Japanese Aircraft in Thai Military Service
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Japanese Aircraft in Royal Thai Air Force and Royal Thai Navy Service During World War II

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During World War II, Thailand was a reluctant ally to Japan. During the 1930's, the USA had been the main supplier of aircraft to Siam, as Thailand was then known. During the Franco-Thai war in 1940-1941, Thailand had received support from Japan, including aircraft. After Japan had attacked Thailand on December 8, 1941, a Pact of Alliance between Japan and Thailand was signed on December 21, 1941. The old Thai proverb, that the tree that bends with the wind is the tree that survives the storm, was indicative of the Thai mentality. Nearly hundred Japanese aircraft of eight different types were delivered to the Royal Thai Air Force between 1940 and 1943. Nine former Netherlands East Indies Air Force, NEIAF, Martin bombers were supplied to the RTAF by Japan as well. The Royal Thai Navy received 27 Japanese aircraft of three different types between 1938 and 1944. Contacts between Japan and Siam, as Thailand was known until 1939, had been tentative. Siam had until the late 'twenties relied heavily on France as the main supplier of military aircraft. By the early 1930s however, the American companies Curtiss and Vought had taken over after the French, delivering Hawk II and III fighters and V.93S Corsair light attack aircraft to the Siamese Royal Aeronautical Service, while the British designed Avro 504N served as primary trainers. Both Curtiss Hawk III's and Corsairs were produced under licence in Siam, as was the Avro 504N.

A non-stop flight between Tokyo and Bangkok had taken place in 1936, along with several proving flights during October and November the same year. During 1936, negotiations between Dai Nippon Koku Kaishu (Greater Japan Air Lines) led to an agreement, signed on November 1, 1936, of opening a regular airline service between Tokyo and Bangkok, beginning February 1, 1940. The route would stretch from Fukuota - Taihotu (Taipei) - Hanoi and Bangkok. Lengthy negotiations with the French authorities in Hanoi meant that the first regular service was flown during June, 1940 instead of February.

The Royal Aeronautical Service was originally organized as part of the Army, not becoming a separate and independent branch of the Siamese armed forces until April 12, 1937. The new service was known as the Royal Siamese Air Force, being re-named yet again in 1939 as the Royal Thai Air Force. By the late 1930's, the Royal Siamese Navy had plans for an air arm as well. Although the Navy had had little interest in aviation matters, their attitude changed after the 1932 coup d'etat. In 1935, two 1,400 ton corvettes, named the Tachin and the Maeklong, were ordered from Japan. A small observation floatplane was to be operated from each corvette. Apart from pilot training, there seems to have been very little contact or co-operation between the Royal Siamese Air Force and the Royal Siamese Navy. In 1937, several Naval officers were sent to the Air Force flying school at Don Muang. Although coastal reconnaissance was the responsibility of the Royal Siamese Navy, no aircraft had been acquired. Three sets of metal floats, manufactured by Edo, were purchased by the Royal Aeronautical Service for use on its Corsairs. No Corsairs were ever equipped with floats, though.

As the plans of acquiring observation aircraft went ahead, the Royal Siamese Navy entered negotiations with the Watanabe Tekkosho K.K. (Watanabe Iron Works, Ltd.) regarding the supply of such an aircraft. The Watanabe company had designed a small observation biplane, known as the Watanabe E9W1, that would be able to operate from large u-boats. The Royal Siamese Navy observation aircraft was based on the E9W1, but with a slightly bigger span and length. Although similar in appearance, the WS-103S and the E9W1 were two different types of aircraft. The WS-103S was designed and built specifically for the Royal Siamese Navy. Two naval officers had been dispatched to Japan in November 1937, to oversee the construction of the new aircraft. The first flight occured during February 1938, with the six aircraft ordered being delivered in May 1938. The Siamese variant was designated the WS-103S (the S standing for Siam), of which six were delivered to the Royal Siamese Navy in May 1938. In the Watanabe WS-103S, the Royal Siamese Navy had an aircraft able to perform its intended duties of observation and scouting, being equipped with a short-wave radio. Its arnament consisted of three Madsen 8-mm machine guns, one in the fuselage and two mounted in the upper wing. One additional machine gun was operated by the observer. As an extra set of flight controls could be installed in the rear cockpit, the WS-103S could also be used as an advanced trainer. Incidentally, small numbers of theWatanabe E9W1 were ordered by the Imperial Japanese Navy. A few were still in service during the early months of the Pacific War, earning the Allied codename "Slim." According to a 1938 report in the Swiss aviation magazine "Interavia", 200 Siamese officers were to receive training by the Imperial Japanese Air Force. Although negotiations were being conducted with both the USA and Great Britain, the Japanese influence on aviation in Siam appeared to be on the increase, with the delivery of the WS-103S's, as well as negotiations on airfield construction.

