modification was carried out from the sixty-second production aircraft. With the move back to the Pacific and carrier operations once more predominating, the question of stowage aboard the new carriers, the deck elevators (lifts in Royal Navy parlance), which were of smaller dimensions, arose. The precise wingspan of the A6M was 39ft 4⁷/16in (12m) so Hirotsugu Hirayama helped to resolve this difficulty by re-designing the wing-tips so that they folded vertically upward about 19¹¹/16in (0.5m) inboard. This modification was introduced on the sixty-fifth production aircraft and resulted in a new designation, changing from land to carrier-based function from the Model 11 to the Model 21. Another change was the result of combat experience in China, where pilots had been complaining, with increasing vehemence, about the heaviness at higher speeds of the controls. The Navy’s answer was to introduce a trailing-edge aileron balance tab linked to the landing gear retraction mechanism, which reduced the problem. This was introduced on the 127th aircraft. Further aileron flutter problems resulted, which was rectified by modification of the tab balance on the 192nd aircraft.

January 1941 had witnessed the annual competition between the fighter aircraft of the Imperial Army and Navy. The A6M, piloted alternately by Lieutenant Yoshitomi and Lieutenant Manbeye Shimokawa, had been entered against the Army types Nakajima-built Ki-27 (‘Nate’), Ki-43 (‘’Oscar’) and Ki-44 (‘Tojo’). In the all-important consideration the (on-paper) superior Ki-44 was totally outclassed by the Zero. The top speed recorded by the A6M during the competition was 329mph (530km/h). All military aircraft of this period had an emergency over-boost capability (also termed war emergency power), which enabled the horse power available to be increased a few vital minutes of combat taking the speed up to around 345mph and, of course, the aircraft were all stripped down. The Japanese sometimes boosted for as long as ten minutes, but the official speed for the A6M2 was listed as 317mph (510.16km/h or 275 knots). Comparisons with the German-built Heinkel He-100 fighter, three examples of the D-O variant having been imported by the Navy as the AXHei, which had briefly having held the world speed record of 394.15mph (634.73km/h), although much faster in level flight, only emphasized the A6M’s superiority, especially in range and reliability.⁸

With regard to design, the steady improvement and progression of the Model 11 A6M received another nasty shock when, on 17 April 1941, a second A6M suffered a similar fate to the earlier accident, the aircraft breaking up in mid-air during a dive. Once again this resulted in the death of the pilot, another very experienced Navy flyer, Lieutenant Manbeye Shimokawa, who was the chief of the Fighter Division with the Yokosuka Kōkūtai. This accident naturally led to another detailed enquiry. It was established that the previous day another test pilot, Lieutenant (j.g.) Yasushi Nikaido, serving aboard the carrier Kaga, was conducting his flight training schedule in a Zero fitted with the new balance tabs, at Kisarazu airfield, Chiba. He reported first seeing skin wrinkling during tight turns. When in a 50-degree dive from an 11,482ft (3,500m) altitude he noted the same problem commencing at 6,561ft (2,000m). As his speed hit 373mph (600km/h) he started to pull out of the dive when the aircraft was shaken and he temporarily blacked out. On almost immediate recovery he found himself flying straight and level at 186.4mph (300km/h) but that the upper surfaces of both his wings had shed large areas of their skinning and also that both ailerons had disappeared while his leading-edge Pitot tube⁹ was bent and buckled. He managed to land safely and made his report, the details of which were immediately transmitted to Yokosuka.

The report was read by Shimokawa who determined to repeat the test using two A6Ms at hand, both machines also returned from the Kaga for examination having suffered similar wrinkling, one fitted with the new balance tab and one without. This was done the next day and Lieutenant-Commander Setsuzo Yoshitomi instructed Shimokawa to cease the test immediately wrinkling was observed. Shimokawa first flew the aircraft, which did not have the balance tab fitted, diving at the same angle and from the same height and reaching a terminal speed of 640km/h without any serious problems. Next he repeated the exercise in the machine fitted with the tab, again without anything untoward occurring. He then conducted a second test, commencing the dive from 13,123ft (4,000m) this time and increasing his dive angle to 60 degrees. On commencement of the pull-out, sheeting from the port wing shed off and then the whole tail section detached and the remainder of the doomed aircraft went straight into the sea, Shimokawa going down with her.

The Navy investigation that immediately followed, under Engineer Kiyoshi Matsudaira, initially came to the conclusion that failure to re-calibrate stress for the wing, aileron and control system at the time the tabs had been added resulted in unacceptable force being exerted on the aileron’s trailing edge. However, this initial verdict conclusion was later reversed following more detailed examinations. Matsudaira announced the team’s final conclusion on 13 June. Careful vibration and stiffness testing and subsequent adjustments had reduced the speed threshold of wing flutter from 466mph (750km/h) to 391mph (630km/h) on a Zero not fitted with the balance tabs. This was further reduced to only 373mph (600km/h) when balance tabs were fitted. This problem was described as the ‘Aileron Rotational Wing Torsional Compound Flutter’ effect.

The solution in the case of the A6M was a mixture of ‘fixes’; the outer wing skinning was made thicker, with deeper countersinking to enable strong rivets; longitudinal stringers were added to increase torsional resistance; a mass balance was introduced on the opposite side of the balance tab; and finally, as a temporary measure, the diving speed of the Zero was reduced to 670km/h and similar restrictions were retrospectively applied to all aircraft. As a result no further accidents of this nature ever took place. From airframe #227 onward the mass balances were once more deleted and replaced with a more aerodynamically acceptable device inbuilt to the leading edge of the aileron.

What were the A6M’s main pluses and minuses? On the plus side, being designed for such close-in air fighting it was manoeuvrable in the extreme, especially at moderate (270mph – 434.52km/h) speeds, although this asset fell away at higher velocities; its great reach, an asset in China that was also to prove equally so in reaching the Philippines in December 1941 and in the long-drawn missions down to Guadalcanal a year later; and while its 20mm cannon gave it some edge there were limitations to their value, as we have seen. One asset the Japanese did have an abundance of initially was that their pilots were combat-experienced and highly skilled. They were not trained in deflection shooting as were their opposite numbers in the US Navy, but then again, nor were the vast majority of their opponents. What the A6M lacked was any decisive speed advantage in level flight over many of its opponents at its critical altitude;¹⁰ a maximum of 332mph (534km/h) at 14,930ft (4,500m) as listed by Robert Jackson and Paul Gunston for the A6M2, was nothing extra-special,¹¹ while Okumiya and Horikoshi list a more modest 328mph,¹² which was similar to her main foe, the US Navy Grumman F4F Wildcat.¹³ Being a lightly constructed machine the A6M lacked the edge in dives over its heavier and more armoured opponents. Its most fatal flaw was its vulnerability, lacking self-sealing fuel tanks and armour protection.