During the Franco-Thai war in 1940-1941, Japan had supported Thailand, supplying bomber aircraft. A series of incidents between French colonial forces in Indochina and Thailand escalated into open war in late November, 1940. The war ended on January 28, 1941, after Japanese diplomatic intervention. France was forced to cede a considerable amount of territory to Thailand. The Royal Thai Air Force had acquitted itself well, claiming five French aircraft as shot down, with another 17 destroyed in bomb raids. The RTAF had suffered losses as well, admitting losing three aircraft as shot down, and between five and ten destroyed on the ground in air raids. Yet another three aircraft were lost in accidents. One Corsair was captured by French forces. Thirteen air crew had been killed and another five wounded. During the evening of December 7, 1941, the Thai Government received a request from Japan to allow Japanese troops to pass Thai territory for attacking British forces in Burma and Malaya. If this request was used, Japan would use force. While the issue was being debated, the Japanese attacked. The first Japanese forces crossed the border around 0400 hours the next day, December 8. The Japanese invasion force landed at four different places along the Thai coastal provinces, including Samut Prakarn south of Bangkok. The Thai forces resisted as best they could, but were overwhelmed by the numerically superior Japanese forces. Three Hawk III's were shot down during take off from Weatatna Nakorn airfield at Aranya Prathet by 11 Nakajima Ki-27bs of the 77th Sentai, while at Prachaub Kirikhan, at least two Hawk III's were lost by the resident 7th Wing. On 0730, a cease-fire was reached. The RTAF had by then lost 39 men, mostly at Prachaub Kirikhan, and at least five aircraft, including five Curtiss Hawk III's. In return, about 400 Japanese soldiers had been killed or wounded by the Thai defenders.

A Pact of Alliance between Japan and Thailand was signed on December 21, 1941. One week earlier, on December 14, the Thai Prime Minister Phibun had agreed to a Japanese request of supporting Japanese forces on the northern Burmese front. This would include both Army and Air Force units. The RTAF was reluctant to co-operate with the Japanese, but there didn't seem to be any alternative. Air Vice-Marshal Athuk stated that the RTAF had no choice but to carry out Prime Minister Phibuns orders and act as professionals. Traditionally, the RTAF had had strong ties with France, Great Britain and the USA, but there was no option other than becoming a reluctant ally to Japan. The Kong Bin Yai Phasom Phak Payab (Northern Combined Air Wing) was formed, being based at Lampang. Three squadrons, Foong Bin 21, equipped with nine Corsairs, Foong Bin 41, equipped with ten Curtiss Hawk III's and Foong Bin 62 with nine Ki-21-Ibs. When the pacific war started, the RTAF changed its insignia from a roundel in the Thai colours, blue, white and red, to a rectangle in the same colours as the Thai flag. During late 1942, the RTAF national markings were changed yet again to a red rectangle upon which a white elephant was superimposed. The change of the national insignia was made to avoid confusion with French and British aircraft.

Type Q'ty Designation In Service Remarks
Martin 139 & 166 9 B.Th3 1943-1945+ Ex NEIAF. Six Martin 139W had been delivered in 1937, giving a total of 15 Martin bombers in RTAF service.
Mitsubishi Ki-21-Ib 9 B.Th4 1940-1945+
Mitsubishi Ki-30 24 B.J2 1940-1950
Nakajima Ki-27b 12+ B.Kh12 1942-1945+?
Nakajima Ki-43 27 B.Kh13 1943-1949
Tachikawa Ki-36/55 24 B.F6 1942-1945+

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First Created: 12 July 2004 - Last Revised: 12 July 2004
Copyright © 2004 Jan Forsgren.     e-mail: john@aeroflight.co.uk